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To a large extent, you are what you eat when it comes to dietary fats. Our bodies can manufacture fats from other substances in the diet when there are excess calories. However, on balance, fat provides many of the excess calories in the American diet. In this section, you will learn not only how the excess fat comes into the diet, but how different fatty acid sources have different effects on physiological processes.

Fats and oils provide the most concentrated source of calories of any foodstuff. Fats provide essential fatty acids (linoleic and linolenic acids) which are precursors for prostaglandins. Fats also require prolonged digestion and contribute to satiety, carry fat-soluble vitamins, and concentrate the tastes of foods to make them more palatable.

Ninety-five percent or more of the fats you eat and store are triglycerides. These fats have a three carbon backbone and three fatty acids esterified at each of the three positions. When all the fatty acids on a triglyceride are the same they are called simple, otherwise they are called mixed triglycerides, which are more common. Excess calories, regardless of source, are stored as triglycerides.

The principal dietary sources of fat are meats, dairy products, poultry, fish, nuts, and vegetable oils and fats used in processed foods. Vegetables and fruits contain only small amounts of fat, so that vegetable oils are only sources of fat due to processing of vegetables. The most commonly used oils and fats for salad oil, cooking oils, shortenings and margarines in the U.S. include soybean, corn, cottonseed, palm, peanut, olive, canola (low erucic acid rapeseed oil), safflower, sunflower, coconut, palm kernel, tallow and lard. These oils contain varying compositions of fatty acids which have particular physiological properties.

Comparison of Dietary Fats

Reference: Agricultural Handbook No.8-4 and Human Nutrition Information Service, USDA, 1979.

In starvation and overfeeding, the body regulates the metabolism of carbohydrates and protein closely, but allows the stores of fat to expand easily in overfeeding and to contract with underfeeding. Since fat yields 9 kcal/gm and requires little water for storage, it is a very efficient store of calories. In the non-obese 70 kg. mythical man, 13.5 kg. of fat will carry 130,000 to 160,000 kcal. while 13.5 kg. of muscle will carry only 54,000 kcal. Utilizing the fat stores, and sparing protein stores is essential to surviving starvation. Since many populations have been exposed to epidemics of starvation and abuse, the tendency to retain fat stores is relatively common and is inherited polygenically with a strong environmental influence. The lipoprotein carrying cholesterol, called apoprotein B is one of the largest proteins in the body with over 1300 possible phenotypes, many of which can affect cholesterol metabolism and transport. The common occurrence of hypercholesterolemia in our population is both a result of environmental influence and common polygenically inherited variants of normal cholesterol homeostasis.

1. Lipids - Definition
Lipids are defined based on their solubility in organic solvents rather than on their structure unlike proteins and nucleic acids. The lipids can be divided into amphipathic and hydrophobic subclasses. Fatty acids and phospholipids are examples of amphipathic lipids in that they dissolve in both aqueous and organic solvents. Examples of hydrophobic lipids are cholesterol,and triacyglycerols.

2. Minor Constituents of Dietary Fats and Oils

3. Brief Overview of Metabolism of Fats and Oils
Since triglycerides are the primary ingredient of fats and oils, the digestion and absorption of triglycerides is central to understanding fat absorption and metabolism. Most lipids are not broken down in the mouth, but are mixed with carbohydrates and protein to form chyme in the stomach. This mix is slowly emptied into the duodenum where lipids stimulate cholecystokinin (CCK) secretion. The CCK slows down gastric emptying time to allow more time for intestinal lipid digestion. CCK causes secretion of pancreatic lipase, which breaks down triglycerides in the alkaline environment of the intestine. CCK stimulates pancreatic exocrine secretion of bicarbonate. CCK also stimulates the gallbladder to contract and release bile salts (e.g. glycocholate-derivative of cholesterol) and lecithin (phosphatidylcholine). This is quickly hydrolyzed so that the fatty acids are removed, forming lysolecithin which helps emulsify fats along with bile salts for digestion. Pancreatic lipase works at the oil/water interface since triglycerides are insoluble. Emulsification increases the surface area of the oil /water interface, promoting the breakdown of triglycerides by pancreatic lipase. The pancreatic lipase preferentially hydrolyzes the 1 and 3 positions forming beta monoglyceride and two fatty acid moieties. As the process of digestion proceeds, micelles form containing beta-monoglyceride, fatty acids, and bile salts. These amphiphilic spheres have polar surfaces and hydrophobic interiors. They can incorporate fat soluble vitamins, but do not contain triglycerides.

