Muscle or fat?

It seems that one of society’s strongest symbols of health and vitality - young men - are the latest to be affected by the global obesity epidemic.

Deakin researchers have discovered that more young Australian men are likely to be obese than previously estimated, due to inaccuracies within the commonly used Body Mass Index (BMI) measurement system.

A recent Deakin study has shown that 24 per cent of young Australian men are obese, as opposed to the previous estimate (based on the BMI) - of only eight per cent.

Professor of Epidemiology, Deputy Director of the IMPACT Strategic Research Centre and Head of the Epi-Centre for Healthy Ageing, Julie Pasco, explained that the study found BMI-based estimates for obesity in women were fairly accurate, but use of the BMI markedly underestimated the amount of body fat in young men aged 20-29.

The fat to weight ratio appears to be disproportionately high in this group, which means that many young men are carrying more fat, proportional to their weight, than expected.

“This is having an effect similar to premature aging,” said Professor Pasco. “It means that an alarming number of young men are exhibiting the physical characteristics of older men, where muscle mass has been replaced by fat. This has particular implications for muscle weakness and potential osteoporosis later in life.”

The results of the study were recently published in the open access journal “BMC Obesity.”

“It seems that the modern sedentary lifestyle and poor diet of this group have contributed to this new trend at a population level,” added Professor Pasco.

“Global and Australian estimates of obesity have been based on Body Mass Index. Yet, as a measure of body weight for height, this technique doesn’t account for different body types. It doesn’t distinguish between fat, muscle or bone, which contribute to body weight differently for men and women, and for different age groups.”

Obesity has nearly doubled, globally, over the past three decades, with women bearing the brunt of the trend. Approximately one in three women and one in five men are obese - and an estimated two thirds of the adult population are either overweight or obese.

“Using BMI has been simple to measure and easy to calculate, so it has become an entrenched means of gauging whether people are over-weight or obese. However, we have discovered that it is not accurate, especially in the case of both young and elderly men,” explained Professor Pasco.

“We knew that the BMI overestimates obesity in muscular body builds and underestimates obesity in the elderly, but the underestimation of obesity in young men is a new finding.”

The survey analysed data from almost 2500 randomly-selected men and women, aged 20–96 years, who are all participants in the long-term Geelong Osteoporosis Study – and can be taken as representative of the Australian white population. Whole-body scans were used to discover actual obesity levels.

Professor Pasco and her colleagues say that, as a result of the findings, instead of using one “cut off” threshold for defining obesity, different sex and age-specific BMI “cut off” values should be used by health professionals, to achieve more accurate estimates for the different groups.

This would improve risk assessment for disease, morbidity and mortality, and help in population health management.

“We have an obesity epidemic on our hands. We need the most accurate information possible so that we can address this issue and encourage people to improve their diet and lifestyle,” she said.

• Body mass index and measures of body fat for defining obesity and underweight: a cross-sectional, population-based study.