It’s one of the most popular tools used in clinical practice and apparently schools are now sending letters home to parents if their child is tracking at a higher percentile (!) But is this a good tool?
For those unfamiliar with BMI, it is calculated by dividing your weight (kg) by the square of your height (m). The number given will then fall into a category - Underweight, Normal Weight, Overweight, Obese.
The limitations of this approach however are somewhat obvious, for example, we know muscle weighs more than fat, and therefore a sports person could be classified as overweight/obese according to this scale just due to their muscle/fat ratio. Also it doesn’t tell us the distributions of body fat, to assess whether it is therefore harmful to health.
The reason we started using this tool was to monitor trends in population over time, not actually as a measure of ‘health’ however it would appear that it is now being used to decide if people are healthy or not. To put this into context, the ‘overweight’ category used to be over 27 (pre 1980s) whereas today it is set at 25 - so what happened when we changed that? Thousands of people didn’t just become ‘overweight’ overnight.
You would assume that using this tool now to dictate ‘health’ would mean there is strong compelling evidence that a higher BMI is attributed to poorer health... errrm not quite. A strong meta-analysis (one of the best studies we have) from 2013 showed that the ‘overweight’ category had the lowest death rate and the ‘obese’ category has the same risk of death as the ‘normal’ category.
So what does this tell us? We can be ‘overweight’ and be healthy?! Exactly that. There are so many contributing factors to health than just weight. How much exercise we do, whether we smoke, how much fruit + veg we eat, how much fibre we eat, what our lifestyle is like.. Did you know that 70% of our weight is determined by genetic factors? Therefore trying to ask someone to ‘lose weight’ isn’t such a simple task and actually may not improve their health, rather than increasing an individuals exercise, fruit + veg intake.. etc.
This photo shows the relative risk to death in exercised individuals ‘fit’ compared to non exercised individuals ‘unfit’, as you can see no matter what BMI category you might be in, the risk is pretty much halved when that individual exercises and a ‘normal’ weight who doesn’t exercise has a higher risk than an ‘obese’ weight who does.
So is BMI a good indicator of health?.. I’ll let you make up your own mind