The micelles are in equilibrium with the constituent substances and move to the vicinity of the intestinal cell where the monoglyceride and fatty acid (but not the micelle) are absorbed. The bile salts are not absorbed but take part in an enterohepatic circulation.

In the intestinal cells, the monoglycerides and fatty acids are recombined to form triglycerides and incorporated into chylomicrons for transport in the blood stream. If the fatty acid chain length is less than 10 (such as medium chain triglycerides), then they are transported by the lacteals directly to the liver. In the hepatocyte these short chain length fatty acids can enter the mitochondria without any special transport and are oxidized for energy. On the other hand, the majority of the triglycerides in the diet have a longer chain length and require transport in the bloodstream by chylomicrons and require carnitineacyltransferase action to move into the mitochondrion for oxidation.

The body can make saturated and monounsaturated fatty acids by modifying other fatty acids or by de novo synthesis from carbohydrate and protein. However, the polyunsaturated fatty acids linoleic and linolenic are essential fatty acids and must be supplied in the diet. Needless to say, the problem of fatty acid deficiency is not widespread in the free-living population in this country. The minimum intake of linoleic acid is said to be 3% of total calories. Since linoleic acid makes up about 60% of corn oil fatty acids, most diets should contain over 10% of total calories as fat. The average American eats between 35 and 40 percent of total calories as fat. High fat diets eaten ad lib account for as much as 600 excess kcal/day compared to low fat ad lib diets. Dietary guidelines for reduction of heart disease and cancer mortality recommend 30% of total calories as fat.

Fat moblized from fat stores (as during a calorie deficit) is released into the blood as free fatty acids. Lipoproteins carrying exogenous or endogenously synthesized triglycerides are broken down by lipoprotein lipase at the endothelial surface of blood vessels and the fatty acids released are taken up into the adipocyte for resynthesis and storage as triglycerides. The reverse process of release of stored fat is under the regulation of hormone-sensitive lipase which releases free fatty acids and glycerol. The free fatty acids released associate with albumin and other proteins and are carried throughout the body. The free fatty acids can also be taken up by the adipocytes again for reutilization. When lipolysis is measured experimentally, glycerol release is followed since it is not taken up by the adipocyte after release.

Dietary Fats: Practical Concerns
Fat in the diet is one of the major contributors to taste and texture. The aroma of sizzling bacon or a steak on the grill comes primarily from the heated fat. The texture of foods is affected by fats, too - butterfat makes ice cream creamy, and fats in baked goods act as shortening (by shortening the strands of gluten in the flour) making the product more tender. But, since fat is such a concentrated energy source, and because there are other health implications to a high fat diet, food technologists are scrambling to find fat replacements that provide the texture and "mouth feel" of fat without the calories. Some fat replacements are protein-based and some are carbohydrate-based, and therefore yield fewer calories per gram (4 vs. 9), and some are fat-based but are not absorbed by the body.

Understanding the food label is an important skill, since some foods can appear low in fat, and yet still provide 100% of their calories from fat. Lowfat margarine, for example, has only 5 calories per tablespoon, but all 5 calories are fat calories - so the product is 100% fat. Fat should be expressed as a percentage of calories, not as a percentage of the weight of the food. Lean ground beef labeled "7% fat" sounds lean, but the 7% is an expression of the amount of fat by weight. Since much of the weight of the meat is water (which has no calories), the true percentage of fat is grossly understated. Reading labels carefully allows you to find hidden fats in foods, and to understand why some advertising claims are misleading.

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