ATLANTA — About a third of men and almost half of women classed as nonobese based on body mass index (BMI) alone had a high percentage of body fat; moreover, they were 10 times more likely to have metabolic syndrome than their peers, researchers report. In addition, gauging risk based on sedentary behavior alone may be misleading, they discovered.

The new findings, based on data from the 2003–2006 National Health and Nutrition Examination Survey (NHANES), were presented in a poster here at Obesity Week 2013: The American Society for Metabolic and Bariatric Surgery and the Obesity Society Joint Annual Scientific Meeting.

"BMI is potentially missing a lot of people who are at risk," since they have "normal-weight obesity," lead author Mark D. Peterson, PhD, from the University of Michigan, Ann Arbor, told Medscape Medical News.

Dr. Peterson does not believe that the BMI threshold to detect obesity will ever be lowered from 30 kg/m2. But he hopes the study results will draw attention to a segment of the population that might be missed when screening for disease risk if waist circumference or percentage of body fat is not considered.

The study also suggests that overall level of physical activity, not time spent being sedentary, is the better predictor of metabolic syndrome, say he and his colleagues.

Does BMI Accurately Predict Metabolic Syndrome?

The best way to screen for obesity, risk for cardiovascular disease, and diabetes remains controversial, and misclassification is common, the researchers explain. While BMI is used as a surrogate marker for adiposity to screen people, it does not distinguish between fat and muscle, Dr. Peterson pointed out.

In addition, time spent being sedentary is believed to contribute to risk for preventable disease and mortality, which is a growing topic of interest, he noted.

To investigate these 2 aspects of screening, the researchers analyzed data from 5576 participants of the 2003–2006 NHANES who were 20 to 85 years old and for whom data were available detailing blood pressure, measures of BMI, waist circumference, body fat, and activity levels.

Nonfasting serum measures of HDL cholesterol and high-sensitivity C-reactive-protein (hs-CRP) concentrations were also taken, as were fasting measures for triglycerides, plasma glucose, and insulin. Metabolic syndrome was defined as the presence of any 3 or more of the following:

  • Abdominal obesity (> 102 cm for men; > 88 cm for women).

  • Elevated triglycerides (> 1.7 mmol/L).

  • Reduced HDL cholesterol (< 1.0 mmol/L in men; < 1.3 mmol/L in women).

  • Hypertension (> 130 mm Hg systolic and/or > 85 mm Hg diastolic).

  • Elevated fasting glucose (> 100 mg/dL).

Total body fat percentage was based on dual-energy X-ray absorptiometry readings, and obesity was defined as body fat of 25% or more for men and 35% or more for women.

Activity levels were based on 4 days of measurements from an accelerometer, which is a small device worn attached to the hip that is similar to a pedometer but that also counts movement intensity, Dr. Peterson explained.

A Very Obese Nation

BMI readings of 30 kg/m2 and above had excellent specificity in both men and women but very poor sensitivity; based on this cutoff, 30.6% of men and 37.5% of women were defined as being obese. Using body-fat percentage instead, the researchers found that 71.1% of men and 79.6% in women in this representative national population sample would be classed as obese (P < 0.001).

And 32.8% of the men and 45.7% of the women with a normal weight based on BMI measurements (18.5–24.9 kg/m2) were identified as obese according to body-fat percentage.

While less than a fifth of the men (17.5%) and women (16.7%) surveyed had metabolic syndrome, those who were classed as obese according to percentage of body fat had about a 10-fold increased likelihood of metabolic syndrome (odds ratio [OR], 9.64 for men; OR, 10.58 for women).

Age, education, percentage of body fat and being in the highest quartile for sedentary behavior (> 9.5 hours for men and > 9.25 hours for women) were all significantly associated with higher odds of having metabolic syndrome. Lower annual income was an independent predictor of metabolic syndrome for women only.

However, after adjustment for overall total activity, sedentary behavior was no longer associated with an increased risk for metabolic syndrome.

Use a BMI of 27.5 to Define Obesity?

"Dr. Peterson has some evidence that maybe we should be using a [BMI] cut point of 27 to 28 — and anything above that would be obese — which correlates better with body-fat percentage than the cut point of 30," William D, Johnson, PhD, from Pennington Biomedical Research Center, Baton Rouge, Louisiana, pointed out when asked to comment. He himself has a BMI of 30 kg/m2, he noted, which brings this study close to home.

"If people would accept that cut point, that would redefine BMI," he added, but like Dr. Peterson, he does not think that is likely, although some underdeveloped countries where people are very lean do use a lower BMI to define obesity, he noted.

This study was supported in part by the National Institutes of Health National Center for Medical Rehabilitation Research, National Institute on Neurological Disorders and Stroke, and National Institute on Aging.

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