V. NUTRITIONAL ASPECTS OF FATS AND OILS

A. General

Fats are a principal and essential constituent of the human diet along with carbohydrates and proteins. Fats are a major source of energy which supply about 9 calories per gram. Proteins and carbohydrates each supply about 4 calories per gram.

In calorie deficient situations, fats together with carbohydrates spare protein and improve growth rates. Some fatty foods are sources of fat-soluble vitamins, and the ingestion of fat improves the absorption of these vitamins regardless of their source. Fats are vital to a palatable and well-rounded diet and provide the essential fatty acids, linoleic and linolenic.

B. Metabolism of Fats and Oils

In the intestinal tract, dietary triglycerides are hydrolyzed to 2-monoglycerides and free fatty acids. These digestion products, together with bile salts, aggregate and move to the intestinal cell membrane. There the fatty acids and the monoglycerides are absorbed into the cell and the bile acid is retained in the intestines. Most dietary fats are 95-100% absorbed. In the intestinal wall, the monoglycerides and free fatty acids are recombined to form triglycerides. If the fatty acids have a chain length of ten or fewer carbon atoms, these acids are transported via the portal vein to the liver where they are metabolized rapidly. Triglycerides containing fatty acids having a chain length of more than ten carbon atoms are transported via the lymphatic system. These triglycerides, whether coming from the diet or from endogenous sources, are transported in the blood as lipoproteins. The triglycerides are stored in the adipose tissue until they are needed as a source of calories. The amount of fat stored depends on the caloric balance of the whole organism. Excess calories, regardless of whether they are in the form of fat, carbohydrate, or protein, are stored as fat. Consequently, appreciable amounts of dietary carbohydrate and some protein are converted to fat. The body can make saturated and monounsaturated fatty acids by modifying other fatty acids or by de novo synthesis from carbohydrate and protein. However, certain polyunsaturated fatty acids, such as linoleic acid, cannot be made by the body and must be supplied in the diet.

Fat is mobilized from adipose tissue into the blood as free fatty acids. These form a complex with blood proteins and are distributed throughout the organism. The oxidation of free fatty acids is a major source of energy for the body. The predominant dietary fats (i.e., over 10 carbons long) are of relatively equal caloric value. The establishment of the common pathway for the metabolic oxidation and the energy derived, regardless of whether a fatty acid is saturated, monounsaturated, or polyunsaturated and whether the double bonds are cis or trans, explains this equivalence in caloric value.

C. Essential Fatty Acids

Experimental work in the 1930’s in animals and humans demonstrated that certain long chain polyunsaturated fatty acids, linoleic and arachidonic, are essential for growth and good skin and hair quality. Now linoleic and linolenic acids are termed "essential" because they cannot be synthesized by the body and must be supplied in the diet. Arachidonic acid, however, can be synthesized by the body from dietary linoleic acid. Arachidonic acid is considered an essential fatty acid because it is an essential component of membranes and a precursor of a group of hormone-like compounds called eicosanoids including prostaglandins, thromboxanes, and prostacyclins which are important in the regulation of widely diverse physiological processes. Linolenic acid is also a precursor of a special group of prostaglandins. The dietary fatty acids that can function as essential fatty acids must have a particular chemical structure, namely, double bonds in the cis configuration and in specific positions (carbons 9 and 12 or 9, 12, and 15 from the carboxyl carbon atom or carbons 6 and 9 or 3, 6 and 9 from the methyl end of the molecule) on the carbon chain.

The requirement for these essential fatty acids has been demonstrated clearly in infants. While the minimum requirement has not been determined for adults, there is no doubt that they are essential nutrients. The current American diet provides at least the minimum essential fatty acid requirement. According to the Food and Nutrition Board’s Recommended Dietary Allowances (2), the amount of dietary linoleic acid necessary to prevent essential fatty acid deficiency in several animal species and also in humans is 1 to 2% of dietary calories. However, for much of the general population, 3% of calories as linoleic acid is considered to be a more satisfactory minimum intake. In the case of linolenic acid, the requirement for humans has been estimated to be 0.5% of calories.

The Committee on Diet and Health of the Food and Nutrition Board (3) has recommended that the average population intake of polyunsaturated fatty acids (primarily linoleic acid) remain at the current level of about 7% of calories and that individual intakes not exceed 10% of calories because of lack of information about the long-term consequences of a higher intake. For many reasons, especially because essential fatty acid deficiency has been observed exclusively in patients with medical problems affecting fat intake or absorption, the Food and Nutrition Board has not established an RDA for omega-3 or omega-6 polyunsaturated fatty acids.

D. Fat Level in the Diet

Fats in the diet are often referred to as "visible" or "invisible." Visible fats are those added to the diet in foods such as salad dressings, spreads and processed foods, whereas invisible fats are those that are naturally occurring in foods such as meats and dairy products.

According to the Economic Research Service of USDA, between 1970 and 1997, Americans increased their consumption of total visible fat from 52.6 to 65.6 pounds per person per year (see Table X). On the other hand, if these data are expressed as a percentage of total calories consumed, fat intake from visible and invisible sources would appear to have decreased from about 43% to about 33% of calories. This apparent paradox of increasing amounts of fat consumed per day while decreasing the percentage of fat calories is explained by the fact that while daily fat consumption has been increasing recently, total caloric intake has been increasing at a greater rate (4). This increased level of calorie intake will reduce the calculated percentage of calories from fat even though actual fat consumption has not gone down. The dietary trend of increased caloric consumption is thought to be the result of increased carbohydrate intake, larger food portions being consumed, more eating occasions, and increased soft drink and alcoholic beverage intake (4).

