IFIC Review: Sorting Out the Facts About Fat
Dietary fats have received more attention from health professionals and the public than any other nutrient in the food supply. Fat and cholesterol can be complex and confusing topics. This issue of IFIC Review provides an in-depth look at this subject, including a review of fat, cholesterol and health.
Fat -- The Nutrient
To many people, fat has negative health connotations. But as nutritionists know, fat is an essential nutrient.
Like carbohydrates and protein, dietary fat is an important source of energy for the body. Fat is the most concentrated source of energy in the diet, providing nine calories per gram compared with four calories per gram from either carbohydrates or protein. Fat is a particularly important source of calories for infants and young children; fifty percent of the calories in human breast milk come from fat.
Dietary fat supplies essential fatty acids, such as linoleic and linolenic acids, which, once again, are especially important to children for proper growth. Fat also is required for maintaining healthy skin, regulating cholesterol metabolism and as a precursor of prostaglandins, hormone-like substances that regulate many body processes.
Dietary fat is needed to carry fat-soluble vitamins A, D, E and K and to aid in their absorption from the intestine.
The body uses whatever fat it needs for energy, and the rest is stored in various fatty tissues. Some fat is found in blood plasma and other body cells, but the largest amount is stored in the body's adipose (fat) cells.
These fat deposits not only store energy, but also are important in insulating the body and supporting and cushioning organs.
What Is Fat?
Technically, we should refer to "fats" in the plural, since there is no one type of "fat." Fats are composed mostly of the same three elements as carbohydrates - carbon, hydrogen and oxygen. However, fats have relatively more carbon and hydrogen and less oxygen, thus supplying the higher fuel value of nine calories per gram.
Fats are actually combinations of many different fatty acids, the building blocks of fats, each exerting characteristic physiological and metabolic effects. These fatty acids are generally classified as saturated, monounsaturated or polyunsaturated. These terms refer to the number of hydrogen atoms attached to the carbon atoms of the fat molecule.
One molecule of a fat can be broken down into three molecules of fatty acids and one of glycerol. Thus, fats are known chemically as triglycerides.
Fats in foods contain both saturated and unsaturated fatty acids. In general, fats containing a majority of saturated fatty acids are solid at room temperature, although some solid vegetable shortenings are up to 75 percent unsaturated. Fats containing mostly unsaturated fatty acids are usually liquid at room temperature and are called oils.
Some common saturated fatty acids in foods include palmitic, stearic and myristic acids. A common monounsaturated fatty acid is oleic acid, and the most common polyunsaturated fatty acid in food is linoleic acid.
Fats are important ingredients in many foods because of their functional properties. In many recipes, fats enhance the taste, aroma and texture of the food. Because fats are digested slowly, they play an important role in satiety, helping to provide a sense of fullness after eating. Saturated fatty acids are more stable than unsaturated fatty acids because of their chemical structure. Stability is important to prevent rancidity and off flavors or odors.
Fats and Cancer
Diets high in fruits, vegetables, whole grains and beans and reduced in fat, partnered with regular physical activity and maintenance of a healthy weight, can help contribute to a reduction in the incidence of certain cancers.
In 1989, the National Research Council (NRC) "Diet and Health" report presented a series of dietary guidelines for reducing the risk of chronic diseases. While the recommendation to reduce dietary fat intake in order to prevent cancer was considered to be important, the scientific evidence to support this was unclear.
According to the 1996 American Cancer Society (ACS) Dietary Guidelines, diets high in fats have been associated with an increase in the risk of cancers of the colon, the rectum, prostate and endometrium. However, it is unclear whether these associations are due to the amount of fat, the type of fat or some other factor in food. The association between dietary fat and the development of breast cancer is weak.
While research continues exploring the relationships between cancer and saturated, polyunsaturated and monounsaturated fats, the ACS advises consumers to moderate their total fat intake rather than emphasize one type of fat over another, and to increase fruit and vegetable consumption. These guidelines are consistent with dietary guidelines for coronary heart disease.
Fats, Cholesterol and Coronary Heart Disease
An excess of fats, especially saturated fats, in the diet is recognized as one risk factor influencing the development of some chronic diseases.
