When I first got back to Dallas I ran in a 5k with some friends. I was not in shape for running then but I was swimming and lifting weights. After the 5k there was a health fair with a BMI calculator. Although I was 5’10 and 165 lbs I was in the borderline obese category. It was infuriating!
BMI can be decent for some people. It’s easy to do as well. You plug in height and weight and then let the numbers do the rest. Since many people don’t exercise or are sedentary it is fair enough for them. But BMI has huge drawbacks as well including not counting for muscle and others:
A 2010 study that followed 11,000 subjects for up to eight years concluded that BMI is not a good measure for the risk of heart attack, stroke or death. A better measure was found to be the waist-to-height ratio. A 2011 study that followed 60,000 participants for up to 13 years found that waist–hip ratio was a better predictor of ischaemic heart disease mortality.
BMI is particularly inaccurate for people who are very fit or athletic, as their high muscle mass can classify them in the overweight category by BMI, even though their body fat percentages frequently fall in the 10–15% category, which is below that of a more sedentary person of average build who has a normal BMI number. Body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing and the limitations of manual measurement have also led to new, alternative methods to measure obesity, such as the body volume index. However, recent studies of American footballl menwho undergo intensive weight training to increase their muscle mass show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea.
BMI also does not account for body frame size; a person may have a small frame and be carrying more fat than optimal, but their BMI reflects that they are normal. Conversely, a large framed individual may be quite healthy with a fairly low body fat percentage, but be classified as overweight by BMI. Accurate frame size calculators use several measurements (wrist circumference, elbow width, neck circumference and others) to determine what category an individual falls into for a given height. The standard is to use frame size in conjunction with ideal height/weight charts and add roughly 10% for a large frame or subtract roughly 10% for a smaller frame.
For example, a chart may say the ideal weight for a man 5 ft 10 in (178 cm) is 165 pounds (75 kg). But if that man has a slender build (small frame), he may be overweight at 165 pounds (75 kg) and should reduce by 10%, to roughly 150 pounds (68 kg). In the reverse, the man with a larger frame and more solid build can be quite healthy at 180 pounds (82 kg). If one teeters on the edge of small/medium or medium/large, a dose of common sense should be used in calculating their ideal weight. However, falling into your ideal weight range for height and build is still not as accurate in determining health risk factors as waist/height ratio and actual body fat percentage.
A further limitation of BMI relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight. (source for all of this)
There are a lot of people who agree that BMI is ridiculous. And there are other ways to calculate whether someone is healthy, such as Body Fat using calipers or water displacement.
So if you exercise and are muscular don’t cop a fit when the body fat numbers aren’t where you need them to be, just understand that body fat is not the end all be all of fitness and health.