Fat intake is generally measured by two methods: (1) surveys of individuals recalling the amounts of foods consumed over a specific time period and (2) "consumption" estimates from food disappearance data as calculated from available sources. The latter method may overstate actual consumption estimates because food disappearance data do not account for foods wasted or discarded and lost to spoilage, trimming, or cooking. It has been estimated that wastage of deep frying fats used in the food service sector may be as high as 50% (5); therefore, food component estimates from food "disappearance" data, particularly fat, may be overestimated. The accuracy of dietary recall data is also difficult to ensure due to errors of memory in recalling foods eaten previously.

Dietary guidelines were first issued by the American Heart Association in the early 1970s in an attempt to educate Americans as to the importance of a healthful diet. The U.S. Department of Agriculture in conjunction with the U.S. Department of Health and Human Services later developed Dietary Guidelines for Americans in 1980. In 1990 these guidelines first included recommended limitations for fat consumption which remained unchanged in a 1995 update (6) of the guidelines. The guidelines for fat call for a total fat intake of no more than 30% of calories with a saturated fatty acid intake of no more than 10% of calories. Although these dietary guidelines for fat have been established for over 20 years, the relatively small changes in fat intake during this time period reflect the difficulty on a national scale in achieving dietary goals such as reducing fat intake.

E. Diet and Cardiovascular Disease

Cardiovascular diseases, which include heart attack and stroke, are the leading causes of death in the United States. The most predominant form of cardiovascular disease is coronary heart disease or CHD (commonly referred to as "heart attack") which, according to the American Heart Association (AHA), is the single leading cause of death in America resulting in 481,287 deaths in 1995 (7). Atherosclerosis, the gradual blocking of arteries with deposits of lipids, smooth muscle cells, and connective tissue, contributes to most deaths from cardiovascular disease.

Cardiovascular diseases are chronic degenerative diseases of complex etiology that often are associated with aging. A number of risk factors for cardiovascular disease have been identified from epidemiological studies. These include positive family history of cardiovascular disease, cigarette smoking, hypertension (high blood pressure), elevated serum cholesterol, obesity, diabetes, physical inactivity, male sex, age, and excessive stress. Although these risk factors have been associated statistically with the incidence and mortality of cardiovascular disease, no cause and effect relationships have been established.

During the 1950s considerable interest began to develop concerning a possible relationship between dietary fat and the incidence of coronary heart disease. This interest has continued to the present time. Since diet can affect serum cholesterol and since heart attack risk increases with increasing serum cholesterol levels, some health advisory organizations (e.g., the American Heart Association, Office of the Surgeon General, the National Institutes of Health, and the National Academy of Sciences) have recommended diet modification to achieve lower serum cholesterol levels in the general population. These diet modifications include reducing consumption of total fat, saturated fat, and cholesterol.

Researchers now recognize that total serum cholesterol is distributed largely between two general classes of lipoprotein carriers, low-density lipoprotein (LDL) and high-density lipoprotein (HDL). The largest portion of total cholesterol is in the LDL fraction, and elevated levels of LDL cholesterol are associated with increased coronary heart disease risk. On the other hand, high levels of HDL cholesterol have been associated with protection against coronary heart disease. One factor that has been related to increased levels of HDL cholesterol is regular exercise. However, it is uncertain whether diet or exercise related modifications of LDL or HDL levels will affect development of coronary heart disease. Long-term studies are currently in progress to address these questions.

The National Cholesterol Education Program (NCEP) was established in the mid-1980s by the National Heart, Lung, and Blood Institute, National Institutes of Health, to increase public and health professional awareness regarding the importance of lowering elevated serum cholesterol levels. In 1987 guidelines were established by the Adult Treatment Panel, which embodied two approaches for a coordinated strategy for reducing coronary risk (8). The first is a "population" approach, which attempts to lower serum cholesterol levels of the entire U.S. population through dietary change. The second approach attempts to identify individuals at high risk of developing heart disease and to provide them with diet and/or drug therapy.

In 1993, a second Adult Treatment Panel updated the earlier guidelines and recommendations for cholesterol management (9). The panel’s recommendations were very similar to previous guidelines. The panel confirmed that low-density lipoprotein (LDL) cholesterol should continue to be the primary target of cholesterol lowering efforts. The most significant of the few additional recommendations included adding age (>45 years in men and >55 years in women) as a risk factor for CHD.

The American Academy of Pediatrics (AAP) in concert with the NCEP guidelines endorses selective cholesterol screening of children older than two years of age whose parents have a history of CHD (10). However, the AAP does not recommend restrictions in fat intake in children younger than two years of age.

The levels of total cholesterol and the LDL and HDL fractions in the blood are influenced by a combination of factors, including age, sex, genetics, diet, and physical activity. Diet and exercise are factors which individuals can modify and thus have been a basis for recommendations to reduce risk factors for chronic diseases such as coronary heart disease. The three major categories of dietary fatty acids (saturated, monounsaturated, and polyunsaturated) appear to influence total, LDL, and HDL cholesterol in different ways. Predictive equations have been developed and applied. In general, diets high in saturated fatty acids increase total as well as LDL and HDL cholesterol levels compared to diets low in saturated fatty acids. The specific saturated fatty acids palmitic (the principal saturated fatty acid in the U.S. diet), myristic and lauric acids are considered to be cholesterol raising, whereas stearic acid and medium-chain saturated fatty acids (6 to 10 carbon atoms) have been considered to be neutral with respect to effects on blood lipids and lipoproteins.

