Energy is stored in the body mostly in the form of fat. Fat is needed in the diet to supply essential fatty acids, substances required for growth but not produced by the human body. Dietary fat is needed to carry the fat-soluble vitamins A, D, E, and K and to aid in their absorption from the intestine. Fats are a group of chemical compounds that contain fatty acids. All fatty acids are molecules composed mostly of carbon and hydrogen atoms. The carbon atoms of a fatty acid are hooked together like a string of beads; a hydrogen atom can hook to the top of each carbon and another can hook to the bottom.
There are three main types of fatty acids: saturated, monounsaturated and polyunsaturated. A saturated fatty acid has the maximum possible number of hydrogen atoms attached to every carbon atom. Therefore, it is said to be "saturated" with hydrogen atoms. Saturated fatty acids are long straight molecules that line up beside each other easily. They "pack" together and form solid fats at room temperature. Some fatty acids are missing one pair of hydrogen atoms in the middle of the molecule; one hydrogen is missing from each of two adjacent carbon atoms. This gap is called an "unsaturation" and the fatty acid is said to be "monounsaturated" because it has one gap. The gap at the point of unsaturation forms a "kink" in the molecule so it won't line up and pack with other molecules. This causes the fat to be liquid at room temperature. Fatty acids that are missing more than one pair of hydrogen atoms are called "polyunsaturated."
Polyunsaturated fatty acids are so kinked that they hardly pack at all making them remain liquid even at refrigeration temperature. Saturated fats contain predominately saturated fatty acids and are found primarily in foods of animal origin. Monounsaturated and polyunsaturated fats which contain monounsaturated and polyunsaturated fatty acids and are found primarily in foods of plant origin and in some seafoods.
Polyunsaturated fatty acids are of two kinds, omega-3 or omega-6. Scientists tell them apart by where in the "unsaturations," or missing hydrogen atoms, occur if you count the number of carbons from the end of the chain. Recently a new term has been added to the fat lexicon: trans fatty acids.
These are by-products of partial hydrogenation, a process in which some of the missing hydrogen atoms are put back into polyunsaturated fats. Until the early 1900s, if you wanted a solid fat for your pie crust, you had to choose between lard, butter, and beef tallow. At the turn of the century a process was discovered that uses heat in the presence of hydrogen and certain metal catalysts to convert natural liquid vegetable oils into solid fats. This change in physical state occurs because some unsaturated bonds become saturated (fully hydrogenated) and others are converted from their natural "cis" arrangement to the "trans" position, creating straight molecules that pack together more solidly.
"Partially hydrogenated vegetable oils," such as vegetable shortening and margarine, are solid at room temperature. In 1911, Procter and Gamble introduced Crisco, a shortening made by hydrogenating a liquid oil (cottonseed). Trans fatty acids form when hydrogen is added to liquid oils. "Trans" and "Cis" refer to the physical structure of the fat. "Cis" means that hydrogen atoms are on the same side of the unsaturated carbon atoms in a fatty acid. "Trans" means across; when hydrogen is added to unsaturated fatty acids, some of the unsaturated bonds become saturated but some of the unsaturated bonds "rearrange" so the hydrogen atoms are on the opposite side of the carbon atoms.
Trans fats are "stiffer" than cis fats because the fatty acids are straighter so they pack together to form solid material necessary for making foods like pastry. In the mid 1980s, the food industry responded to recommendations from health authorities and interest from consumers to reduce the amount of animal fats and tropical oils in the food supply. Both are high in saturated fatty acids which is linked to a number of public health risks. The best available alternative in many cases was to reformulate products by substituting partially hydrogenated vegetable oils for the highly saturated fats.
Because butter is rich in both saturated fat and cholesterol, it is potentially a highly atherogenic food. Another concern is that trans fatty acids have been linked to increased risk of coronary heart disease (CHD). The effects of hydrogenated dietary fats on raising fats in the blood were initially investigated in the 1960s.
Study results in the 1970s did not show clear relationships of trans fatty acids to increased heart disease risk. Since 1990, several studies in women and men have shown trans fatty acids to increase total blood cholesterol and LDL (low density lipoprotein) while decreasing HDL (high density lipoprotein) cholesterol, the protective form of blood cholesterol. The Nurses' Health Study, a prospective trial involving more than 85,000 women, showed a positive association of trans fatty acids with coronary heart disease. By analyzing individual foods, this study suggested that trans fatty acids formed during the partial hydrogenation of vegetable oils used in margarine, cookies, cakes, and white bread accounted for all of the increased risk of CHD.
There are naturally occurring trans fatty acids in animal fat, but these foods did not have the same association to risk of CHD in this study. An increased risk of heart attack was evident only among women consuming more than .5 pats of margarine per day. The risk remained significant after adjusting for other risk factors for CHD, including cigarette smoking, body mass index, hypertension, alcohol intake, dietary fat intake, and family history of early heart attack. However, the study did not address the apparent inverse relationship between trans fatty acid intake and the intake of carotene, dietary fiber, saturated fat, monounsaturated fat, vitamin/mineral supplementation and the amount of daily exercise.
