The Big Myth That Losing Weight Will Extend Your Life
Understanding the relationship between BMI and Mortality
Posted Mar 08, 2017
"No, as it turns out, I really like being congratulated on my weight loss. I like it so much, it's tragic. " Carrie Fisher
Very few of us are satisfied with our weight. We search for the latest miracle diet and see the barrage of weight loss headlines on magazines in the grocery checkout aisle and on the Internet. They tell us we can lose weight and flatten our abdomens or tighten our buns in less than six weeks. Although these messages are often cloaked in health claims, the root of our obsession with weight loss stems more from our perceptions of what society deems beautiful or attractive. In terms of health and longevity, it turns out that having 10 or 20 extra pounds can actually be protective. A number of longitudinal studies back this up, including the Framingham Massachusetts study, the 90 Plus Study and the National Health and Nutrition Examination Survey II sponsored by the National Center for Health Statistics of the U.S. Centers for Disease Control and Prevention (CDC).
The scientific determination of body heft is a measure called the Body Mass Index (BMI). The BMI, a simple measure of the body weight in kilograms divided by the square of the height in meters, was devised by Adolphe Quetelet in the early 19th century as a statistical measure to compare body weight across populations. Underweight is defined as a BMI under 18.5 and obesity is a BMI over 30. Morbid obesity is a BMI over 40. Several BMI tables and calculators are readily available online.
The relationship between BMI and mortality is essentially a shallow “U” shape (see Adams,KF et. al, reference 3). Those on the extremes of body weight have much higher mortality rates than those in the middle where the curve is almost flat. Morbid obesity (BMI over 40) is a life limiting condition. The epidemic of childhood obesity is of concern because it indicates inactivity and increases the likelihood of developing some chronic diseases, which both shortens the lifespan and decreases quality of life. The higher death rates at the extreme low end of the weight curve are likely explained by serious illness (either diagnosed or undiagnosed) or eating disorders such as anorexia nervosa.
The middle ranges of the curve reveal some interesting nuances. Numerous studies from around the world consistently show that thin individuals have higher mortality rates than moderately obese individuals even when the studies control for underlying illness, smoking and other factors. In addition, those who are slightly (10-15%) overweight are less likely to have osteoporosis, which contributes to the risk of hip fractures. Risk for lung cancer also may be reduced for this group. CDC researchers concluded in 2005 that body weight may not be a risk factor for coronary artery disease except at the highest weights. These data are consistent with a number of large epidemiological studies that have not found a connection between obesity and heart disease.
In addition, the act of losing weight can actually reduce your longevity. People who lose about ten percent of their body weight have an increase in their mortality rate anywhere from 40-65 percent compared to those whose weight remains constant. This increase is due primarily to heart disease and means that in a very real sense dieting may be hazardous to your health. As a geriatrician I worry when one of my patients starts to lose weight and I make every effort to try to reverse that trend while looking for an illness that is not otherwise manifest.
The major exceptions to my “don’t worry about dieting” mantra are those over on the steep side of the “U” with a BMI over 35, who may benefit from modest weight loss. I also recommend weight loss for those with significant obesity who also have diabetes, hypertension or painful osteoarthritis of the hips or knees. In these conditions losing five to ten percent of body weight often can significantly reduce the number of medications needed to control the disease and, in the case of osteoarthritis, can improve function and reduce pain.
It seems paradoxical that we appear to be having an obesity epidemic in the midst of a period of historically unprecedented longevity. This raises the interesting question, “Who determines what an ideal body weight is?” Is it based on science or social desires? After all, the lusty full-figured bodies of women painted by Peter Paul Rubens, the Flemish baroque painter of the 17th century, seem to border on morbid obesity by modern standards. Why should we base our self-image on today’s thin, lean, muscular, tanned stereotypes of Madison Avenue and Hollywood glamour models? As far as I am concerned, as long as you stay away from the arms of the “U” you are fine.
Myrskyla, M. & Chang, V. W. (2009). Weight change, initial BMI, and mortality among middle- and older-aged adults. Epidemiology, 20, 840-848
Ingram, D. D. & Mussolino, E. E. (2009). Weight loss from maximum body weight and mortality: The Third National Health and Nutrition Examination Survey Linked Mortality File. International Journal of Obesity, 34, 1044-1050
Adams KF, Schatzkin A, Harris TB, Kipnis V, Mouw T, Ballard-Barbash R, Hollenbeck A, Leitzmann MF. Overweight, obesity and mortality in a large prospective cohort of persons 50 to 71 years old. N Engl J Med 2006; 355:763-778.