A recent study in the Journal of the American Medical Association (JAMA)1 suggested that overweight people live longer. It certainly got a lot of press. And that’s too bad, because it appears both the media and the authors of the study have not figured out that correlation should not be confused with causation.
Put simply, the only thing the researchers found was a link between carrying a few extra pounds and slightly lower risk of death. They did not find that extra weight caused longer life, or that being normal weight caused earlier death.
It could well be that there is something related to carrying less weight, like long and degenerative disease, that is the actual cause of earlier mortality.
Many serious ills cause weight loss and then death. People with Alzheimer’s, congestive heart failure, emphysema, renal failure, and numerous other ills lose weight – often over the course of several years – before dying. Unintentional weight loss is generally associated with a marked increased risk of mortality.
We all know people who were overweight or obese most of their lives, but unless they die suddenly from a heart attack or stroke, they typically lose a lot of weight before they die.
Bottom Line: Dying slowly causes weight loss. By contrast, healthy people who lose weight with a healthy eating and exercise program reduce the risk of dying and increase longevity.
Put another way, it is more correct to say unintentional or illness-related weight loss leads to dying than it is to suggest that being overweight or obese protects against dying.
Finally and most importantly, the preponderance of research has found that being overweight or obese promotes many types of cancer, raises blood pressure, promotes Type 2 diabetes, and greatly increases the risk of gallstones, osteoarthritis, senility, heart failure, and numerous other ills that increase both morbidity and mortality.
The type of superficial analysis of correlational data published in JAMA tells us little of any real value.
A far better meta-analysis of the impact of body mass index [BMI] and the risk of dying was published by Walter Willett, MD, and colleagues in the New England Journal of Medicine2on December 2, 2010. The article, containing some excellent figures, clearly shows that as BMI rises into the overweight range and higher, the risk of dying increases exponentially in people who have never smoked, especially when the follow-up period is 15 or more years. All-cause mortality was lowest with a BMI in the normal-weight range of 20 to 24.9.
Long-term follow-up helps eliminate non-causal correlations that can distract us from reality.