The truth about BMI

Does Body Mass Index really tell you how healthy you are?

The Truth About BMI
Body mass index (BMI) is a good way to check if you're a healthy weight. For adults, BMI is a measure of whether you're a healthy weight for your height.

The BMI is defined as the body weight divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from weight in kilograms and height in metres.

It's an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then determine if that person is underweight, normal weight, overweight, or obese based on that value.

If you have a BMI above the healthy range you are at raised risk of the serious health problems linked to being overweight, such as type 2 diabetes, heart disease and certain cancers, the NHS says.

But is BMI really a reliable measure of health?

One criticism aimed at BMI is that someone with a lot of muscle and low fat could have a high BMI. A professional rugby player, for example, can have an 'obese' BMI result despite having very little body fat.

Also, taller individuals, even if they had exactly the same body shape and relative composition, always have a larger BMI than shorter individuals.

Another is that BMI was designed for population studies, not individuals. Its creator, Belgian Adolphe Quetelet, wasn't a doctor but a mathematician who developed the measurement in the 1830s to discover who represented the 'average man' and was not interested in health issues.

In the 1980s, BMI values were designated as being underweight, normal, overweight and obese. But studies have shown that there is not always a clear link between which category you are and how likely you are to develop health issues like heart disease, particularly for those in the middle categories.

So, your BMI can measure your weight relative to your height, but not always how fat or how healthy you are.