Obesity is defined simply as too much body fat. Your body is made up of water, fat, protein, carbohydrate and various vitamins and minerals. Obesity is the excessive accumulation of adipose tissue to an extent that health is impaired.

Obesity is usually determined using the body mass index or BMI. If you have too much fat — especially in your waist area — you're at higher risk for health problems, including high blood pressure, high blood cholesterol, diabetes, heart disease and stroke.

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Link: http://www.americanheart.org/presenter.jhtml?identifier=4639