According to the Third National Health and Nutrition Examination Survey (NHANES III), conducted by the Center for Disease Control's National Center for Health Statistics and published in 1993, current average fat intake in the United States is about 34 percent of total calories, with approximately 12 percent of calories as saturated fat. Most health authorities advise limiting fat consumption to 30 percent or less of total calories, with saturated fat providing 10 percent or less of total calories. It is not advised that children under the age of two be restricted in fat consumption.
Cholesterol travels through the blood via particles called lipoproteins - combinations of fats (also called lipids) and proteins. Too much cholesterol can build up in the blood and accumulate in the walls of the blood vessels, a condition known as atherosclerosis. This condition can ultimately reduce the flow of blood in major arteries, leading to heart attack.
According to the American Heart Association, coronary heart disease (CHD) claims the lives of more Americans every year than any other disease or group of diseases. Although the death rate from heart attack declined 28.7 percent from 1985 to 1995, almost 500,000 people die annually as a result of coronary heart disease.
The main concern about excess saturated fat in the diet centers on its potential role in raising blood cholesterol. High serum or blood cholesterol has been identified as one of the significant risk factors in the development of coronary heart disease. Although diet is important, it is just one factor influencing blood cholesterol levels. Many experts suspect that dietary influence on serum cholesterol may be more complex than just the cholesterol and fatty acid content of the diet alone. In past clinical trials, dietary habits, individual responsiveness, baseline serum cholesterol levels and other components of the diet have been identified as variables that can influence the effect of diet modification on blood cholesterol levels.
In fact, for some people at risk, heredity is a stronger predictor of cholesterol levels than diet. Regardless of how little fat or cholesterol they eat, their bodies produce excess amounts of cholesterol that can spell trouble. Scientists may one day be able to identify a gene or phenotype that is carried by such "cholesterol responsive" individuals.
Other risk factors beyond our control include age, race and gender. But there are many risk factors that we can influence. For example, in addition to reducing high blood cholesterol levels, we can reduce our risk of heart disease by maintaining healthy weight, getting adequate physical activity, controlling high blood pressure, avoiding cigarette smoking and managing stress. For those with diabetes, controlling blood glucose levels also is important.
Facts about Blood Cholesterol
Blood cholesterol reflects the amount of three major classes of lipoproteins: very-low-density lipoprotein (VLDL); low-density lipoprotein (LDL), which contains most of the cholesterol found in the blood; and high-density lipoprotein (HDL). LDL seems to be the culprit in CHD and is associated with cholesterol deposits on artery walls.
In contrast, HDL is seen as desirable. Studies indicate that the more HDL in the blood, the lower the risk of developing CHD. HDL apparently carries cholesterol out of the blood and back to the liver for breakdown and excretion.
The average level of blood cholesterol in the U.S. adult population is 203 milligrams/ deciliter (mg/dl), according to data collected from 1988-1994 NHANES III. The National Heart, Lung and Blood Institute's National Cholesterol Education Program (NCEP) classifies risk for heart disease based on total blood cholesterol levels as follows:
The NCEP recommends that individuals with cholesterol readings of 200 mg/dl and above - about half of all Americans - should have these readings confirmed by their physicians and follow-up on specific lipoprotein measurements. In addition, all adults over the age of 20 with desirable levels should have their blood cholesterol rechecked every five years.
Dietary Management for High Blood Cholesterol
In June 1993, the NCEP released the Second Report of the Expert Panel on Detection, Evaluation and Treatment of High Blood Cholesterol (Adult Treatment Panel II).
NCEP recommends dietary modification as the first treatment for elevated blood cholesterol. The recommendations are designed to reduce intake of saturated fat and cholesterol and to promote weight loss for those who are overweight.
NCEP recommends limiting total fat to 30 percent of calories or less, saturated fat to 8-10 percent of total calories and cholesterol to less than 300 milligrams daily - similar to the U.S. Dietary Guidelines. Controlling weight and increasing physical activity have also been added to the list of recommendations.