Monounsaturated (e.g., olive, canola) and polyunsaturated (e.g., sunflower, corn, soybean) fatty acids are cholesterol lowering when they replace significant levels of saturated fatty acids in the diet. Clinical and epidemiological studies indicate that polyunsaturates lower LDL and total cholesterol. Some studies have found that diets high in monounsaturated fatty acids compared with polyunsaturated fatty acids decrease LDL cholesterol while maintaining HDL cholesterol levels (11,12). Other work has suggested that the effect of consuming polyunsaturated fat and monounsaturated fat is similar and results in a decrease in both LDL and HDL cholesterol (13). There is currently disagreement among health authorities on whether unsaturated fatty acids in the American diet should be reduced in favor of complex carbohydrates.

Studies have shown that dietary components other than fats and oils, including proteins, carbohydrates, fiber and trace minerals, may also affect blood lipid levels and the development of atherosclerosis. Thus, a possible relationship between diet (particularly fats) and coronary heart disease remains uncertain, and the appropriateness of specific dietary recommendations for the general population is not agreed upon. Additional research will be necessary to clarify the uncertainties. In the meantime, nutritionists emphasize moderation in the consumption of fat as well as other nutrients.

While atherosclerosis is the slow, progressive narrowing of an artery that gradually reduces blood flow, the actual precipitating event of a heart attack is frequently thrombosis, the formation of a blood clot that can lodge in an artery blocking blood flow. If the blockage of the artery is complete, a heart attack or stroke may result. There is current scientific interest in whether atherosclerosis (including elevated serum cholesterol levels) is related to thrombotic risk. With regard to how specific dietary fatty acids might affect thrombotic tendency, it has been demonstrated that polyunsaturated omega-3 fatty acids (e.g., from fish oils) have antithrombotic effects. The mechanisms of these effects appear to involve the metabolism of compounds related to eicosanoids. In contrast to the effects of polyunsaturated omega-3 fatty acids, there appears to be no direct evidence that dietary long-chain saturated fatty acids, such as stearic acid, are thrombogenic to humans. Although some epidemiological evidence suggests that saturated fatty acids may play a role in thrombotic events, more research is needed to establish whether there is a relationship to thrombotic risk and to elucidate the possible mechanism of action.

Lipoprotein(a), or Lp(a), a particle similar to low density lipoprotein (LDL), has been suggested by some researchers to be a risk factor for CHD due to its high serum levels in many heart attack victims who otherwise have no obvious risk factors. Lp(a) levels are thought to be largely controlled by genetic factors, however, some reports indicate that the diet also may influence Lp(a) levels (14, 15).

Recent research has revealed that LDL particle size may influence one’s susceptibility to CHD (16). Specifically, an abundance of smaller size LDL particles has been associated with increased CHD risk. It has been determined that approximately 67% of men and 80% of women in the U.S. have a high proportion of LDL particles whose size is relatively large (approximately 270 angstroms in diameter). The remaining population has LDL particles that are somewhat smaller (approximately 250 angstroms in diameter). The larger LDL is characterized as Phenotype A and the smaller LDL as Phenotype B. Phenotype B has been shown to be particularly atherogenic and is associated with lower HDL levels, higher triglycerides, higher levels of intermediate density lipoproteins, and an increased risk of CHD. Although LDL Phenotypes A and B are thought to be determined by genetics only, some recent work suggests that substantial reductions in energy from fat could be detrimental to certain individuals.

The American Heart Association (AHA) has taken the position that dietary factors influence the risk of CHD (17). The AHA believes the three most important dietary risk factors for atherogenesis are saturated fat, cholesterol, and obesity. The AHA has established a two-step dietary guidance program designed to reduce total fat, saturated fat and cholesterol, which it believes could reduce average blood cholesterol levels in the U.S. by 5-15%. The AHA encourages carbohydrate intake be increased to replace those calories lost through reductions in fat intake.

A trend of considerable interest to epidemiologists and other health professionals is the continuing decline in the death rate from cardiovascular diseases. During the period 1984 to 1994, the U.S. age-adjusted death rate from cardiovascular diseases (considered as a whole) decreased about 22% (7). In particular, the mortality from coronary heart disease decreased 29% during this period (7). Specific reasons for decreasing mortality due to cardiovascular disease are not known. However, recognition and increased public awareness of major risk factors (cigarette smoking, hypertension, and elevated serum cholesterol) and more effective treatment of heart disease have probably played roles. The decline in heart disease mortality in the U.S. has been observed in all decades of life, in all races, and in both sexes.

F. Diet and Cancer

Cancer is the second leading cause of death in the United States, exceeded only by heart disease. The American Cancer Society predicted that about 564,800 Americans would die of cancer in 1998 (18). The three most common sites of fatal cancer in men are lung, prostate, and colo-rectal. In women, the three most common sites are lung, breast, and colo-rectal. In men and women, cancers at these top three sites account for about half of all cancer fatalities.

Cancer is a group of diseases characterized by uncontrolled growth and spread of abnormal cells. If the spread is not controlled, it can result in death. Cancer is caused by both external factors (e.g., chemicals, radiation, and viruses) and internal factors (e.g., immune conditions and inherited mutations). Causal factors may act together or sequentially to initiate or promote carcinogenesis. Frequently the time period between exposures or mutations and appearance of cancer is very long, often 10 years or longer. Risk factors contributing to cancer development include cigarette smoking, certain dietary patterns, exposure to sunlight, exposure to radioactive materials or specific chemicals, and family history. All cancers caused by cigarette smoking and heavy use of alcohol are considered preventable. Many cancers related to dietary factors or to sunlight exposure are also felt to be preventable. Unlike heart disease in which blood cholesterol levels serve as an indictor of risk, there are no similar types of markers to indicate a cancer may be developing. Early detection of cancer, for instance through regular screening examinations, greatly increases the chances of successful treatment.

