Body mass index (BMI) is an estimate of body fat based on height and weight. It doesn’t measure body fat directly, but instead uses an equation to make an approximation. BMI can help determine whether a person is at an unhealthy or healthy weight.

A high BMI can be a sign of too much fat on the body, while a low BMI can be a sign of too little fat on the body. The higher a person’s BMI, the greater their chances of developing certain serious conditions, such as heart disease, high blood pressure, and diabetes. A very low BMI can also cause health problems, including bone loss, decreased immune function, and anemia.

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A calorie is a measurement, just like a teaspoon or an inch. Calories are the amount of energy released when your body breaks down (digests and absorbs) food. The more calories a food has, the more energy it can provide to your body.