How well does body mass index correlate with excess fat?
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Q. I recently read that a person's body mass index (BMI) might not be the best way to determine if your weight is healthy. Why is BMI being questioned? Are other measurements better?
A. Your BMI is a number that is calculated based on your height and weight. (Many online calculators are available to determine your BMI.) According to the CDC, a normal BMI is 18.5 to 25. BMI helps determine if someone is underweight, as a BMI below 18.5 might indicate a medical problem or nutrient deficiency.
However, the more common use of BMI is to determine whether a person is overweight or obese and as a proxy for excess body fat. A very high BMI of 40 or higher (severe obesity) almost always indicates excess fat. However, many people with a BMI greater than 25 but less than 30 (overweight), and even people with a BMI of 30 or higher (obesity), might carry extra fat without it affecting their daily life or health. Also, some people with normal BMI still have excess fat in places that put them at increased risk of chronic conditions.
That's why, on Jan. 14, 2025, a group of 58 world experts convened by The Lancet Diabetes & Endocrinology published a new definition of obesity that is not based solely on BMI. Does that mean we should ignore BMI between the extremes of underweight and severe obesity? Not necessarily. But BMI needs to be combined with at least one additional measurement, because the location of fat is just as important as the amount of fat.
Visceral fat lies deep inside the belly and surrounds organs. Unlike subcutaneous fat, which is directly under the skin, visceral fat can infiltrate the liver and increase the risk of diabetes, cardiovascular disease, kidney problems, and liver failure.
BMI is not a good way to measure visceral fat. A better approach is to measure your waist size just above your navel at the end of a natural breath while standing tall. In men, a waist size of 40 inches or more almost always indicates excess visceral fat. Ideally, men between 5 feet, 6 inches and 6 feet, 6 inches tall should aim for a waist size of no more than one-half their height.
An alternative way to assess visceral fat is waist-to-hip ratio. For men, waist size divided by the circumference measured around the hips should be less than 1.0. (Some references suggest an ideal ratio for men is no more than 0.9). In simple terms: your waist should be smaller than your hips.
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About the Author
Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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