How Is Fat Stored and Burned as Energy in the Human Body?

A small amount of fat is an essential part of a healthy diet. Excess fat is stored in the body's cells until it is needed for energy. When the body requires more energy, it will burn stored fat in a chemical process known as metabolism. As well as providing the body with energy, fats play an important role in the regulation of body temperature, the reduction of inflammation, blood clotting and brain development.

Protein, carbohydrates and fats are the three essential nutrients that provide the body with caloric energy. Although carbohydrates are the main source of energy for the body, fats are the most energy dense of these nutrients. Containing 9 kcal per gram, fats provide roughly twice as much energy and calories as proteins and carbohydrates which only provide 4 kcal per gram, according to the Centers for Disease Control and Prevention. This energy is used for exercising and for basic biological processes, known as the basal metabolic rate, that the body performs while at rest. These include functions like blood circulation, the regulation of hormones, cell growth and digestion. Any calories that are not immediately metabolized for energy are stored in the body as fat for future use.

Fat is stored throughout the body in fat cells known as adipocytes. The number of adipocytes an individual has is determined by late adolescence and rarely changes during adulthood. However, fat cells can increase and decrease in size depending on the amount of fat that the body is storing. If the body stores more fat then it uses, the fat cells will expand causing weight gain. If the body is forced to rely on stored fat reserves for energy, whether because of diet or exercise, the fat cells will shrink causing weight loss.

The fat stored in the body is broken down through a complex process known as metabolism. Metabolism is the chemical process that converts fat molecules into energy. It does this by breaking fat or triglycerides into glycerol and fatty acids. These are then absorbed by the liver, kidneys and muscles tissues until they are completely broken down by the chemical process. The byproducts of this process include heat which helps to maintain body temperature and the waste products water and carbon dioxide.

Although fats are an essential part of a healthy diet, certain fats are healthier than others. Saturated fats, which can be found in animal products such as whole-milk dairy products, fatty meats, butter and cheese and the trans fats found in fried food can cause a big increase in levels of bad cholesterol, or LDL, levels. Unsaturated fats, on the other hand can actually help lower LDL levels and increase HDL, or good cholesterol, levels. Unsaturated fats include olive and canola oils, fish, nuts and avocados. All fats are high in calories, and consuming too much of any type of fat can lead to serious health issues such as heart disease, diabetes, obesity, cancer and high cholesterol.