If that approach fails to reduce blood cholesterol after a six-month trial, NCEP recommends a harder line. Saturated fat is further decreased to 7 percent of calories and cholesterol dropped to no more than 200 milligrams per day for another six months. If LDL levels cannot be reduced adequately through diet alone, drug therapy may be tried along with continued adherence to the diet.
Dietary Fats and Obesity
According to NHANES III, approximately one-third of American adults are overweight. This incidence represents a significant increase over the previous decade.
The cause of the increased prevalence of obesity in the U.S. is not clear, but it is known that obesity is multi-factorial in origin. Evidence suggests an increasingly sedentary lifestyle plays an important role in the problem. The wide availability of and easy access to food in this country has also been implicated. Investigations into the role of a high-fat diet in obesity, however, do not single out dietary fat as a culprit independent of energy intake.
While studies show animals fed high-fat diets gain more weight compared to those fed low-fat diets, other research suggests these findings do not necessarily apply to humans. For example, as the percent of calories from fat has declined in the U.S. diet, there is no evidence that body weight is also declining. Further, diets in many European countries contain more calories from fat than in the U.S., but people in these countries experience less obesity. In addition, because the fat content of the diet generally does not increase with aging, dietary fat is not considered a primary factor in the weight gain seen with aging.
It has also been theorized that fats are more efficiently metabolized than carbohydrates and therefore contribute more energy than is accounted for by standard energy calculations (i.e., fats contain 9 calories per gram; carbohydrates contain 4 calories/ gram). But long-term studies comparing high-fat and high-carbohydrate diets of equal calorie value fail to show high-fat diets contribute to weight gain to a greater or lesser extent than high-carbohydrate diets.
To stem the rising prevalence of obesity in the U.S., many experts recommend a reduction in energy intake along with an increase in energy expenditure. This recommendation may imply a decrease in fat intake since dietary fats contain twice as many calories as carbohydrates and protein. However, over-consumption of any calorie-containing nutrient, such as protein, fat, carbohydrate or alcohol, can contribute to the development of obesity.
Sources of Fats and Cholesterol
The 1988 NRC Designing Foods report indicated that products from animal sources provide about 57 percent of the total fat and 75 percent of the saturated fat available in the diet. Primary animal sources of fat include red meats, poultry and fish; milk and milk products; and eggs.
Fat from vegetable sources has increased in recent years, now accounting for about 43 percent of fat available in the diet. Vegetable fat is consumed mostly in the form of plant oils such as soybean, canola, olive, safflower, sunflower and corn.
In addition to direct consumption of meat products, fat is consumed in fried foods and other products such as butter, margarine, dairy products, cheese, nuts, baked goods, salad oils, shortenings, mayonnaise, salad dressings, frostings, gravies and sauces.
Cholesterol is not a fat, but rather a fat-like substance classified as a lipid. Cholesterol is vital to life and is found in all cell membranes. It is necessary for the production of bile acids and steroid hormones.
Dietary cholesterol is found only in animal foods. Abundant in organ meats and eggs yolks, cholesterol is also contained in meats, chicken and shell fish. Vegetable oils and shortenings are cholesterol-free.
Actually, most of the cholesterol in the blood is manufactured by the body, at a rate of about 800 to 1,500 milligrams a day, compared with 300 to 450 milligrams consumed daily by the average American in foods. As the amount of dietary cholesterol is decreased, the body compensates by increasing its production.
Hydrogenation is the process of adding hydrogen molecules directly to a monounsaturated or polyunsaturated fatty acid, making them more saturated.
Hydrogenation is used to convert liquid oils to a semi-solid form for greater functionality. For example, vegetable oils are often partially hydrogenated to produce shortenings or margarines. Hydrogenation also is used to increase the stability of a fat or oil, which is important in cooking and extending a product's shelf-life.
All fats, particularly polyunsaturated fats, have a tendency to break down or oxidize when exposed to air and heat. Oxidized fats impart an undesirable rancid flavor and odor. By adding hydrogen molecules, the fatty acids become more stable and resistant to oxidation. This stability is especially important for fats used in deep-fat frying.