According to the American Cancer Society (18), between 1991 and 1995, the national cancer death rate fell 2.6%. Most of the decline was attributed to decreases in mortality from cancers of the lung, colon-rectum, and prostate in men, and breast, colon-rectum, and gynecologic sites in women. The declines in mortality were greater in men than in women, largely because of changes in lung cancer rates; greater in young patients than in older patients; and greater in African-Americans that in Caucasians, although mortality rates remain higher in African-Americans.

Some scientific studies have reported associations between dietary factors, such as low intake of dietary fiber or high intake of fat, and the appearance of cancer at certain sites. Such studies are called epidemiological studies. They assess the existence of relationships between factors, such as diet and development of diseases like cancer. These studies do not prove cause and effect. For example, incidence of breast and colon cancer have been correlated with variations in diet, especially fat intake. However, a causal role for dietary factors has not been firmly established. A recent study of approximately 90,000 women in the U.S. found no significant association between diets high in fat and breast cancer (19). Also in developed countries, certain types of cancer may be related more closely to excessive calorie intake than to any specific nutrient. Overall, there is no evidence of a causal relationship between macronutrients in the diet and cancer.

Laboratory animal studies on diet and cancer have dealt largely with the response of chemically-induced or transplanted tumors to increased calories from extra fat in the diet. Some of these studies have suggested that dietary calories and type of fat consumed may be related to cancer incidence, particularly with breast and colon cancer. Other animal studies have indicated that moderate caloric restriction may result in lower cancer incidence and, for a given level of fat in the diet, animals may develop a higher incidence of cancer when the fat is unsaturated. Some studies suggest that a high level of dietary fat may act as a promoter of carcinogenesis rather than as an initiator of tumors. A promoter is a compound that by itself is not carcinogenic but which enhances the ability of a carcinogen to produce cancer. The existence, however, of a direct relationship between caloric content, fat unsaturation, and carcinogenesis is still unclear.

In addition to interest in effects of the total amount of fat in the diet, there is also interest in whether individual types of fatty acids can affect cancer risk. A recent assessment of currently available data suggests that specific saturated, monounsaturated, or polyunsaturated fatty acids do not affect cancer risk (20). Although animal studies have suggested that polyunsaturated fatty acids may increase tumor growth, no relationship has been found between polyunsaturated fatty acids and cancer in humans (21). Similarly, studies in animals have found that omega-3 fatty acids (e.g., from fish oils) suppress cancer formation, but at this time there is no direct evidence for protective effects in humans. Oleic acid and saturated fatty acids have not been found to have any specific effects on carcinogenesis. Available scientific evidence also does not support a relationship between trans fatty acids and risk of cancer at any of the major cancer sites. On a positive note, recent studies have shown that conjugated linoleic acid, found primarily in lipids from ruminant animals, appears to be unique among fatty acids because low levels in the diet produce significant cancer protection. This effect seems to be independent of other dietary fatty acids. (For further discussion of conjugated linoleic acid, see section H, Areas of Current Research Interest.)

Accumulating evidence suggests that diets rich in antioxidant vitamins (in particular, vitamins A and C) may help reduce the risk of some cancers. Vitamins A and C are abundant in many fruits and vegetables. Vitamin E is an antioxidant vitamin found principally in vegetable oil products. There is less epidemiological evidence regarding vitamin E, however, laboratory and animal data support its anticarcinogenic activity. Further research is needed to establish dietary levels of antioxidants that are both safe and effective.

The American Cancer Society has suggested (18) from existing scientific evidence that about one-third of the cancer deaths that occur in the U.S. each year are due to dietary factors. Another third is due to cigarette smoking. Thus, for the majority of Americans who do not use tobacco, dietary choices and physical activity become important modifiable determinants of cancer risk. Evidence also indicates that although genetics are a factor in the development of cancer, heredity does not explain all cancer occurrences. Behavioral factors such as tobacco use, dietary choices, and physical activity modify the risk of cancer at all stages of its development. Adopting healthful diet and exercise practices at any stage of life can promote health and likely reduce cancer risk.

Many dietary factors can affect cancer risk: types of foods, food preparation methods, portion sizes, food variety, and overall caloric balance. The American Cancer Society believes that cancer risk can be reduced by an overall dietary pattern that includes a high proportion of plant foods (fruits, vegetables, grains, and beans), limited amounts of meat, dairy products, and other high-fat foods, and a balance of caloric intake and physical activity. Plant foods contain fiber, which is believed to reduce the risk of cancers of the rectum and colon. They also are rich in antioxidant vitamins, minerals, and phytochemicals that may play a role in reducing cancer risk.

Overall, in the area of diet and cancer, a significant challenge is relating promising data from animal and cell culture studies to the prevention of cancer in humans.

G. Health Effects of Trans Fatty Acids from Hydrogenation

Hydrogenation is the process of chemically adding hydrogen gas to a liquid fat in the presence of a catalyst. This process converts some of the double bonds of unsaturated fatty acids in the fat molecules to single bonds, thereby increasing the degree of saturation of the fat. The degree of hydrogenation, that is, the total number of double bonds which are converted, determines the physical and chemical properties of the hydrogenated oil or fat. An oil that has been "partially" hydrogenated often retains a significant degree of unsaturation (i.e., double bonds) in its fatty acids. Hydrogenation also results in the conversion of some cis double bonds to the trans configuration (see Part IV, Section C, Isomerism of Unsaturated Fatty Acids) and in the formation of cis or trans positional isomers in which one or more double bonds has migrated to a new position in the fatty acid chain. The levels and types of these isomeric fatty acids formed depend on the type of oil and conditions (e.g., temperature, pressure, catalyst, and duration) of the hydrogenation processing.

