How Does Fat Leave Your Body When You Lose Weight?

Exercise and diet can help you lose weight by creating a calorie deficit. Your body will begin to break down stored fat through a series of complicated metabolic pathways. The byproducts of fat metabolism are expelled through your lungs, sweat and urine, while the energy liberated from fat is used to maintain your body's biological function.

Whether it's solid or liquid, all fat is found in a form known as a triglyceride. Triglycerides can be broken down to produce glycerol and fatty acids to be used in energy metabolism or stored in fat tissue. Fatty acids can participate in a metabolic pathway known as the citric acid cycle and electron transport chain to produce large amounts of energy. The longer the fatty acid, the more energy produced. The citric acid cycle and electron transport chain occur in the mitochondria, the "powerhouse of the cell."

Excessive caloric consumption combined with a sedentary lifestyle cause triglycerides to be stored in fat tissue. When you eat less and exercise more, a hormone known as lipase begins breaking down stored fat. Tissues throughout the body, such as the muscle and liver, metabolize the previously stored fat through the citric acid cycle and electron transport chain. Byproducts from these metabolic reactions include carbon dioxide, water, heat, and adenosine triphosphate, or ATP. Carbon dioxide is exhaled from the lungs, water leaves as urine and perspiration, and heat helps maintain body temperature. The ATP produced is used throughout your body, from the moving muscles to organ function.

Weight loss and maintenance is difficult. An article published in the "American Society for Clinical Nutrition" in 2005 looked at the National Weight Control Registry, which is a compilation of strategies from individuals who have lost weight and kept it off. Those successful in their weight-loss goals followed a healthy diet and exercised. For example, registry members exercised for about an hour each day. Walking was the most popular exercise. Dietary strategies include eating breakfast, following a 1,300 to 1,800 calorie diet, following their diet during the week and on weekends and eating in, averaging two and a half meals each week in restaurants.

Where you lose fat is important. A 2006 review published in "Current Diabetes Reviews" found that fat that accumulates around the stomach, known as visceral fat, is correlated with Type 2 diabetes and coronary artery disease, while fat that accumulates in the butt and hips, known as subcutaneous fat, is not correlated with those complications. This relationship was found regardless of age and overall body weight. Visceral fat also releases pro-inflammatory cytokines such as tumor necrosis factor-alpha, which causes adverse metabolic changes such as insulin resistance and hardening of the arteries, while subcutaneous fat does not.