Hydrogenation also contributes important textural properties in food. The degree of hydrogenation can help influence the firmness and spreadability of margarines, flakiness of pie crust and the creaminess of puddings.
Hydrogenation is not a novel food process, but in fact has been used since the turn of the century. In 1911, cottonseed oil was first hydrogenated in the United States to produce vegetable shortening as a replacement for lard. The manufacturing process became more popular in the 1930s with the development of margarine.
Today, hydrogenation plays an important role in the stability and texture of various margarine products, shortenings, baked goods, snack foods, cake mixes and a wide assortment of other foods.
Extent of Hydrogenation
Oils can be hydrogenated to a greater or lesser extent, depending on their desired uses in foods. Most oils used in foods, shortenings and institutional cooking are partially hydrogenated.
Partially hydrogenated oils are routinely used in place of fats with higher proportions of saturated fatty acids. For example, partial hydrogenation of soybean oil to produce margarine may raise its saturated fatty acid content from 15 percent to approximately 17 to 21 percent. But by comparison, butterfat is 53 percent saturated fat.
Liquid vegetable oils can be lightly hydrogenated to improve stability and still remain highly unsaturated and in liquid form. Greater degrees of hydrogenation are used to make semisolid forms such as margarines and shortenings. Many of these products still contain a high percentage of unsaturated fatty acids.
Some foods require a higher proportion of semi-solid fat. The flaky texture, taste, and mouthfeel of pie crust, for example, simply cannot be achieved when an unhydrogenated vegetable oil is used. Manufacturers use only the minimum degree of hydrogenation necessary to achieve the desired functional properties in food.
Also, hydrogenation of oils rich in linoleic acid (a polyunsaturated fat) and oleic acid (a monounsaturated fat) results in the formation of oleic acid and stearic acid. Oleic acid has been found to have a lowering effect on blood cholesterol levels when compared to saturated fats. And unlike other saturated fatty acids, stearic acid does not appear to raise blood cholesterol levels.
Scientists often discuss hydrogenation in terms of "cis" versus "trans" fatty acids. The phrase "cis" describes the typical configuration of hydrogen atoms in unsaturated fatty acids, which are located on the same side of the carbon-to-carbon double bond.
During hydrogenation, some hydrogen atoms move to the opposite side of a double bond. This newly formed configuration of fatty acids is referred to as "trans", meaning "on the other side of." The position of a double-carbon bond within the fat molecule also may be changed. Trans fatty acids (trans fats) contribute to the stability of hydrogenated fat thus preventing rancidity.
Trans fats from all sources provide two to four percent of total calories compared with 12 percent from saturated fat and 34 percent from total fat in the American diet. The majority of trans fats come from processed foods. Food such as beef, lamb and dairy products contribute about 20 percent of total trans dietary intake. Trans fats are digested and used for energy in the same manner as cis fatty acids.
The physiological effects of trans fats are being investigated on a number of levels, from epidemiological to clinical studies. An epidemiological study published by Willett and colleagues in 1993 suggested that a high intake of trans fats may be associated with an increased risk of heart disease. Given the nature and limitations of epidemiological research, this study did not demonstrate a cause-and-effect relationship between trans fats and CHD. However, the study provided important clues to potential associations.
Clinical studies have demonstrated that consumption of trans fats and partially hydrogenated fats result in higher concentrations of total and LDL blood cholesterol than cis fatty acids or naturally occurring oils. Human studies conducted by Judd and colleagues demonstrate that trans fats raise total and LDL cholesterol similarly to saturates. Many other studies have indicated trans fats have lesser effects on blood cholesterol levels than do saturated fats.
The American Heart Association, in a 1997 Science Advisory, continues to recommend polyunsaturated and monounsaturated fats over saturated fats, and softer margarines, which contain less trans fats, over stick margarines or butter.
Innovations in Low-Fat Foods
With fat contributing about 34 percent of total calories in the average diet, Americans have yet to meet dietary recommendations regarding fat. But with nutrition education and a variety of lower-fat food choices, consumers should be able to make progress in achieving dietary recommendations as noted in the USDA/DHHS Healthy People 2000 objectives. In fact, more choices of foods lower in fat are available in the supermarket today than ever before. Experts see that trend continuing with low-fat foods dominating new product introductions in the near future.