Small amounts of trans fatty acids occur naturally in foods such as milk, butter, cheese, beef, and tallow as a result of biohydrogenation in ruminants. USDA has estimated that up to 20% of the trans fatty acids in the American diet are from ruminant sources (22). Traces of trans isomers may also be formed when nonhydrogenated oils are deodorized under certain conditions (e.g., prolonged heating at high temperatures).

The hydrogenation process is very important to the food industry to achieve desired stability and physical properties in such food products as margarines, shortenings, frying fats, and specialty fats. Examples of enhanced stability provided by hydrogenation include increased shelf life of commercial snack foods and prolonged frying stability of food service deep frying fats. An example of a desired physical property is the semi-solid consistency at refrigerator and room temperatures of margarines and spreads.

Trans isomers were a principal objective in solid shortening and margarine products of the 1950s and 1960s because of their ability to contribute higher melting properties while maintaining an unsaturated character. More recent concerns about trans isomers acting physiologically like saturated fatty acids has encouraged the industry to pursue reduced levels of trans isomers in many food products. Current partially hydrogenated restaurant and food service frying oils typically contain about 10-35% trans isomers. Tub margarines and spreads (on a product basis) typically contain trans fatty acid levels up to 16%, whereas stick margarines typically have trans fatty acid levels around 15-22%.

Widespread use of partially hydrogenated vegetable oils in the U.S. during the past six or seven decades has raised questions about possible adverse consequences of consuming the isomeric fatty acids present in these products. The principal isomeric fatty acids of interest have been trans fatty acids rather than positional isomers of cis fatty acids. Studies concerning health effects of trans fatty acids have focused primarily on their levels in the U.S. diet and their effects on parameters related to coronary heart disease risk.

The Institute of Shortening and Edible Oils (ISEO) has reported an estimate of trans fatty acids available for consumption in the U.S. diet for 1989 to be about 8 g/person/day (22). This estimate is considered to be relevant to the present day because few changes have occurred in the U.S. diet since around 1989 that would have modified appreciably the estimate. The ISEO’s estimate was based on a comprehensive analysis of products made from partially hydrogenated fats and oils that were available for consumption. Availability estimates do not represent actual consumption but rather indicate what could be consumed on average. Using data from the USDA’s Continuing Survey of Food Intakes by Individuals, Allison, et al (23) estimated the mean intake of trans fatty acids by the U.S. population to be 5.3 g/day, which is substantially lower than the ISEO’s availability estimate of 8 g/person/day. Estimates of trans fatty acid levels in the U.S. diet by various investigators have indicated that trans acids contribute only about 2 to 4% of total energy. This is a small percentage compared to saturated fatty acids, which contribute 12-14% of energy intake (24, 25).

A major study involving 14 Western European countries was conducted by the TNO Institute of the Netherlands assessing trans fatty acid intake using compositional data developed during the study and available national food consumption data (26). Trans fatty acid intake ranged from 1.2 g/d in Portugal to 6.7 g/d in Iceland. The overall mean of trans fatty acid intake was 2.4 g/d, smaller than expected by the investigators.

Prior to 1990, there were numerous reviews and studies on the nutritional and biological effects of trans fatty acids. Most of these studies focus on the development of atherosclerosis and on the effects of trans fatty acids on serum cholesterol levels. Generally these studies indicated that trans fatty acids were not uniquely atherogenic nor did they raise total cholesterol compared to cis fatty acids. However, these findings were challenged in 1990 by a Dutch study (27) which indicated that a diet high in trans fatty acids (11.0% energy) raised total and LDL cholesterol and lowered HDL cholesterol in human subjects compared to a high oleic acid diet. A follow-up study by these investigators using a somewhat lower level of dietary trans fatty acids (7.7% energy) reported that dietary trans acids raised total and LDL cholesterol and lowered HDL cholesterol compared to a high linoleic acid diet but not compared to a high stearic acid diet.

The results of these studies stimulated a major U.S. clinical trial which examined the health effects of both high (6.6% energy) and moderate (3.8% energy) levels of trans acids compared with high oleic acid (16.7% energy) and high saturated fatty acid (16.2% energy as lauric, myristic and palmitic acids) levels (28). The results were that the high and moderate trans fatty acid diets increased total and LDL cholesterol compared to the oleic acid diet but reduced total and LDL cholesterol compared to the high saturated fatty acid diet. The high trans diet, but not the moderate trans diet resulted in a minor reduction in HDL cholesterol. Lp(a) levels were not affected by the trans diets compared to the oleic diet when all subjects were considered collectively (29).

Judd, et al, (30) have conducted a follow-up study to address limitations noted in previous studies. A high carbohydrate diet was included as a control to assess if direct addition of trans fatty acids to the diet has an independent cholesterol raising effect. Other dietary treatments included high oleic acid, high stearic acid, high trans, moderate trans, and high saturated fatty acids. Compared to the control diet, the trans fatty acid diets raised total and LDL cholesterol to about the same extent as the high saturated diet but they had no effect on HDL cholesterol. The stearic acid diet had no effect on LDL cholesterol but lowered HDL cholesterol.

Most of the studies on health effects of trans fatty acids conducted during the 1970s and 1980s found no significant associations between trans fatty acid intake and risk of coronary heart disease. On the other hand, recent epidemiological studies have reported that trans fatty acids have a positive association with CHD risk (31). There are a number of limitations of these studies including the difficulty of measuring trans fatty acid intake through the use of food frequency intake questionnaires, the lack of a dose response relationship between trans fatty acid intake and heart attack risk and the inconsistency of study results. Above all it must be remembered that epidemiological studies do not show cause and effect and are simply indicators of where clinical studies may be needed. Furthermore, the mortality from CHD in the U.S. has continued to decrease during the past 25-30 years (32), a period in which the availability for consumption of trans fatty acids has remained relatively constant (22).

