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15-01-2016 | Endocrinology | News | Article

BMI obesity threshold should be lowered in older people

medwireNews: Body mass index (BMI) is an inaccurate measure of obesity in older people, show data from NHANES.

“Our study highlights the challenges of using BMI as the most widely used and accepted method to diagnose obesity in clinical care setting, particularly in older adults”, say John Batsis (Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, USA) and co-researchers.

The team found that BMI at the usual cutoff of 30 kg/m2 was specific but had low sensitivity for increased body fat in 4984 participants of the US NHANES (National Health And Nutrition Examination Survey), who were all at least 60 years old.

The selected participants had all undergone dual-energy X-ray absorptiometry (DEXA), and the researchers used thresholds of 25% and 35% body fat in men and women, respectively, to define obesity.

Based on DEXA results, 87.5% and 89.1% of men and women, respectively, were classed as obese, whereas BMI identified a corresponding 28.9% and 34.3%. The specificity of BMI 30 kg/m2 for obesity was at least 98.8%, but the best sensitivity was just 44.5%, achieved in women aged 60–69.9 years.

And sensitivity was especially poor in the oldest participants, aged 80 years or more, at 14.5% in men and 23.4% in women.

Reducing the BMI threshold for obesity to 25 kg/m2 improved sensitivity to 63.2% and 65.7% in men and women, respectively, over the age of 80 years. Although the corresponding specificities fell to 86.9% and 91.3%, there was a large increase in the number of people correctly classified, from 23.9% to 65.8% of men and from 38.4% to 70.8% of women.

The ideal BMI cutoffs for identifying obesity in the overall cohort were markedly lower than the accepted threshold, at 24.91 kg/m2 in men and 24.10 kg/m2 in women, the team reports in the International Journal of Obesity.

“With the changes observed in body composition in this patient population, our data provide an opportunity to caution clinicians in solely relying on this anthropometric measure for counseling patients on reducing their weight and lowering their cardiovascular risk”, they say.

The researchers observe that a BMI of around 25 kg/m2 has been linked to reduced mortality in a number of studies, which, combined with the current findings, “suggest that traditional BMI cutoffs are likely inaccurate and conceivably should be revisited.”

They concede that BMI can be a helpful measure, but urge caution in its use. Given the cost of methods such as magnetic resonance imaging, they advise the use of DEXA in older adults, “particularly when this test is performed for other indications, such as osteoporosis screening or monitoring.”

By Eleanor McDermid

medwireNews is an independent medical news service provided by Springer Healthcare Limited. © Springer Healthcare Ltd; 2016