Why BMI is inaccurate for persons in sports

If you are athletic, you are likely to be more muscular and have higher bone mineral density than a sedentary or nonathletic person, and that adds up to extra pounds. When you put your weight into the BMI equation, you may get a value greater than 25, classifying you as having overweight, or perhaps greater than 30, classifying you as having obesity. The CDC admits that some athletic individuals may have a high BMI but a low percentage of body fat.

Body mass index (BMI) isn't well understood. People with small frames, large frames, muscular and athletic people, can fall into the "overweight" or obese categories all too easily. It is useful as a tool to track the average changes over time in populations, but at an individual level it doesn't really serve as a reliable tool. Nor does BMI take into account a person's body composition and body shape - the individual characteristics of bone, muscle, and fat. It's widely confused as a measure of fat, or fitness. It's not. Instead it's an indicator of possible future health risks, such as being overweight, diabetic, or obese. It's much better to think of the measure, and other similar measures that use height, weight, and sex differences, as a guide.

To illustrate this, consider athletes and muscular sport stars or basketball players. Former All Black great Richie McCaw did not fall within the "average" range of BMI, for example. At 1.88m and a weight over 100kg, McCaw's BMI was around 30. Sprinter Usain Bolt's BMI, calculated as 24.5, is just within the healthy range.

Of course, BMI shouldn't be used to measure athletes' health. BMI is the measurement of a person's weight in kilograms, divided by their height squared in centimetres. It has been used to classify people as underweight, healthy, overweight or obese since the 1980s.

Since BMI doesn't take body mass composition or distribution into account, it is basically blind to fitness level and body shape. You probably won't be found to be obese when you're actually underweight, but for people who are borderline, being misplaced is easy. For example, you can be an athlete in great shape and be deemed overweight or even obese. Or, you could have an average BMI but carry a significantly higher amount of mass in your torso, which as some studies show can be a predictor of health risks such as cardiovascular disease or diabetes.

An example of the biggest flaw in using BMI

Wrestler Steve Austin, at the height of his career weighed 114kg (252lbs). He was 6ft 2ins tall. His BMI at 32.4 would class him as having obesity.

Body Mass Index’ biggest flaw is that it does not take into account the person’s body fat versus muscle (lean tissue) content. Muscle weighs more than fat (it is denser, a cubic inch of muscle weighs more than a cubic inch of fat). Therefore, BMI will inevitably class muscly, athletic people as fatter than they really are.

A 6ft-tall Olympic 100 meter sprinter weighing 90kg (200lbs) may have the same BMI (26) as a couch potato of the same height and weight. A BMI calculation would class both of them as overweight. That calculation is probably right for the sedentary couch potato, but not for the athlete.