Scientists have also performed numerous studies using animal models to investigate the health effects of trans fatty acids. Available data from short-term studies involving rabbits, hamsters, pigs, and monkeys have demonstrated that trans fatty acids in the presence of adequate essential fatty acids did not produce atherosclerosis.

Four recent comprehensive reports have addressed possible health effects of trans fatty acids. A British Nutrition Foundation Task Force (33) concluded that "the risk attributable to the current low intake of trans fatty acids (4-6 g/person/day) which accounts for 2% of dietary energy in the UK appears to be small, although extreme consumers may experience higher risk." A report by the International Life Sciences Institute (ILSI) (24) emphasized that since trans fatty acids are often substituted for unsaturated fatty acids in experimental diets, it is unclear whether the responses reported reflect the addition of trans fatty acids to the diets or the reduction in dietary unsaturated (i.e., cholesterol-lowering) fatty acids. Another ILSI report (34) addressed whether dietary trans fatty acids compromise fetal and infant early development. The report concluded that there is little evidence in animals or humans that trans fatty acids influence growth, reproduction, or gross aspects of fetal development. An ASCN/AIN Task Force on Trans Fatty acids (25) concluded that "compared to saturated fatty acids, the issue of trans fatty acids is less significant because the U.S. diet provides a smaller proportion of trans fatty acids and the data on their biological effects are limited." The Task Force recommended that data be obtained on the intake of trans fatty acids, their biological effects, mechanism of action, and relation to disease that are comparable to those for saturated fatty acids.

In contrast to the amount of literature on trans fatty acids in relation to coronary heart disease, relatively few investigators have studied trans fatty acids with respect to cancer. Ip and Marshall (35) published a comprehensive review of more than 30 reports addressing this issue. With respect to breast cancer, Ip and Marshall noted that epidemiologic evidence shows only slight to negligible impact of fat intake in general on breast cancer risk and no strong evidence that intake of trans fatty acids is related to increased risk. In addition, there is no evidence indicating that intake of trans fatty acids is related to increased risk of either colon cancer or prostate cancer. Overall, the available scientific evidence does not support a relationship between trans fatty acids and risk of cancer at any of the major cancer sites.

In summary, recent comprehensive reviews of the literature indicate that trans fatty acids at their current level of intake are a safe component of the diet. At relatively high levels of intake, trans fatty acids appear to raise LDL and lower HDL cholesterol. When substituted for unhydrogenated oils high in unsaturated fatty acids, trans fats increase total and LDL cholesterol. However, trans fats lower total and LDL cholesterol when substituted for animal fats and vegetable oils high in saturated fatty acids. Hydrogenated oils are used mainly as a substitute for more highly saturated vegetable oils and for animal fats containing both saturated fatty acids and cholesterol. Since trans fatty acids appear to raise total and LDL cholesterol levels to about the same extent as saturates but are present in the U.S. diet at a much lower level compared to saturates (2-4% versus 12-14% of energy), they pose less of a health concern than do saturated fatty acids. Consumers lowering their fat intake to 30 percent of calories, as recommended in the U.S. Dietary Guidelines for Healthy Americans (6), will simultaneously reduce their intake of saturated as well as trans fatty acids.

H. Areas of Current Research Interest

A group of isomers of the essential fatty acid linoleic acid, collectively termed "conjugated linoleic acid" (CLA), has received considerable attention in recent years because these isomers appear to have both anticarcinogenic and antiatherogenic properties and may affect body composition. CLA differs from linoleic acid by the position and geometric configuration of one of its double bonds. CLA isomers are found primarily in lipids originating from ruminant animals (beef, dairy, and lamb) and are reported to range from about 3 to 11 mg/g fat (36). Fats from nonruminants (pork and chicken) and vegetable oils contain lower amounts of CLA ranging from 0.6 to 0.9 mg/g fat (37). The availability for consumption of CLA in the U.S. has been estimated to be on the order of several hundred milligrams per day (38).

Animal studies have indicated that CLA reduces the incidence of tumors induced by carcinogens such as dimethylbenz[a]anthracene and benzo[a]pyrene (39-45). CLA appears to be a unique anticarcinogen because it is a naturally occurring substance found primarily in food products derived from animal sources. Most other naturally occurring substances that have been demonstrated to have anticarcinogenic activity are of plant origin. CLA is also unique because it is a fatty acid mixture and anticancer efficacy is expressed at concentrations close to human consumption levels. Inhibition of tumor development in animals has been seen with CLA at concentrations as low as 0.1% in the diet.

In addition to its anticarcinogenic properties, CLA appears to be antiatherogenic as well. Recent studies involving rabbits (46) or hamsters (47) indicated that incorporation of CLA into the diets suppressed total and LDL-cholesterol and also atherosclerosis. Furthermore, dietary CLA is able to affect body composition (48, 49). Diets supplemented with 0.5% CLA have been found to decrease body fat content and increase lean body mass in several species including poultry, pigs, and rodents. Pigs fed CLA also showed improved feed efficiency (weight gain per unit weight of food consumed) and improved immune systems compared to controls. Thus, in the future CLA could be a useful additive for animal feeds. Further research is needed to elucidate the mechanism of CLA to inhibit cancer and atherosclerosis. Human studies on body composition effects are underway at this time.

Another area of recent research interest is in the possibility of using dietary intake of certain plant sterols, such as sitosterol, to help reduce the risk of coronary heart disease. Sitosterols were found to be serum cholesterol lowering agents in the 1950s, and their mode of action appeared to be due to the inhibition of cholesterol absorption during the digestive process (50).

