BMI poor indicator of mortality risk

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Body Mass Index or BMI is a widely used indicator of obesity, and obesity is known to influence health and possibly risk of death. Studies based on BMI, however, have not universally demonstrated that a high BMI is linked to an increased mortality risk. In spite of that, BMI is a widely used tool for deciding whether an individual is eligible for bariatric surgery, which has been shown to ameliorate obesity-related ailments. A new study just published online in JAMA Surgery presents a new means of predicting if a person s obesity may raise mortality risk, and thus whether they should be considered eligible for bariatric surgery.

Dr. Raj S. Padwal from the University of Alberta, Canada and colleagues examined data from over 15,000 people who met the current BMI criteria for bariatric surgery either a BMI over 35 or BMI 30.0-34.9 with an obesity-related illness such as type 2 diabetes. These individuals were between 18 and 65 years of age, and their data were collected between 1988 and 1998. On average, they were about 47 years old, with an average BMI of 36.2 (which would be considered severely obese), and 63 percent were women.

The researchers main outcome was ten-year all-cause mortality. They examined the relationship between that occurrence and four factors besides BMI: Age, gender, current smoking, and type 2 diabetes mellitus. These factors all indicated a significantly increased risk of all-cause mortality: BMI by 3 percent per BMI unit, age by 9 percent, type 2 diabetes by 125 percent, current smoking by 62 percent, and male sex by 50 percent.

A prediction rule based upon age, sex, smoking and diabetes was then generated to estimate all-cause mortality, with 4 levels of risk dependent upon the presence or absence of older age, current smoking, type 2 diabetes, and male sex. They found that compared to these other factors, BMI was not an important mortality predictor.

In their discussion the authors noted Our results challenge conventional wisdom emphasizing the importance of BMI as a prognostic indicator for mortality in individuals eligible for bariatric surgery, and of equal import, the practice of using BMI as a surrogate for long-term prognosis.

ACSH s Dr. Ruth Kava was impressed by the study. Use of BMI alone to indicate obesity has always been problematic, since it doesn t take into account a person s activity level or distribution of body fat, both of which are important. If a better-tailored index such as this can be widely used, we should be able to see better-defined and more accurate indicators of the utility of bariatric surgery.