The body mass index is the standard method for evaluating body weight according to height. A “normal” BMI range is between 18.5 and 25. Between 25 and 30 is considered overweight, and above 30, obese (over 40 is considered extreme obesity). Below 18.5 is underweight, which can be as bad for health as obesity, especially in people over 65. To compute your BMI, you divide your weight (in kilograms) by your height (in meters) squared. Another way to do it: Multiply your weight (in pounds) by 705; divide the result by your height (in inches); then divide again by your height. This government website will compute your BMI for you.
Bear in mind that BMI, weight tables, and other indices serve only as approximate guides to evaluating weight. The cutoffs between weight categories are somewhat arbitrary. And some experts believe that the cutoffs for the normal range may be too low. Indeed, some studies suggest that BMIs in the overweight range (but not obesity) are not unhealthy and may even be the “sweet spot” on the weight spectrum in terms of longevity, particularly after age 65.
Moreover, the BMI doesn’t make allowance for some factors that affect body fatness and health. For instance, it doesn’t take into consideration how muscular you are, whether you are large- or small-boned, or whether you have lost height with age.
Perhaps most important, the BMI doesn’t gauge where your body fat is stored, which affects your health risks. If your body is apple-shaped (weight primarily around the waist), you’re at greater risk for various chronic problems, notably cardiovascular disease, diabetes, and some types of cancer, than if you are pear-shaped (fattest in the hips, buttocks, and thighs). It’s possible to have a normal BMI and carry too much weight around the waist and thus still be at elevated risk.
Several factors, notably gender, influence fat distribution. Men store most excess fat in the midsection, while women tend to accumulate it lower on the body. Still, women can gain abdominal fat, too, particularly after menopause (largely because of hormonal changes). Heredity and activity level also affect body shape.
While most fat in the hips and thighs is stored just under the skin (subcutaneous fat), more fat in the midsection is stored in and around the liver and other organs (visceral fat). These fat cells deep in the abdominal area are more “metabolically active” than subcutaneous fat. That is, they release certain fatty acids, hormones, and pro-inflammatory compounds, which are believed to account for some of the adverse health effects. Visceral fat increases estrogen production, for instance, which may partly explain the increase in breast cancer risk in some postmenopausal women. And by inducing chronic inflammation in the body, excess visceral fat may further boost cardiovascular risk.
To determine if you carry too much abdominal fat, measure your waist circumference: 40 inches or more indicates high risk for men, 35 inches for women, though risk starts to rise before those cutoffs.