The role of dietary fats is frequently misunderstood. Because of the name, many people associate dietary fats with body fat, but the relationship is much more complex than that. Fats are just building blocks. They constitute an entire type of nutrient called a lipid.
Definition
The lipid is a larger class of macromolecule that includes fats, waxes, oils, sterols and phospholipids. All lipids are hydrophobic, meaning they repel water. This makes them soft and greasy. Lipids are essentially hydrocarbons, which are molecules made predominantly or exclusively with hydrogen and carbon atoms; however, lipids also contain oxygen atoms.
Dietary Fats
Most lipids are structural components synthesized within the cells--phospholipids are found in cell membranes, and oils are used to lubricate parts of the skin--but in the diet, lipids take the primary form of dietary fats. The main dietary fats are saturated and unsaturated. The latter can be broken down into monounsaturated and polyunsaturated. The difference is the number of double bonds in the chemical structure. Saturated fat contains no double bonds and can pack tightly together. Monounsaturated fat contains one double bond, and polyunsaturated fat contains multiple double bonds. Also in the diet are trans fatty acids, hydrogenated fats and cholesterol.
Function
The role of these fats in the diet is simply to provide the structural components to lipids in the body. They contain essential fatty acids, which are long chains of carbon and hydrogen atoms that cannot be manufactured by the body and therefore must be obtained through the diet. Lipids are found in a number of molecules and tissues: cell membranes, hormones, hair and many others. As MedlinePlus notes, they also play an important role in blood clotting, brain development and controlling inflammation.
Energy
Fat is an energy-dense nutrient. It contains nine calories per gram, whereas proteins and carbohydrates contain only four calories per gram. A calorie is a measure of energy in foods. Carbohydrates are a preferred energy source because they are easily broken down into glucose, a molecule that is converted into a usable form of energy in the process of cellular metabolism. According to the textbook "Biochemistry," however, fatty acids can be converted into a molecule known as acetyl CoA, which can fulfill the task of cellular metabolism. Normally glucose is converted into pyruvate and then acetyl CoA, but fatty acids can bypass this step.
Considerations
The consumption of fats does not necessarily mean body fat will accumulate. Adipose tissue is actually composed of molecules known as triglycerides stored within fat cells. Triglycerides contain three fatty acids and must be specifically metabolized by an enzyme first. Unless your body is getting too much energy, it will not store up energy in fat tissue. Even the cholesterol--a waxy steroid found in cell membranes--that you consume isn't quite the same thing as the cholesterol transported through the blood. Only about 15 percent of this kind of cholesterol comes from dietary sources. The rest is made by the body.
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