The Skinny on Fat

Fat, fat, fat! Would all of our weight loss problems be solved if we just eliminated fat from our diets? Unfortunately, it's not that simple. We actually need fats -- can't live without them, in fact. Fats are an important part of a healthy diet: They provide essential fatty acids, keep our skin soft, deliver fat-soluble vitamins, and are a great source of energizing fuel. But it's easy to get confused about good fats vs. bad fats, how much fat we should eat, how to avoid artery-clogging trans fats, and the role omega-3 fatty acids play in heart health.

The U.S. Department of Agriculture's 2005 Dietary Guidelines recommend that adults get 20%-35% of their calories from fats. At a minimum, we need at least 10% of our calories to come from fat.

The problem is that the typical American diet is higher in fat: Roughly 34% to 40% of our calories come from fat. From a nutritional standpoint, the body utilizes six different dietary components: protein, carbohydrates, fats, vitamins, minerals, and water. Protein, fats, and minerals serve to make up major structural components, while protein, carbohydrates, and fat can serve as fuels by providing calories for the execution of physiologic functions.

Another function of fat is as a source of energy. Fatty acids are very calorically dense with 9 calories per gram of fat. Carbohydrates and protein on the other hand are only 4 calories per gram. In addition, the body has limited ability to store carbohydrate (in the form of glycogen) because it requires a lot of supporting water to maintain the structure. In fact, the average person carries around about 1 pound of glycogen with an additional 4 pounds of water. This is why calorie restricted diets are diuretic in nature for the first few days; as the glycogen is used up (and not replaced), the water is lost along with it. The body does not store protein and so must breakdown protein for energy if necessary. Muscle is the most readily available source. But again, muscle tissue is 72% water by weight, so a little loss of muscle protein, causes a large loss of total weight. Unfortunately, when eating levels are returned to normal, these compartments are restored with a concomitant retention of water.

Alternatively, fat can be efficiently stored (much to the dismay of dieters). Because of the chemical structure, fat does not require large amounts of water to sustain it. In adipose tissue (the storage form of fat), about 85% of the total weight is actual fat. Thus, from the body's energy perspective, fat is the obvious choice as a storage form of excess energy.

Thus, fat has a place in our diets and a major role to play in basic physiology. From a nutritional standpoint, fat may appear a non-issue since our bodies can actually manufacture fat as needed. The problem here is that not all the necessary fats can be produced. There is a subset of dietary fatty acids, specifically linoleic acid (known as omega-6) and linolenic acid (known as omega-3) that is essential to consume. These two types of polyunsaturated fatty acids that our bodies cannot manufacture are not interconvertible with each other and must be derived from the diet.