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HealthDay News -- Only obese people with a BMI of 35 or higher have a significantly higher risk for death than normal weight people, data from a large metaanalysis suggest.
Death rates were similar among normal weight (BMI 18.5 to 24.9), overweight (BMI 25 to 29.9) and moderately obese (BMI 30 to 34.9) individuals, Katherine M. Flegal, PhD, of the National Center for Health Statistics in Hyattsville, Md., and colleagues reported in the Journal of the American Medical Association.
Ninety-seven studies were included in the analysis, with a combined sample size of more than 2.88 million individuals, more than 270,000 of whom died during follow-up.
In fact, BMI values in the overweight range appeared to be slightly protective relative to normal weight with a hazard ratio of 0.94 (95% CI 0.91 to 0.96), the researchers found.
Results from earlier studies have also shown lower mortality associated with moderate overweight and obesity. "Possible explanations have included earlier presentation of heavier patients, greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves," the researchers suggested.For all grades of obesity combined, the random-effects summary all-cause mortality hazard ratio was 1.18 (95% CI: 1.12 to 1.25) relative to normal weight. The hazard ratio for grade 1 obesity was 0.95 (95% CI: 0.88 to 1.01) and for grades 2 and 3 obesity, 1.29 (95% CI: 1.18 to 1.41), relative to normal weight.
These findings persisted when the analysis was limited to studies that were adequately adjusted for measured weight and height. Using self-reported, rather than measured weight and height, tended to result in higher hazard ratios.
In an accompanying editorial, Steven Heymsfield, MD, and William Cefalu, MD, both of Pennington Biomedical Research Center in Baton Rouge, La., said the study findings highlight the limitations of BMI as an indicator of unhealthiness.
"Sole use of BMI as a health risk phenotype aggregates people with substantial differences in nutritional status, disability, disease and mortality risk together into similar BMI categories," Heymsfield and Cefalu wrote.
They emphasized that establishing BMI "is only the first step toward a more comprehensive risk evaluation," and added that traditional risk factors, such as BP, blood lipid levels and fasting blood glucose levels should also be considered when assessing risk in individuals who appear overweight.
Study limitations included the potential for publication bias, lack of data on cause-specific mortality or on body composition parameters other than BMI, and potential errors or omissions in study selection and data abstraction, the researchers acknowledged.