The trend also is apparent in meats and dairy foods, which contribute the majority of fat and saturated fat in the American diet. Closer fat trimming by butchers has resulted in a 27 percent reduction in separable fat on retail cuts of beef, and the fat content of ground beef is 10 percent lower than 40 years ago, according to the National Research Council report, Designing Foods.
Producers are cross-breeding to achieve leaner animals and also are using different grains, combinations of grains and/or other ingredients to lower the fat content of animals.
In the dairy case, low-fat and fat-free milk now outsell whole milk, and choices abound in reduced-fat cheese, cottage cheese, sour cream, spreadable butter, margarine products and yogurt. Sales of low-fat and fat-free products such as snacks, pastries, cookies, yogurt, salad dressing, processed cheese and frozen desserts increased rapidly in the early to mid-1990s. Some of these products have been developed by combining familiar ingredients such as milk and egg proteins, skim milk, water and/or a variety of carbohydrates in a process of heating, acidifying and blending to simulate the textural properties of fat and achieve the taste and sensory quality demands.
Research shows even the most motivated individuals have difficulty with long-term compliance to very low-fat diets, in part because they dislike the taste of some low-fat foods. Although fat substitutes and reduced fat foods will not compensate for poor dietary habits, these foods and ingredients may be a help to many persons trying to eat healthful, balanced diets.
Clearly, consumers have expressed a demand for foods lower in fat. As companies continue applying their research and development skills to this challenge and as farmers employ new technologies on the farm, a number of innovative products are sure to follow.
The Latest on Fatty Acids and Health
Will eating more fish reduce an individual's risk for heart disease? Can a diet high in some types of fat actually be the best dietary prescription for certain people at risk for heart disease? Does eating conjugated linoleic acid help protect against cancer? These are just a few of the questions that researchers and many nutrition-savvy consumers are asking these days. The questions concern the impact on health of omega-3 and omega-6 fatty acids. And the answers are generating a good deal of excitement.
Omega-3 Fatty Acids
Omega-3 fatty acids are a class of polyunsaturated fatty acids that includes alpha-linolenic acid, found in walnuts, soy and canola and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), both found primarily in fish and fish oils. Studies suggest these fatty acids may help reduce risk of chronic diseases such as coronary heart disease (CHD), stroke and cancer.
Found in abundance in oily fish such as salmon and mackerel, omega-3 fatty acids--specifically EPA and DHA--have been shown to reduce levels of triglycerides in the blood of both people with high triglyceride levels and those with normal levels. Elevated triglyceride levels may increase CHD risk. Research also shows large amounts of omega-3 fatty acids may modestly reduce blood pressure in untreated people with high blood pressure and decrease factors involved in blood clotting and stroke. Animal studies suggest EPA and DHA may inhibit the development of cancer.
Alpha-linolenic acid is a plant source of omega-3 fatty acids. Humans can convert alpha-linolenic acid to a limited extent into EPA and DHA in the body. Alpha-linolenic acid also appears to act like omega-6 fatty acids and monounsaturated fatty acids in its effect on blood lipids (see below).
Omega-6 Fatty Acids
Omega-6 fatty acids are another class of polyunsaturated fatty acids. They include the essential fatty acids linoleic and arachidonic acids. Linoleic acid comprises the majority of the polyunsaturated fat eaten in the U.S. and comes from a variety of commonly-consumed animal and vegetable products. Linoleic acid is the predominant fatty acid in vegetable oils such as soybean, corn and safflower oils.
Research shows omega-6 fatty acids may reduce the risk of CHD. Clinical studies show linoleic acid decreases total cholesterol and LDL-cholesterol blood levels while increasing HDL-cholesterol levels, much like monounsaturated fatty acids such as those found in olive and canola oils.
Much of the research conducted to determine whether there is an association between dietary fat and cancer risk has examined unsaturated fat intake, specifically linoleic acid. Yet a clear consensus does not exist about unsaturated fatty acid or linoleic acid intake and cancer risk.
