How Does Your Body Burn Fat?

Close up shot of runner's shoes
Exercise helps create a fat-burning environment in your body. (Image: Ivanko_Brnjakovic/iStock/Getty Images)

Whether you want to lose weight for health or to look slimmer in your jeans, you must burn fat to achieve your goal. Your body loses fat when you create an environment in which it's not getting enough energy and must turn to fat stores to fuel all its function. When a cell needs energy, a complex chemical process releases the fat from fat cells and converts it to usable fuel. You end up slimmer and healthier.

The Fat Burning Process

Your body stores fat in adipose tissue -- made up of fat cells -- as triglycerides. This form of fat isn’t immediately usable for energy, however. When your body senses an energy deficit, your adipose cells react. An enzyme inside them, called hormone-sensitive lipase, helps break down the triglycerides, releasing fatty acids and glycerol into the blood stream. Cells requiring energy take in these compounds and transform them into usable fuel, plus carbon dioxide and water.

This explains why you can’t “spot reduce” a specific area. Fat cells all over your body respond to an energy deficit, not just the ones located in your “problem” spots.

Where Does the Fat Go?

Carbon, hydrogen and oxygen make up fat. When triglycerides are broken down into the usable energy of glycerol and fatty acids, carbon dioxide and water are also released. You exhale out the carbon dioxide and excrete the water in urine and sweat.

When fat cells “burn” fat, they shrink but don’t entirely disappear. When you eat in excess of what you burn, they refill, and you gain fat. You avoid gaining fat by keeping a balance between calories taken in and those burned. To lose fat, create a deficit by eating fewer calories than you burn. This drives your body to turn to stored fat to supply the missing calories.

Why You Might Burn More Than Fat

Although fat supplies about 60 percent of the energy you use when resting, and sometimes even more during prolonged light- to moderately-intensity exercise, it can’t sustain all of your body’s functions. Your brain, nerves and red blood cells need glucose to operate, and fat supplies minimal glucose. If you go on a starvation or very-low calorie diet, your body will turn to lean muscle, which it will burn to get some glucose.

A pound of body fat is equal to 3,500 calories, so when you eat this many calories fewer than you burn, you lose a pound. But, even if you burn 2,000 calories per day, and take in no food or drink -- besides water -- you won’t successfully lose 0.6 pounds of fat per day. This is because your body will also eat into lean muscle to fuel necessary functions. About 25 percent of every pound lost when you don’t exercise, will come from lean muscle mass. You cannot lose more than one-half of a pound of pure fat per day, even if you drink nothing but water.

Maximizing Fat Loss

To help prevent your body from using lean mass for energy to fuel your brain and central nervous system, strength train as you reduce calories. This sends a message to your body that the muscle is necessary for basic function. Aim for at least two workouts per week that address all the major muscle groups.

Your body needs some carbohydrates in your diet to completely break down fat, too. You may cut back on carbohydrates slightly when reducing calories, but don’t eliminate them altogether. Whole grains, vegetables and fruits are healthy sources. The Institute of Medicine recommends 45 to 65 percent of your daily calories consists of carbohydrates.

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