Body mass index (BMI) has become the most commonly used measurement to assess the relative healthiness of one’s weight. It delineates where normal weight crosses into overweight, and where obesity—and its associated health risks—begins. But taken as a single measure, its accuracy and validity can vary widely.
BMI is calculated as (weight in kilograms [kg]) / (height in meters [m] squared). Ideal BMI is between 18.5 kg/m2 and 25 kg/m2 (BMI is usually written without the unit of measure.) A person with a BMI between 25 and 30 is considered overweight, and a person with a BMI 30 or over is considered obese. Generally, people with higher BMIs have a higher risk of diabetes, arthritis, fatty liver disease, hypertension, some cancers, high cholesterol, and sleep apnea.
Studies show that BMI can accurately reflect the health risks of obesity at high levels (over 30), but is less reliable in the overweight (25 to 30) category. That’s because BMI only represents weight by height. It doesn’t differentiate between lean and fat mass. A weightlifter or athlete, for example, could be very lean and muscular but show a high BMI because of overall weight.
Further, BMI doesn’t take into account the location of fat. Visceral (or belly) fat is associated with more health risks than higher fat in other parts of the body. The value of BMI has also been questioned in older adults because height and body composition change as we age.
Even one’s ethnicity can affect the validity of BMI. Asians have higher health risks at substantially lower BMI levels, so different risk cutoffs are used. In addition, studies have shown that at the same BMI, people who are African American have lower levels of body fat than those who are Caucasian.
Other measures to consider
Waist size. The National Institutes of Health concluded that a waist larger than 40 inches for men and 35 inches for women increases the chances of developing heart disease, cancer, or other chronic diseases. Waist measurement is fairly easy to use, but there is less information available about race/ ethnic variations.
Waist to hip ratio. You can also take your waist measurement and divide it by your hip measurement to get a waist-to-hip ratio. According to the World Health Organization, a healthy result is 0.9 or less in men and 0.85 or less for women. Waist-to-hip ratio is not substantially more accurate at determining risk than waist size alone but is more difficult to interpret.
Relative fat mass index (RFM). Researchers at Cedars Mount Sinai developed a formula to compare height and waist circumference:
- Men: 64 – (20 x height/waist circumference) = RFM
- Women: 76 – (20 x height/waist circumference) = RFM
Men who score over 30 and women who score over 40 would be considered obese and at elevated risk for health problems and even death.
There are more technical ways to measure body fat, but many are fairly expensive and therefore reserved for research purposes. BMI remains one of the easiest ways to determine risk and is accurate in a large percentage of people. Having said that, it is probably more useful for health providers and researchers than it is for patients.