Being Fat May Not Be All Bad — if You're 70

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Putting a smile on chubby faces everywhere, a new study by Australian researchers finds that being overweight may be a boon for the elderly: among the 9,240 adults ages 70 to 75 in the study, those who were overweight were the least likely to die over 10 years, compared with people who were of normal weight or obese.

The study, published Thursday in the Journal of the American Geriatrics Society, looked at data from two long-term population-based studies and found that adults who were overweight had an average 13% lower risk of death from any cause over 10 years, compared with those who were of normal weight. Those who were underweight were 76% more likely to die, while the obese had the same mortality risk as those of normal weight. Researchers also found that being sedentary increased the risk of death in men by 28%; in women, the risk was doubled.

The new report adds to the ongoing scientific debate over how to define ideal weight in adults and whether the widely used measure of weight categories — body mass index (BMI), a measure of body fat based on a ratio of weight and height — is equally useful for all age groups. The World Health Organization defines normal weight as a BMI of 18.5 to 24.9 kg/m2; overweight is defined as a BMI of 25 to 29.9 kg/m2. A BMI of 30 or higher is considered obese, and under 18.5 underweight.

In 2007 a study by U.S. researchers at the Centers for Disease Control and Prevention and the National Cancer Institute found that overweight adults had a slightly lower risk of death than their normal-weight peers, largely because they were less likely to die from a variety of diseases, including Alzheimer's, infections and lung disease. Another study in 2005, published in Obesity, analyzed data on more than 11,000 Canadian adults for over 12 years and found that people who were overweight were 17% less likely to die than those of normal weight. Underweight adults, by contrast, had a 73% higher risk of death.

The current study looked only at major causes of mortality, which the researchers grouped together as cardiovascular disease, cancer, chronic respiratory disease or other. In all weight categories, the leading causes of death were cardiovascular disease and cancer. The lowest risk of death from either cause occurred in overweight adults.

Why exactly excess weight would protect older adults is unclear, but the study's lead author, Leon Flicker, a professor of geriatric medicine at the University of Western Australia, offers a theory. "We can only hypothesize, but it may be that as we age, the presence of nutritional and metabolic reserves [that is, fat] are advantageous. If you develop an illness, a little more reserve gives you a greater chance to recover from that illness," he says.

The study has a few limitations, however. First, it used height and weight measurements that were reported by participants — which can be imperfect. (People tend to overestimate height and underestimate weight, which skews their BMI.) It also collected height and weight information only once, at the start of the study; researchers could not have known, for instance, whether people might have unintentionally lost weight before the study or during the follow-up as a result of underlying disease. Furthermore, the study's participants had a lower overall mortality rate than the general population, suggesting they were healthier to start with. Many overweight and obese people die at younger ages, and participants in Flicker's study necessarily had to survive until 70 in order to be included.

Preventive health experts also point out that there is a difference between survival and quality of life. Being overweight is a major risk factor for many health problems, including Type 2 diabetes, hypertension, stroke, coronary disease, cancer and loss of physical function. "These are strong enough reasons to strive for a healthy weight and avoidance of obesity," says Dr. JoAnn Manson, a Harvard Medical School professor and chief of preventive medicine at Brigham and Women's Hospital. "Moreover, given all of the adverse effects of obesity on health, it isn't biologically plausible that overweight would lower mortality risks."

Where researchers may agree is that BMI is an unreliable marker for body fat, or health, in the elderly. As people age, they lose muscle mass and bone density, which leads to weight loss and a declining BMI, despite an increase in body fat. Manson suggests measuring waist circumference instead, which is a more accurate gauge of abdominal obesity and tends to predict a higher risk of death in all age groups.

For the authors' part, they acknowledge that being overweight is not a condition to which we should aspire. "The evidence is overwhelming that in younger age groups being overweight increases your mortality risk and lots of other nonfatal conditions," says Flicker. But he adds that if you are among the population's successful agers, you are probably doing something right: "Having reached the age of 70 years, and you are overweight — not obese — there is no reason why you should lose weight, unless you have a condition that is associated with being overweight, such as diabetes mellitus or severe osteoarthritis."