Body Mortality Index

It looks like a little extra weight might go a long way. A new Canadian study has found that people who are slightly overweight live longer than those who are of a normal weight. Once again, I’ll emphasize the slightly in case the bold and italics weren’t enough. The study examined approximately 11,000 Canadian adults for a period of 12 years. HealthDay explains:

“Compared to normal-weight people, those who were underweight were 70 percent more likely to die and those who were extremely obese were 36 percent more likely to die, the researchers found.

On the other hand, overweight people were 17 percent less likely to die than those of normal weight. The risk for obese people was the same as for people of normal weight, the study authors noted.”

If you are already reaching for that extra snack, you may want to put it down. The researchers clarify that the results of this study do not mean that people of a normal weight should try and put on extra pounds.

Mark Kaplan, coauthor and a professor of community health, clarified that their study only examined life span and did not focus on quality of life. It is well known that there are many negative health conditions that result from being overweight.

The results of this study were published online in the Obesity journal. It was conducted by researchers at Statistics Canada, Kaiser Permanente Center for Health Research, Portland State University, Oregon Health & Science University, and McGill University.

3 Responses to Body Mortality Index

  1. Luk Arbuckle says:

    It was also an observational study, which are notoriously poor. Observational studies are especially poor in the case of diet and health because people are choosing for themselves what they eat. In a randomized controlled trial (RCT), people would not choose what they eat, eliminating all sorts of hidden bias.

    There are many examples in the health sciences where the results of an observational study were reversed under a properly conducted RCT–the only type of study that can demonstrate causality. The classic example is hormone replacement therapy, where a prospective cohort study suggested a decreased risk of coronary heart failure, but proper RCTs later revealed that the treatment in fact increased the risk.

    Observational studies can only indicate causality if the effect is very large–like smoking and lung cancer. But such examples are rare, and this is not one of them. A basic rule of thumb I learned is to treat with suspicion the relative risk from an observational study if it is less than 2, or greater then 1/2 for alleged protective effects. And the relative risks quoted in the study are all within 1/2 and 2 (except the upper limit of the confidence interval for the underweight category).

  2. Joe says:

    Multiple studies over many years have found the same results, including a well done Japanese study. All these studies found something else worth repeating; people with very low weight have extremely high risks of disease and death.

  3. Luk Arbuckle says:

    As the authors of the study themselves admit, “although a clear risk of mortality is associated with obesity, the risk of mortality associated with overweight is equivocal”, i.e., ambiguous. Although there are (observational) studies that agree with their results (as there were for HRT), there are also many studies that disagree with their results (watch out for updated systematic reviews and meta-analyses).

    I’m assuming this is the Japanese study being indirectly referenced (in the same issue as the Canadian study): BMI and All-cause Mortality Among Japanese Older Adults: Findings From the Japan Collaborative Cohort Study.

    As the researchers point out in this Japanese study, “overweight/obesity is related to excess mortality among both younger and middle-aged populations, and the cut-off points recommended by World Health Organization are mainly based on them.” They then reference papers that suggest increased risk for older adults, plus a meta-analysis that suggested no risk, which is actually what they concluded in their study (not a decreased risk, as in the Canadian study). They go on to explain why they may not have found an increased risk for older adults (for example, survivor bias).

    And finally, this recent paper relates to another point the authors of the Canadian study make, that of quality of life (although co-morbidity and mortality are also highly correlated, suggesting a higher risk of death): The incidence of co-morbidities related to obesity and overweight: A systematic review and meta-analysis.

    I may be wrong, but I don’t believe there’s any disagreement in the literature with regards to the increased risk of mortality for the underweight. The risk factors being quoted in the Japanese study, and some it references, seem large enough to be considered practically significant.

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