By Michael Ormsbee, Ph.D., Florida State University

Edited by Kate Findley, The Great Courses Daily

Do you have a friend who hunts down every avocado in sight or add nuts to every snack? Or maybe it’s the friend who tries to avoid fat at all costs—they say no to butter, oils, or mayonnaise. Fat has been widely debated in the nutrition world, but few understand what it actually does in your body. Dr. Ormsbee explains.

What Are Fat’s Roles?

In order to address the controversy of whether you should include fat in your diet or avoid it at all costs, it’s important to address what fat actually is and what fat does for us physiologically. Fats play many different roles in our body that go beyond how we look. We need fat for optimal health.

Most people think of fat as what is underneath your skin. In this case, it is an insulator acting to keep you warm. However, fat is also found in your cell membranes, surrounding your internal organs, and in your muscles. 

It also helps cells to communicate with each other because fats are a major component of cell membranes, cell receptors, and chemical signals in the blood. Certain fats are even used by your body to make other important compounds, including some essential hormones. 

For example, the cholesterol found in many fatty foods is required to produce your steroid hormones. This is important for body composition because some of the steroid hormones, like testosterone, can have a big impact on muscle mass and fat mass for men and women.

Does Fat Make You Fat?

It is often thought that the more fat you eat, the more fat you store on your body. The truth is that excessive calorie consumption, whether from fat, protein, or carbohydrates, will result in fat storage. 

The American Council on Exercise classifies the average healthy woman as having about 25 to 31 percent body fat and the average healthy man with 18 to 24 percent body fat. Fit individuals and athletes typically have even lower body fat than this. 

Of all the functions of fat, its primary role is to provide you with an energy source or an energy reserve that is much greater than any other fuel that you can tap into. For example, if you weigh 180 pounds, or 81 kilograms, and have 25 percent body fat, then approximately 45 pounds, or 20 kilograms, of fat is stored on your body.

Fat and Energy Use

Because one pound of stored fat is thought to provide 3,500 calories, then you essentially have about 160,000 calories of fat to use at any given time.

Assuming that you can burn about 100 calories per mile while walking, then, in theory, you could walk about 1,600 miles on your stored body fat. That would get you from Atlanta to Denver. 

Of course, a number of other factors could challenge this, but it is important to understand what a large fuel source we have in the form of stored fat. It’s far greater than stored carbohydrate that can only provide a theoretical 2,000 calories or slightly more for energy.

Fats are used to fuel most of the cells in your body including contracting muscles in the arms and the legs. Fat provides 80 to 90 percent of the fuel you use while at rest, like working at a computer all day, driving in your car, or watching TV. 

Other Fat Functions

Almost two to four percent of your body fat serves to protect and insulate vital organs like your heart, liver, spleen, kidneys, brain, and spinal cord. The fat-soluble vitamins A, D, E, and K are dependent on fat in your diet for absorption and movement in your body. This is why you should take these vitamins with your meals.

In fact, if you have a diet very low in fat, then you can’t absorb these key vitamins. This would then impact how well your body can use other nutrients in your diet. 

One advantage to fat is that it empties very slowly from your stomach, and due to this fact, eating fatty foods like nuts, seeds, or avocado can also decrease your hunger. Many people report feeling fuller for longer by simply having fat included in their meals. 

Just as carbohydrates are composed of monosaccharides linked together, fats are composed of fatty acids. This is the simplest form of fat, and fat goes by a few names. You may hear about fat as a lipid or even a phospholipid. 

All of these are considered fats and, in general, they are composed of carbon, hydrogen, and oxygen atoms, just like carbohydrates. However, the carbon chain in fats is much longer than the carbon chain in carbohydrates. 

This longer carbon chain is a unique feature of fats and plays a pivotal role in energy production. All fats contain nine calories per gram, making them the macronutrient that provides more calories per gram than carbohydrates or proteins, which provide only four calories per gram. Thus, fats have many roles, and because of this they must be included in a healthy diet. 

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Dr. Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his M.S. in Exercise Physiology from South Dakota State University and his Ph.D. in Bioenergetics from East Carolina University.