BMI: A Well Proven Tool for Maintaining Health
A recent study published in the Journal of the American Medical Association (JAMA) that suggests that being normal weight is associated with a higher risk of death than being overweight or mildly obese should not lead us to reject a preponderance of scientific work that has demonstrated significant medical risks associated with being overweight and obese.
There are a number of technical limitations in this publication that deserve review. In this publication, the authors used body mass index (BMI) to define normal weight (defined as BMI 22.5-24.9) or mildly obese (defined as BMI 25.0-29.9). BMI, which categorizes weight ranges according to height, is a good measure of obesity, but it is not a perfect measure. For example, the same elevated BMI could describe athletes who have a lot of muscle mass and overweight couch potatoes with loads of abdominal girth and yet these two groups of people may have significantly different risk of developing heart disease or dying prematurely. Furthermore, the BMI ranges used in this study are not the same ranges used by other investigators who have published in this field. Normal weight is more frequently defined as a BMI from 18.5 to 24.9 rather than 22.5 to 24.9. So, it is possible that dividing up the weight groups differently lead to these surprising results. There are a number of other risk factors for death that are associated with being overweight or obese (such as levels of physical activity or fitness) which were not evaluated in this study. The authors combined the findings from a number of large studies to generate a study population for 2.88 million, which often requires creative statistical manipulations to homogenize the groups which can in turn introduce bias in the results. Finally, in a very large study population, even very small differences in outcomes can reach statistical significance but may not always have clinical significance.
Independent of the methodological limitations of any analysis, it is also important to consider whether the conclusions of a study are biologically plausible and reflect clinical experience. A preponderance of scientific work supports the conclusion that significant medical risks are associated with being overweight and obese. People who are overweight suffer from more diabetes, high blood pressure, heart disease and certain cancers such as breast and endometrial cancer. Further, there is clearly a greater death risk in those who are significantly obese.
BMI, or other measures of fatness versus leanness, are clinical measures that do not exist in a vacuum. There are important relationships among blood pressure, blood sugar, cholesterol profiles, physical activity, physical fitness, vascular inflammation and BMI. In addition to diabetes, high blood pressure, high cholesterol and heart disease, obesity increases the risk of certain cancers, arthritis and related immobility, and difficulty fighting infections. So, maintaining a BMI in the normal range as recommended by the CDC can reduce the risk of a variety of health problems.
Finally, even within groups of individuals who are overweight or obese, there are factors which influence the likelihood of elevated BMI leading to the development other medical conditions. Numerous studies from The Cooper Institute demonstrate that obese men and women who are physically fit have longer lifespans compared with those who are less fit.
So, let’s not discard a robust scientific literature which details the many health hazards of being overweight or obese based on the findings of single study. Staying or becoming physically fit and achieving or maintaining a normal BMI are not only well proven tools for maintaining wellness, they just make sense.