Why BMI isn’t best measure of obesity

It’s a vexed question. I’ve always been sceptical about BMI as the gold standard for the measurement of obesity. The BMI measures your weight but makes no distinction between fat and muscle.

Obesity, defined as abnormal accumulation of fat such that health is impaired, is still most commonly assessed using the body mass index (BMI). Now, however, experts are questioning whether the BMI is the best measure.

BMI (weight in kg divided by the square of height in m) is a relatively simple and low-cost measure for assessing weight and obesity. BMI cut-off levels to define obesity are based on risks for heart disease, Type 2 diabetes and premature mortality.

However, although BMI is strongly correlated with body fat measures, it’s basically “weak” because it can’t distinguish between lean muscle and fat. Plus, it gives no indication of body fat distribution.

We now know that where your fat accumulates is at least as important as how much fat you have.

The relationship between BMI, total body fat and Type 2 diabetes also differs by ethnic group, leading to different recommendations for obesity thresholds by race.

The loss of muscle mass in elderly people also means that BMI is less accurate. Several studies have suggested that compared with BMI, other crucial obesity measures – including waist circumference, waist:hip ratio and waist:height ratio – are better at predicting heart disease, Type 2 diabetes and mortality.

These measures of obesity are important because they’re associated with illness and death independently of BMI. And they’re especially important in women.

Waist:hip and waist:height ratios are probably better predictors than waist circumference, though above a BMI of 35, their accuracy is questionable.

Ideally, the waist:hip ratio should be below one – ie the waist is smaller than the hips, for example 28”/40”.

If the waist is the same as the hip – it’s one, eg 40”/40”, not so good.

If the ratio is more than one, eg 48”/40”, it probably means you have a beer belly – very bad news.

Other measurements, such as skinfold thickness, can act as a marker of central-to-peripheral fat distribution that’s associated with heart disease. Peripheral fat is deposited on the body in places other than the trunk. That would be thighs, hips and upper arms. This fat isn’t “toxic”.

Central fat on the trunk (beer belly) and hidden inside the abdomen is the toxic kind and will shorten your life.

Taking all these factors into consideration, BMI is only one of the markers for predicting disease risk, but it must be considered along with waist measurement and waist:hip ratio.

For a long life, ideally that ratio should be less than one.

Fat location is as important as the quantity