Edited by Kate Findley, The Great Courses Daily
Even if you’ve calculated how many calories you burn during a particular exercise, you probably don’t know how much fat you’re burning. Dr. Ormsbee explains the science behind fat-burning and why what you eat before exercise matters beyond “calories in, calories out.”
How We Burn Fat
Fat is fuel—one gram of fat yields nine calories, and fat is much better at satiating hunger than carbohydrates. You may be wondering, though, how you burn fat as fuel. Ideally, you want to use your stored fat to provide energy and ultimately improve your body composition.
Fat provides the most abundant source of energy that we have; it fuels your body at rest and during light- to moderate-intensity activity. During this type of exercise, a muscle’s main source of energy is adipose tissue, commonly known as body fat.
Muscles can also rely on fat that is stored inside of the muscle. Fats which are stored inside muscle are called intramuscular triglycerides or IMTG.
The amount of intramuscular triglycerides used during exercise will vary with your fitness level. The more fit you are, the more fat you’ll use.
This is a useful adaptation to exercise. With fat stored in your muscle, it is already located where it needs to be in order to be used as energy. The amount of IMTGs will also vary with the intensity and the duration of exercise, where longer and slower activity will rely more on fat use than short and fast activity.
Barriers to Fat-Burning
The trick is for your body to get the fat from your fat cells into the blood and then into your muscles in order to be burned or oxidized for energy in the mitochondria of your cells. To do this, fats enter something called the carnitine fatty acid transport system. This system works as a gatekeeper to allow fat into the cell’s mitochondria.
However, fat is not able to enter the mitochondria effectively when levels of glucose are high because another molecule called malonyl-CoA shuts down the carnitine transport system. This means that after a high carbohydrate meal—for example, a muffin—you literally impair the ability to transport fat into your cell’s mitochondria to be used as energy.
Likewise, some hormone changes occur with high carbohydrate intake that can also work against you being able to burn fat. Be aware of what you eat and when you eat it; it can impact your body composition and performance dramatically.
Using Fat as Fuel
Once the free fatty acids released from the stored triglyceride are inside the mitochondria, these fatty acids are broken down to provide energy during a process called beta-oxidation, which means fat is burned as a fuel.
Therefore, to use fat as a fuel during exercise, your body needs to be able to break down stored triglycerides into free fatty acids to make adenosine triphosphate (ATP) for energy. The intensity of the exercise will determine if fat will be used during exercise.
The more intense the exercise, the less your body will be able to utilize the fat stores and vice versa. As mentioned, fat oxidation or burning is regulated in part by how much glucose you have in your blood, so eating lots of carbohydrates will increase blood glucose and decrease the breakdown of stored body fat, which is called lipolysis, and fat’s use or oxidation for energy.
However, fat oxidation is enhanced by exercise training, and this occurs because exercise training can increase the number of mitochondria you have, as well as the capillary density.
The increase in mitochondria will help the body process the fat at a higher volume, and an increase in capillary density will help increase the blood flow and oxygen delivery to the exercising muscle, which is required to burn fat.
Exercise training will also increase the activity of the carnitine fatty acid transport system that essentially allows more fat to enter the increased number of mitochondria that you have from exercising. Some supplements may also work through this transport system to help burn fat.
Not surprisingly, then, exercise does help burn more fat, but it depends on the intensity and the duration of exercise, as well as the overall mix of foods that you have recently eaten.
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.