Why athletes need higher BMI thresholds
Research found that standard measurements exaggerate whether those of a sporty disposition are overweight.
The study of more than 600 young male athletes – all of whom took part in competitive sports – classed more than one quarter as obese or overweight, judged by their body mass index (BMI).
When scientists used scans, which measure body composition, it was found that fewer than 4 per cent fell into this category.
The study, led by Italian researchers, then used statistical modelling to create more accurate BMI cut-off points for those with athletic habits.
Among athletes, the threshold for overweight raised from a BMI of 25 kg/m2 to 28.2 kg/m2, while for obesity it shifted from 30 kg/m2 to 33.7 kg/m2.
Researchers from the University of Modena and Reggio Emilia said the best way to measure body composition was to use a dual X-ray absorptiometry (DXA) scan.
But in the absence of this, athletes should be classed on higher BMI thresholds, they said.
'Wrongly classified'
The research, to be presented at this year's European Congress on Obesity in Malaga, follows a long debate about the use of BMI to spot excess fat in those with high levels of muscle.
Professor Marwan El Ghoch, of the university's department of biomedical, metabolic and neural sciences, said: 'BMI doesn't distinguish between body fat and lean mass, which includes muscle.
'As a result, a muscular athlete with low body fat can be wrongly classified as being in the overweight or obese range. Despite this, many sports organisations still rely on the traditional BMI classification system.'
The cross-sectional study involved 622 males, whose average age was 25.7 years, and who took part in sports including football, rugby, basketball, volleyball, cycling, cross-fit, karate and triathlon at a competitive level.
The participants were categorised using the current BMI system and by their body fat percentage.
Using the current BMI system, more than a quarter of the individuals were categorised as overweight or obese. While 72.5 per cent of cases were of normal weight, 23.8 per cent were overweight and 3.7 per cent were obese.
'Cut-off points are clearly flawed'
Total body fat percentage was measured using dual X-ray absorptiometry (DXA) scans – known to be a highly accurate tool for measuring body composition – according to age- and gender-specific cut-off points.
Body fat of 21 per cent or above was classified as overweight and body fat of 26 per cent or above was classified as obesity.
Using this system, 96.1 per cent of individuals were classed as of normal weight, 3.1 per cent were deemed overweight and just 0.8 per cent were classed as obese.
The findings were published in the journal Nutrients.
Professor El Ghoch, who led the study, says: 'This demonstrates that the current BMI cut-off points are clearly flawed in determining weight status in athletes, as many of the athletes were misclassified as living with overweight or obesity, where in reality, very few had body fat levels in this range.'
Study co-author, Professor Chiara Milanese, of the University of Verona, said: 'Although DXA scans measure body composition accurately, they are not always available in sports settings. In contrast, weight and height, the two measures needed to calculate BMI, are easy to obtain and, with the new BMI cut-offs that we identified, BMI could be a highly useful tool in sports clubs, both at training grounds and in competitions.
'A direct assessment of body composition remains the gold standard but, in its absence, we encourage sports organisations and committees to adopt the new BMI classification system.'
Researchers said more studies were needed to identify new cut-offs for female athletes and, potentially, specific cut-offs for different sports, particularly those that were not included in the current study.
Body mass index (BMI) is a method used for measuring people's weight status, defining whether they are normal weight, overweight or obese. It is easily calculated by dividing an individual's weight in kilograms by the square of their height in metres.
A BMI of 25 kg/m2 or above is an indicator of overweight and a BMI of 30 kg/m2 or above indicates obesity in white men and women of all ages, according to the World Health Organisation's (WHO) categorisation system.
However, several experts have said that waistlines are a better marker of health risks.
The National Institute for Health and Care Excellence says that people should check their waistline and keep it to less than half their height, to stay in good health.
It follows warnings that abdominal fat carries specific health risks, such as Type 2 diabetes, high blood pressure and cardiovascular disease.
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