Fat is one of the three general classes of energy sources ingested as food. As with the other sources, carbohydrates and proteins, fat is processed by the body in the manner that makes it most useful for energy production both immediately and in the longer term.
The term "fat" encompasses a number of subcategories of fats, each of which is utilized by the body in different ways. The uses to which fats are directed within the body include: a source of stored energy fuel; a vehicle by which important nutrients, such as the fat-soluble vitamins A, D, and E are absorbed; and the production of certain cholesterols, known as high density lipoproteins (LDLs), or "good cholesterol," which assist in the reduction of artery-clogging compounds in the blood.
The stored fats within the body are an important source of energy. Fats are utilized by the body in a manner complimentary to carbohydrate use for many functions. Carbohydrates are the chief source of fuel for the energy required to propel the movements of the musculoskeletal system during athletic activities; carbohydrates are also the exclusive source from which the brain and central nervous system are supplied with their energy needs. When fats are first digested in the intestine, they are converted for both ease of movement as well as storage into fatty acids. When the amount of fatty acid in the bloodstream reaches a level that the body senses create an imbalance, a mechanism is triggered whereby these fatty acids are essentially captured by available adipose tissue, specialized fat storage cells located in various parts of the body, with particular concentrations at the abdomen and buttocks. Adipose tissue is capable of absorbing and indefinitely storing fatty acids.
The male and female anatomies are constructed differently with respect to adipose tissue storage. The breasts, waist, hips, and buttocks are the primary adipose tissue locations on the female body. Men generally have fat cell storage in the chest, abdomen, and buttocks. Both men and women have internal adipose tissue located around the kidneys and the liver. The body also has a limited intramuscular fatty cell storage capability. Contrary to the claims made by certain elements of the weight-loss industry, "spot reducing," the notion that certain areas of adipose tissue can be utilized to extract fat cells in priority to others, is without scientific foundation. The body accesses its stores of fats for energy on a general, and not location-specific, basis. When the energy requirements of the body exceed available energy stores, the body signals the release of fat cells for energy conversion, irrespective of physical location.
Once a fat cell is released from adipose tissue storage to be converted into energy, the fat cell will be reduced into its two constituent parts, glycerol, an energy component that is directed through the bloodstream to the liver to be reprocessed into glucose, and fatty acids. The fatty acids are transported to the mitochondria, the local powerhouse of the working muscle, where it is used in the same fashion as available glucose, the carbohydrate energy product, to generate adenosine triphosphate (ATP), the actual fuel consumed for energy.
Vitamins are essential to healthy human function. Vitamins that are water soluble are not stored within the body. Each must be replenished on a daily basis through food consumption. A number of vitamins are fat soluble, meaning that each requires the presence of fatty acids in the digestive process as a medium for absorption into the body. Fat-soluble vitamins, which are stored in the liver, include vitamins A and D (each essential to bone growth and general health), vitamin E (a protective substance within the cardiovascular system), and vitamin K (a material essential to blood clotting).
In contrast to the essential functions of fat as an energy source and medium for vitamin absorption, other fats contained in the human diet are harmful to overall physical performance. When the fat ingested is a saturated fat (such as those contained in animal products), it will be digested and processed into a form that lends to the creation of low density lipoproteins (LDLs), a fat-related structure that assists in the creation of plaque and other substances that clog and narrow the vessels of the cardiovascular system. Narrow or constricted arteries do not function correctly, and serious conditions such as high blood pressure and stroke are significant risks of this condition. By contrast, various unsaturated fats (including a number of plant-based fats) will precipitate the creation of high density lipoproteins (HDLs), which counter the action of LDLs, reducing the buildup of plaque otherwise generated by saturated fat consumption.
Fat as a insulator is a much misunderstood and overplayed aspect of fat utilization in the body. Fat is essential to human function, given its critical role in the absorption of vitamins and the production of energy. Fat is not essential to insulate the body either in cold or heat; the construction of the cardiovascular system and the presence of vessels near the skin surface perform this function. A body fat percentage of between 5% and 15% is desirable in a lean athletic male, depending on the physical demands of the sport. Physically fit women, by virtue of their structure, would be similarly expected to possess a body fat percentage of between 7% to 20%.