Early this year the Journal of the American Medical Association published a meta-analysis with a rather controversial finding: overweight people live longer than normal weight individuals. But before we use this study as an excuse to pack on a few extra pounds (or just keep the pounds on), we need to dig into the science a bit more.
For the meta-analysis, Dr. Flegal and Dr. Kit, along with other researchers, searched the PubMed database for studies examining the relationship between body mass index (BMI), which is a measure of the relation of body weight and height, and lifespan. The researchers then combined 97 eligible studies to examine the relationship between BMI and longevity for over 2.8 million study participants. Researchers felt that such a large study was bound to find an association between weight status and longevity if there really was one.
After controlling for age, sex, and smoking, the researchers concluded that overweight people (defined by having a BMI between 25 and 29.9) had a 6% decreased risk of death from all causes compared to people of a normal weight status (BMI 18.5-24.9). Obese people in general (BMI over 30) had an 18% increased risk of death. However, when examined more closely, the risk for all-cause death among mod
erately obese people (BMI 30-34.9) did not significantly differ from those with normal weight status. In contrast, obese people with a BMI over 35 had a 29% increased risk of death. Thus, from this study, it appears that some, but not too much, extra weight is associated with longer lives.
Should we be surprised by these findings? No. The phenomenon that overweight people live longer has been well documented in other meta-analyses, and it even has its own name: “the obesity paradox.” Some potential explanations for the paradox and the benefits of extra fat include that it supplies energy during wasting-associated illnesses and provides extra cushioning for traumatic injuries.
However, just because being overweight is associated with living longer does not mean being overweight causes one to live longer. According to the CDC, being overweight increases one’s risk of heart disease, type 2 diabetes, many cancers, stroke, liver disease, sleep apnea, arthritis, impotence, and infertility. But, if being overweight increases the risk of all of these diseases, how is it that studies associate being overweight with longer life spans? One explanation is that these studies do not exclude data from people with pre-existing diseases that have ca
used them to lose weight. For example, alcoholism, anxiety, depression, autoimmune diseases, drug addiction, digestive disorders, and cancer can result in both unintentional weight loss and shorter life spans. While the researchers in this meta-analysis controlled for age, sex, and smoking, they did not control for any other diseases. Thus, if a significant portion of people in the normal weight category had these diseases, it could help explain why normal weight status is associated with shorter life spans. Indeed, the prevalence of these diseases is rather high in the United States:
- Alcoholism: 12.5%
- Anxiety disorders: 18%
- Depression: 8%
- Autoimmune disorders: 8%
- Irritable Bowel Syndrome: 10%
Additionally, as with any study using the body mass index, it is important to recognize that BMI is not the best measure of body fat since it is merely a measure of the relationship between body weight and height. Tissues other than fat can increase BMI, and muscular individuals (such as trained athletes) may technically have high BMIs and considered to be in the “overweight” category even though they have low
levels of body fat. For example, an active basketball player with 10% body fat can have a higher BMI than someone who leads a sedentary lifest
yle with 30% body fat. Additionally, the distribution of the fat is even more important, as the fat that accumulates around one’s middle is much more dangerous than fat deposited in the thighs.
Over 40% of adult men and 30% of adult women are overweight in the United States. Even more disturbing, over 36% of Amer
icans are considered obese. Thus, we need researchers like Dr. Flegel to study the consequences of extra
weight; however, we must keep in mind that meta-analyses like the one published merely measure association rather than causation. We must take care to not extrapolate the findings too far. Perhaps
being overweight may not be as harmful as some claim, but I would not use this study as an excuse to overindulge.
Natalie Obermeyer is a first year student in the Nutrition Communication and Masters of Public Health programs. When she is not studying, reading, or writing, she loves to run, hike, ski, play outdoors in the sunshine, and experiment in the kitchen.