Their study suggests that the extent to which obesity has grown over the last 20 years may be underestimated.
NUI Maynooth researchers found that waist circumference outperformed body mass index (BMI) — a key index for relating height to weight — and bioelectrical impedance analysis (BIA) — the measurement of body fat in relation to lean body mass both BMI and BIA.
Donal O’Neill from the Department of Economics at NUI Maynooth examined data on a nationally representative survey of 33,994 US citizens who were evaluated with all three measures.
While BMI and BIA misclassified a large number of individuals, this was not the case when waist circumference was used.
The statistical analysis found estimated error rates of 3% for waist circumference compared to as high as 45% to 50% for BMI and BIA.
Prof O’Neill said reliable measures of body composition were essential to develop effective policies to tackle the costs of obesity.
“The problem with body mass index is not that it misclassifies non-obese people as obese, but rather that it fails to truly detect obesity when it is present. The issue with BIA is the very high probability of a false positive,” he said.
“The classification of latent obesity based on waist circumference exhibits high degrees of accuracy, suggesting that this measure may provide a cheap and effective measure of latent obesity.”
Prof O’Neill said the findings had important policy implications, both in terms of how to measure the growth in obesity over time and how to evaluate racial gaps in obesity.
“Since our analysis suggests that measures based on waist circumference accurately reflect the true prevalence of obesity, and since waist circumference measures of obesity have grown in excess of what would be predicted given the growth in body mass index, it is quite possible that we are underestimating the extent to which obesity has grown over time,” he said.
The research revealed that BMI tends to return the lowest obesity rate of all three tests, while BIA returns the highest rate.
Though small, the false positive rate for BMI was higher for men than for women, which might be expected given that men tend to have more muscle and fat-free mass.