Recent studies with humans have examined the serum cholesterol lowering ability of sterol esters incorporated into margarines. A Finnish study utilized sitostanol esters, a hydrogenated sitosterol obtained from a wood pulp byproduct, fed in margarine to mildly hypercholesterolemic subjects at levels ranging from 1.9-2.6 g sitostanol/day. A mean reduction in plasma serum cholesterol of 10% was observed after one year (51). A second study used esters of sitosterol that were extracted from soybean oil and incorporated into margarine, and compared their cholesterol lowering effect directly with that of sitostanol esters. This study, using normocholesterolemic and mildly hypercholesterolemic subjects, found a reduction of 8-13% of plasma total and LDL cholesterol levels, and both the soybean sterols and the sitostanols were equally effective compared to the control diet (52).

In 1995, a margarine containing sitostanol esters from wood pulp was introduced commercially in Finland. The product proved to be very popular and initially demand outstripped supply. This success has stimulated proposals to launch the product in the U.S. in 1999. At the same time, a commercial margarine containing sitosterol esters obtained from soybean oil has been developed for the U.S. and European markets. Both the sitosterol based and sitostanol based margarines are expected to be introduced in the U.S. in mid-1999, pending regulatory approval.

I. Nonallergenicity of Edible Oils

Food allergies are caused by the protein components of food. Edible oils in the U.S. undergo extensive processing (sometimes referred to as "fully refined", discussed in section VII Processing) which removes virtually all protein from the oil. Refined edible oils therefore do not cause allergic reactions because they do not contain allergenic protein. Food products containing refined edible oils as ingredients are also non-allergenic unless the food products contain other sources of protein.

Some edible oils may be extracted and processed by procedures that do not remove all protein present. While the vast majority of oils found in the US are refined by processes which remove virtually all protein, mechanical or "cold press" extraction is occasionally used, which may not remove all protein. These cold pressed oils are rarely used domestically and are usually found only in health food or gourmet food stores. Studies using cold pressed soybean oil have shown it to be safe; however, insufficient testing has been done to ensure that all cold pressed oils can be safely consumed by sensitive individuals.

Edible oils have been blamed for causing allergic reactions in people, but there are conflicting views and inadequate scientific evidence regarding their allergenicity. Many reports alleging edible oil allergenicity have been testimonial in nature. Of those reports that have been scientifically recorded, most lack evidence that edible oils were indeed the causative agent or were even ingested. For example, many investigators did not perform tests to confirm an allergic response from the oil in question nor were analyses conducted to determine if protein was present in the oil. Also many reports do not indicate if the oils were cold pressed or not. There is also a lack of scientific data to determine the levels of proteins needed to cause an allergic reaction; therefore such tolerance levels in humans have not been established. Furthermore, the sensitivities of food allergic individuals may vary widely, and not all allergenic foods have the same tolerance level.

While some consumers are convinced they are allergic to edible oils, there are usually alternate explanations for these reactions. For example, foods containing peanuts, a common allergenic food ingredient, may be cooked in peanut oil. An allergic reaction experienced as a result of eating this food may be mistakenly blamed on the oil. Also foods containing inherent allergens may be cooked in edible oils resulting in traces of the allergenic protein being left behind in the oil. Restaurants and food service facilities should therefore exercise caution in cooking techniques and be able to readily identify not only the oils used but also a complete list of all foods cooked in the oil.

The vast preponderance of edible oils consumed in the US are highly refined and processed to the extent that allergenic proteins are not present in detectable amounts. The majority of well-designed and performed scientific studies indicate that refined oils are safe for the food-allergic population to consume (53).

J. Biotechnology

Biotechnology has been defined broadly as the commercial application of biological processes. It includes both hybridization and genetic modification of plants and animals. The goal of biotechnology is to develop new or modified plants or animals with desirable characteristics. The earliest applications of this technology have been in the pharmaceutical, cosmetic and agricultural sectors. Agricultural applications to food crops have resulted in improved "input" agronomic traits, which affect how the plants grow. Such traits include higher production yields, altered maturation periods, and resistance to disease, insects, stressful weather conditions, and herbicides. Currently researchers and seed developers are placing more emphasis on improving "output" quality traits which affect what the plant produces. An example of this application is the custom designing of nutrient profiles of food crops for improved nutrition and reduced allergenic properties.

The more notable biotechnology applications within the oilseed industry include herbicide tolerant soybeans and canola, high and midoleic sunflower, low linolenic/low saturate soybeans, high linoleic flaxseed oil, low linolenic canola, high laurate canola, high oleic canola, and high stearate canola. Exciting opportunities for edible oil crop nutrient content and functionality improvement include reduction in saturated fatty acid content, improved oxidative stability resulting in a reduced need for hydrogenation, reduced calories or bioavailability, creation of specific fatty acid profiles for particular food applications, and creative "functional" foods for the population at large or for medical purposes. Other applications may include increased oil yield, improved extraction of oil from oilseeds through enzyme technology, industrial production of fatty acids, and improved processing methods.

Genetic engineering is a specific application of biotechnology. This technique is also called recombinant DNA technology, gene splicing, or genetic modification, and involves removing, modifying, or adding genes to a living organism. New plant varieties that result from genetic engineering are referred to as transgenic plants. The most recognized examples in the U.S. are herbicide resistant soybeans, corn which is resistant to the European corn borer, and cotton which is resistant to the bollworm. The acceptance and utilization of these and other transgenic food crops in the U.S. have been very rapid. For example, transgenic herbicide resistant soybeans after being first introduced commercially in 1996 on 1 million acres were planted on about 25 million acres in 1998. Genetically modified insect resistant corn was also commercially introduced in 1996, and it occupied about 16 million acres in 1998. Transgenic soybeans and corn are expected to be about 50% of the U.S. total planted acreage for these crops in 1999.

