Can We Trust BMI to Measure Obesity?

BMIAround 60% of adults in the UK are overweight or obese, according to a 2013 study by the Institute for Health Metrics and Evaluation. They are labelled this way because of their body mass index, or BMI.

We are used to the idea that being fat is harmful. But some people who have a high BMI are relatively safe when it comes to the risk from serious conditions such as diabetes or heart disease. Meanwhile, people with a supposedly ‘healthy’ BMI could still have a high risk of developing health problems.

A study published in the journal PLoS One documented such inconsistencies and questioned the accuracy of using BMI to classify weight status of 1,400 men and women.

Among the study participants, about half of women who were not classified as obese according to their BMI, actually were obese when their body fat percentage was taken into account. Among the men, in contrast, about a quarter of obese men had been missed by BMI.

Furthermore, a quarter who were categorized as obese by BMI, were not considered obese based on their body fat percentage. Overall, about 39% of participants who were classified as overweight by their BMI were actually obese, according to their percent body fat.

Some argue that BMI, which dates back to the mid-19th Century, is no longer fit for purpose. Research suggests there may be other, more accurate measures of individual health and this provoked a few comments along these lines: “Why, in this day and age, are you using BMI to tell people they are overweight? It is an outdated method that does not take into consideration muscle and actual health! I am extremely fit and healthy with a low body fat percentage, yet your BMI tells me that I am overweight!”

BMI doesn’t distinguish between fat, muscle and bone. This means it doesn’t necessarily tell us how much body fat we have.

People with a lot of muscle bulk can have a high BMI, even if their body fat is low. On the other hand, some older people who lose their muscle with age could see their BMI fall into the ‘healthy’ range, despite carrying too much fat.

BMI also doesn’t take into account where the body fat is. Research shows that people who carry a lot of fat around their waists are at higher risk of health problems than those with more fat around their thighs and buttocks.

So waist size may be a better way to monitor your health than BMI.

A professor explains why BMI is still extremely relevant, but does it work for everyone?

Tim Cole, professor of medical statistics, at University College London Great Ormond Street Institute of Child Health, said BMI was “still extremely relevant”.

There are some people who carry a lot of muscle and little fat, like bodybuilders, boxers and rugby players.

Muscle is much denser than fat so they may end up with a BMI that classes them as obese, despite the fact they may be fit and healthy. But this is thought to apply to fewer than 1% of the population. Most people aren’t extreme athletes.

“You don’t see many bodybuilders around but you do see lots of people with large waists. Many people get exercised about that wrongly.”

As people age, they lose muscle and may be classed in the ‘healthy weight’ range even though they may be carrying excess fat. This is particularly true of smokers.

BMI also doesn’t apply to pregnant women, and some ethnic groups have a higher risk of health problems at lower BMI levels.

With all this being taken into account BMI can help measure our height and weight, although not everyone will trust this method. If anyone has any concerns about weight then you should seek medical advice with your own GP or a pharmacist.

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