Conjugated linoleic acid, which is produced from linoleic acid by rumen bacteria and consequently is found in dairy foods and ruminant meats such as beef, has received considerable attention for its potential to inhibit tumor incidence in animals compared to its parent fatty acid.
Experts agree fatty acids play a key role in the development and progression of some chronic diseases. But significant gaps in understanding remain. Until research more clearly defines the effects of individual fatty acids on health, the Dietary Guidelines for Americans and the Food Guide Pyramid represent the best advice for eating to reduce chronic disease risk. Frequent consumption of fish or the prudent use of oils rich in specific fatty acids may offer health benefits and fall well within guidelines for healthy eating.
Role of Saturated Fatty Acids Fine-Tuned
Ancel Keys and Mark Hegsted, pioneers in fat research, were among the first to suspect it, others have confirmed it. All saturated fatty acids are not equal in their cholesterol-raising effects.
Nearly 40 years ago, researchers first noticed the different effects of saturated and polyunsaturated fats on serum cholesterol. In the early 1950s, scientists observed that diets rich in unsaturated vegetable fat resulted in lower levels of serum cholesterol than diets rich in the more saturated animal fats.
Stearic Acid A Unique Saturated Fatty Acid
Keys and Hegsted each developed equations to predict the effect of dietary modifications on serum cholesterol. Each equation showed that saturated fat significantly raises cholesterol levels. But both researchers found that fats rich in stearic acid (C18) did not fit the predictive equation.
Additionally, Hegsted's formula indicated that saturated fatty acids with shorter carbon chains (C6-C12) were not cholesterol raising and that myristic acid (C14) was the most hypercholesterolemic of the saturated fatty acids. Key's calculations, on the other hand, identified palmitic (C16) as the most potent saturate.
Over the next 25 years, other researchers confirmed the neutral effect of stearic acid on blood cholesterol levels in animal and human studies. As a result, The National Research Council's report "Diet and Health" and "The Surgeon General's Report on Nutrition and Health" noted the neutral effect of stearic acid on cholesterol.
In light of the findings about stearic acid, some researchers recommend no longer grouping it with other saturated fatty acids. Beef, cocoa butter and fully hydrogenated vegetable oils have stearic acid content.
Decisions on Labeling
In food label regulations, both FDA and USDA define saturated fat strictly by chemical composition, as "the sum of all fatty acids containing no double bonds." Regulators in Canada and the European Community similarly define saturates by chemical composition.
Despite convincing evidence on stearic acid's effects, the federal government continues to include it in the definition of saturated fat for labeling purposes. In recognition of the neutral effects of stearic acids, USDA allows the voluntary label declaration of stearic acid as a subcomponent of saturated fat.
Although the evidence about stearic acid is strong, the effects of other saturated fatty acids, such as lauric, myristic and palmitic acids, are still under investigation. Some research indicates that there is a difference among these saturated fatty acids on blood lipids.
Confounding variables have led researchers to question effects beyond fatty acid content, which influence effects on serum cholesterol. The relationship between saturated fatty acids and other dietary components, such as protein or fiber appears more complicated than previously supposed.
The results of fatty acid research are likely to have practical application for food scientists. Changing the fatty acid profile of oils is a likely beginning. In fact, some companies are developing genetically engineered canola plants to produce an oil with increased levels of stearic acid for use in soft margarine products and confectionary production.
Some researches see the structure of individual triglycerides, as opposed to the fatty acid composition of an oil, as the next frontier. Every triglyceride, or fat, contains three fatty acids. The rearrangement of these three fatty acids within a triglyceride molecule potentially could effect the atherogenic potential of such a fat.
Scientists continue to search for the best dietary fatty acid profile for health promotion.
Moderation in fat and cholesterol consumption, while important, is only one aspect of good nutrition. Variety, moderation and balance of all foods are the most prudent approach for the general population. Moreover, eating a well-balanced diet while getting plenty of physical activity, maintaining a healthy weight, avoiding smoking and controlling diseases such as heart disease and diabetes is the best approach to a healthy lifestyle.
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