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What Happens to Body Fat When You Lose Weight?

author image Andrea Cespedes
Andrea Cespedes is a professionally trained chef who has focused studies in nutrition. With more than 20 years of experience in the fitness industry, she coaches cycling and running and teaches Pilates and yoga. She is an American Council on Exercise-certified personal trainer, RYT-200 and has degrees from Princeton and Columbia University.
What Happens to Body Fat When You Lose Weight?
Fat cells shrink when you lose weight. Photo Credit PhotoStock-Israel/Cultura/Getty Images

You've just lost weight, and you're left wondering where that part of you went. Fat burning is a complex chemical process that creates carbon dioxide and water, and, essentially, any fat you lose, you exhale away. A minor percentage of fat lost is in the form of water, through your urine, sweat, tears and feces. Fat doesn't convert to muscle, nor is it excreted primarily through fecal waste or sweat, as some people believe.

Understand Fat "Burning"

When you gain weight in the form of fat, the adipocytes -- or fat cells -- in your body swell with the stored triglycerides, or fat molecules. When you reduce your calorie intake and exercise, creating a calorie deficit, your body converts these triglycerides into free fatty acids and glycerol, which are then used for energy. As the fat cells free fatty acids and glycerol, they begin to shrink -- but never disappear completely.

When you lose weight with extreme dieting, such as fasting or a calorie deficit that has you consuming fewer than 1,200 calories per day, you might start to use some lean muscle, as well as fat, for energy. Your body senses starvation and holds onto fat to help you survive; it takes more energy for your body to sustain muscle mass. The chemicals that used to be muscle are excreted via exhaling and and as water waste, too.

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The Outcome of Fat Burning

Using the fatty acids and glycerol for energy, or fat "burning," results in a loss of water, carbon dioxide and energy. To lose 10 pounds of fat, you must inhale 29 kilos of oxygen, which then produces 28 kilograms of carbon dioxide and 11 kilograms of water. You lose approximately 84 percent of your fat as carbon dioxide and the other 16 percent as water. This makes your lungs an essential organ in the process of fat metabolism and excretion, since they breathe out the carbon dioxide that used to be fat.

Fat Cells Can Always Expand

Losing weight doesn't mean you lose fat cells, it means you shrink them. While liposuction can actually remove fat cells from specific areas, if you consume more calories than you burn consistently, the remaining ones on your body will just get bigger. The average person has 10 billion to 30 billion fat cells, but if you're obese, you may carry as many as 100 billion.

Between birth up to age 6, the number of fat cells you have triples; they also expand in size threefold. You get a brief respite in fat cell expansion, but after age 8, girls experience a notable expansion in the size of their fat cells. You don't gain more fat cells; the ones you have just swell due to signalling from hormones. If you're obese as a child, you may continue to grow new fat cells throughout adolescence regardless of your gender, though.

Eat Less, Move More to Lose Weight

The common weight-loss advice of "eat less, move more," makes sense when you consider how your body uses fat. Exercise that uses a lot of energy is more effective at helping you lose weight than small, targeted toning exercises. Your body must sense it needs energy to burn fat off in the form of gas and water. Pinpointing a place on your body from which you want to use fat is also impossible; you lose weight proportionally and your body decides where to pull fat from first when it senses a deficit.

It takes a 3,500-calorie deficit to lose a pound. A combination of exercise and reduced caloric intake helps you create this deficit, but it should never exceed much more than 500 to 1,000 calories per day, or you may lose weight too quickly. When you lose weight quickly, you release too much of the waste products into your system, often leading to gallstones or other complications. Some special very low-calorie diets do cause extreme weight loss, but they're prescribed in a medical setting and entail consistent monitoring.

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