Sonya Bolch Angelone, M.S., R.D., C.L.C. Responds:

Sonya Angelone, MS, RD, a registered dietitian with a master's degree in nutritional sciences and researcher on the nutritional effects of a low-fat, low-cholesterol diet at the Stanford Heart Disease Prevention Program, responds: Body composition refers to the makeup of your body—the part that's fat and the part that's lean. Taken together, they account for your body weight. Body fat is measured as the percentage of fat or portion of your body that is not "lean" (muscle, organs, bones, and so on). Average body fat percentages for men fall within 15 to 18% body fat, and for women they range from 22 to 25% body fat. Generally, 25% body fat for men and 30% body fat for women are considered cutoffs for obesity. Take a look at the many ways to measure body fat. 

The best standard for estimating body fat is hydrostatic, or underwater, weighing. This process consists of comparing a person's weight while fully submerged in water to a person's "dry weight" on land. Hydrostatic weighing is costly and not widely available. A lower-cost, relatively accurate, and more widely used technique is bioelectrical impedance (BIA). This process uses a computer-assisted device that transmits a harmless electrical current between the wrist and ankle and measures the resistance encountered by the current. Fat tissue resists the passage of the current and, fat-free weight conducts the current more easily. This method is often available at gyms and rehabilitation centers or on many commercial scales. A long recognized method to determine body fat is a series of skin-fold measurements. Special calipers are used to "pinch" the skin at specific places on the body, and then calculations are made to determine your percentage of body fat. The accuracy and reliability is dependent upon a skilled and experienced tester. 

The proportion and distribution of fat varies from person to person and affects risk of disease. The waist-to-hip ratio, or WHR, measures fat distribution and reflects this proportion of fat. A ratio of greater than 0.8 for women and greater than 0.9 for men is considered undesirable. A simple measurement anyone can perform is waist circumference, which is strongly associated with abdominal fat. For women, aim for less than 35 inches, and for men, less than 40 inches. The smaller the waist circumference, the better. Special data tables are available to determine if you are overweight. They show desirable weights for height, gender, and body frame. 

Generally, overweight begins at 120% of recommended weight and obesity begins at 130%. This technique is not always accurate. For example, these height and weight tables could indicate that a lean, muscular person is "overweight" (muscle weighs more than fat), though a person whose weight is within the "normal" range might actually be carrying around more fatty tissue and less muscle than is healthy. Body mass index (BMI) is a measure of height to weight appropriateness and can be used to help indicate risk for disease. For men and women, a healthy BMI is between 19 and 25. A value of 25 to 29 is considered overweight, and greater than 30 is considered obese.