The findings are thought provoking, but conclusions cannot be drawn based on this study alone; dietary patterns and lifestyles are what may be related to increased risk of CHD. Most margarine is made from vegetable fat and provides no dietary cholesterol. The more liquid the margarine (i.e. tub and liquid forms) the less hydrogenated it is and the less trans fatty acid it contains. Therefore, margarine is still a preferable substitute for butter, and soft margarines are better than hard ones.
The American Heart Association recommends that consumers shop for margarine with no more than two grams of saturated fat per tablespoon. They should choose soft margarine over stick forms to limit their intake of cholesterol-raising trans fatty acids. Partially hydrogenated vegetable oil is found in solid vegetable fats, such as shortening and margarine, and foods made with these types of fats. Trans fatty acids are naturally occurring in some animal fats. Trans fatty acids from all sources are estimated to account for approximately 5 to 8 percent of calories in the American diet.
The typical American diet currently provides about 12 percent of calories from saturated fat and 34 percent of calories from total dietary fat. The American Dietetic Association and the American Heart Association are NOT recommending that everyone substitute butter for margarine due to these studies. The total amount of fat in the diet is still more important than the contribution trans fatty acids makes to heart disease risk. Sara C. Parks, President of the American Dietetic Association, responded to the controversy over trans fatty acids with the following statements: "The body of available scientific evidence on trans fatty acids is inconclusive. What's known, after extensive scientific research, is that a diet high in fat increases the risk for a number of diseases. By reducing their fat intake to the recommended level of no more than 30 percent (of calories), Americans can reduce the health risks posed by all types of fat."
The American Heart Association's Nutrition Committee continues to monitor research in this area. The committee advises that healthy Americans over the age of two limit their total fat intake to less than 30 percent of total calories. If people limit their daily intake of fats and oils to 5-8 teaspoons, they are not likely to get an excess of trans fatty acids. Trans fatty acids are not listed specifically on the Nutrition Facts Label. There is one regulation for a descriptive word that takes trans fatty acids into account; to make a "saturated fat free" claim, the product must contain less than 0.5 grams of trans fatty acids per serving. Information on the trans fatty acid content of foods is limited due to a lack of USDA composition data and changes in food manufacturing processes.
Here are brief definitions of the key terms important to an understanding of the role of fat in the diet
A chemical compound manufactured in the body. It is used to build cell membranes and brain and nerve tissues. Cholesterol also helps the body make steroid hormones and bile acids.
- Dietary cholesterol
Cholesterol found in animal products that are part of the human diet. Egg yolks, liver, meat, some shell- fish, and whole-milk dairy products are all sources of dietary cholesterol.
- Fatty acid
A molecule composed mostly of carbon and hydrogen atoms. Fatty acids are the building blocks of fats.
A chemical compound containing one or more fatty acids. Fat is one of the three main constituents of food (the others are protein and carbohydrate). It is also the principal form in which energy is stored in the body. Hydrogenated fat: A fat that has been chemically altered by the addition of hydrogen atoms (see trans fatty acid). Vegetable oil and margarine are hydrogenated fats.
A chemical compound characterized by the fact that it is insoluble in water. Both fat and cholesterol are members of the lipid family.
A chemical compound made of fat and protein. Lipoproteins that have more fat than protein are called low- density lipoproteins (LDLs). Lipoproteins that have more protein than fat are called high-density lipoproteins (HDLs). Lipoproteins are found in the blood, where their main function is to carry cholesterol.
- Monounsaturated fatty acid
A fatty acid that is missing one pair of hydrogen atoms in the middle of the molecule. The gap is called an "unsaturation." Monounsaturated fatty acids are found mostly in plant and sea foods. Monounsaturated fat: A fat made of monounsaturated fatty acids. Olive oil and canola oil are monounsaturated fats. Monounsaturated fats tend to lower levels of LDL-cholesterol in the blood.
- Polyunsaturated fatty acid
A fatty acid that is missing more than one pair of hydrogen atoms. Polyunsaturated fatty acids are mostly found in plant and sea foods. Polyunsaturated fat: A fat made of polyunsaturated fatty acids. Safflower oil and corn oil are polyunsaturated fats. Polyunsaturated fats tend to lower levels of both HDL-cholesterol and LDL-cholesterol in the blood.
- Saturated fatty acid
A fatty acid that has the maximum possible number of hydrogen atoms attached to every carbon atom. It is said to be "saturated" with hydrogen atoms. Saturated fatty acids are mostly found in animal products such as meat and whole milk.
- Saturated fat
A fat made of saturated fatty acids. Butter and lard are saturated fats. Saturated fats tend to raise levels of LDL-cholesterol ("bad" cholesterol) in the blood. Elevated levels of LDL-cholesterol are associated with heart disease.
- Trans fatty acid
A polyunsaturated fatty acid in which some of the missing hydrogen atoms have been put back in a chemical process called hydrogenation. Trans fatty acids are the building blocks of hydrogenated fats.