Limitations of BMI

BMI can’t distinguish between muscle mass, bone density, and fat

A 6 foot, 185 pound bodybuilder will have a BMI of 25.09 which is squarely inside the range that the National Institutes of Health defines as overweight. This is because BMI is calculated by dividing a person’s weight by the square of his or her height (72in x 72in = 5184 inches squared) and then multiplying that by 703. Is that person overweight? Of course not. BMI fails to account for a person’s body composition: his or her percentage of muscles, fat and bones. Instead, it’s a catch-all that can label an NFL player or other muscular athlete as overweight – or even obese (a BMI higher than 30) simply for having more muscle mass.

BMI can’t account for age or sex

BMI is a universal assessment however, there are important distinctions in the relationship between BMI and health risks that depend on age and sex. On average, older adults aged over 70-75 years have a consistently higher percentage of body fat, irrespective of the level of BMI. The same is true of women compared with men, who hold more body fat than men even at a given level of BMI. So the same BMI category does not indicate the same level of health risk for (older) women or (older) men, compared with (younger) women or (younger) men.

BMI doesn't account for the different types of body fat

You’re better off knowing where you carry your weight than how much you weigh. Those with so-called apple-shaped fat distributions — i.e., who store it around the waist — are far more likely to develop Type 2 diabetes, heart disease, breast and uterine cancers and other health problems than those who are pear-shaped, which is when fat accumulates around the hips and thighs. But you can’t tell where fat is stored from BMI.

BMI is inaccurate for people who have circumstances that cause uneven distribution of weight

BMI isn’t perfect; pregnant women and those who are lactating are likely to have meatier arms. Body builders and others with large muscle mass are also prone to inaccuracies. Persons with ascites, a fluid that accumulates in the abdomen due to cirrhosis or another health condition, have an uneven distribution of their weight as well.

BMI can be used as a starting point

BMI is a good way to screen for risk factors – but it’s not the most important measure of health out there. Looking at your diet, exercise and family history can also help to give you some idea of your health, and can be done alongside BMI measurement. If you are worried about your weight or health, make sure to speak with a doctor.