Global plantings of transgenic crops are also increasing at very rapid rates. While almost 7 million acres were planted internationally to transgenic crops in 1996, about 31.5 million acres were planted in 1997 (54). All indications are that worldwide acreage will continue to expand at a rapid pace for the next several years. The most popular crops as a percent of total global acreage planted to transgenic crops are the following: soybeans (40%), corn (25%), tobacco (13%), cotton (11%), and canola (10%). The United States is the leader in agricultural applications of genetically engineered crops representing 64% of the total global acreage devoted to transgenic crops in 1997, followed by China with 14%, Argentina with 11%, Canada with 10% and Australia and Mexico with about 1% each (54).

In the U.S., the Food and Drug Administration has principal regulatory responsibility for approving the introduction of foods and food additives from transgenic plants into the marketplace. The agency has maintained a biotechnology policy since 1992, which states that foods derived from new genetically engineered plant varieties will be regulated essentially the same as foods created by conventional means. Labeling of such foods or food additives is not required unless the nutrient composition is significantly altered, allergenic proteins have been introduced into the new food, or unique issues have been posed which should be communicated to consumers.

While U.S. consumers appear to have accepted biotechnology and recognize its potential benefits (e.g., foods, drugs), Europeans have been less willing to embrace biotechnology due to concerns regarding the safety of genetically modified foods. The European Union requires foods containing genetically altered components to be labeled as such and has been very slow in approving new genetically modified plant varieties as imports. This position has caused much anxiety between Europe and the U.S. from an agricultural trade standpoint. The prospects for resolution of these differences between Europe and the U.S. appear to be improving.

As new varieties of oilseeds are developed to incorporate specific fatty acid components, historical fatty acid profiles for source oil identification will become less useful. A challenge for food manufacturers will be how to identify these food components through food labeling.

The age of biotechnology is here with a vast array of improved plant varieties already commercially available. The future looks bright particularly for the emergence of new oilseed varieties that will have improved agronomic characteristics, nutrient profiles, and functionality in foods and food ingredients as well as in industrial products. It is therefore important for consumers to understand the many benefits of biotechnology and that the application of genetic engineering technology to foods will render them safe, more functional, and nutritious.

K. Fat Reduction in Foods

Americans have been advised during the past two decades by many health organizations, including the Office of the Surgeon General, American Heart Association, U.S. Department of Agriculture, and Department of Health and Human Services, to reduce total dietary fat intake to less than 30% of calories and saturated fat intake to less than 10% of calories. High intake of total and saturated fat has been associated with increased risk of obesity and coronary heart disease.

Healthy People 2000, a program of the U.S. Public Health Service to promote health and prevention of disease, recommended food manufacturers double the availability of lower fat food products from 1990 to the year 2000. That goal was quickly met by 1995. One of the primary methods employed by the food industry in creating newer food products containing less fat has been the use of fat replacers or fat substitutes. The technology used in the development of these fat replacers allows key sensory and physical attributes and functional characteristics of affected foods to be maintained.

Fat replacers are generally classified into three basic categories: fat-based, protein-based and carbohydrate-based. These categories and the most important examples within them are discussed below:

• Fat-based substitutes.
  
  Sucrose fatty acid polyesters (SPEs) are mixtures of compounds called esters made by combining sucrose esters and fatty acids, the most common example of which is olestra. Because of its large molecular size, olestra is not absorbed or metabolized by the body, thus it contributes no calories to the diet. Olestra is currently approved by FDA as a frying medium for savory snacks but has the potential to be included in frying oils and shortenings.

Sucrose fatty acid esters (SFEs) are similar to SPEs however, their molecular size is smaller. As a result, they are partially or fully absorbed thus providing up to 9 calories per gram to the diet, the same as provided by conventional fats. SFEs are used as emulsifiers and stabilizers in a wide variety of foods and as components of coatings used to retard spoilage of fruits.

Structured lipids are triglycerides which may be made from a variety of combinations of short, medium and long chain fatty acids. They are primarily used to reduce the amount of fat available for absorption, thereby reducing caloric value. Salatrim is an example of a structured lipid which is partially metabolized thus providing only about 5 calories per gram energy.

• Protein-based fat replacers. These materials are derived from a variety of protein sources including eggs, milk, whey, soy and wheat gluten. Generally these proteins undergo a process called microparticulation in which they are sheared under heat into very small particles to impart similar mouthfeel and texture as conventional fats. They are used in frozen dairy desserts, cheese baked goods, sauces and salad dressings and may provide only 1-4 calories per gram depending on the water level incorporated into them.
   

• Carbohydrate-based fat replacers. A number of carbohydrates including gums, starches, pectins and cellulose have been used for many years as thickening agents to add bulk, moisture and textural stability to a wide variety of foods including puddings, sauces, soups, bakery goods, salad dressings and frozen desserts. Digestible carbohydrates such as modified starches and dextrins provide 4 calories per gram, while nondigestible complex carbohydrates provide virtually no calories.

In response to consumer demand of recent years, food manufacturers have developed a wide variety of reduced fat food products utilizing fat substitutes as a primary method of fat reduction. Since 1990 an average of over 1,000 new low fat foods have entered the marketplace annually bearing nutrient content claims of lowered fat (55). In 1996 the introduction of new low fat foods reached a peak at 2,076 products. Since 1996, however, introductions of new reduced or low fat products have declined to 1,405 products in 1997 and 1,180 in 1998.

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