It’s hard to avoid eating extra food at work, from “fun size” Snickers at the front desk to cookies and muffins during meetings and plentiful meals on the road. In this environment — and for anyone carrying a few too many pounds — the headlines couldn’t have been more alluring. “That spare tire might make you live longer” and “Do you want to live longer? Then put on weight” were just two of the dozens of articles gleefully reporting the results of a new study from Canada published online in the journal Obesity.
In the scientific ping-pong match over the health effects of excess weight, this is a brief volley; bigger, better studies showing shorter survival with excess weight still have the advantage. Before you make vending machine raids a daily sport, here’s what you should know.
The study that delighted headline writers and flashed through the blogosphere included just over 11,000 men and women who took part in Canada’s National Population Health Survey back in 1994 and 1995. They answered dozens of questions about their health, habits, employment, stressors, and other issues. Ten years later, researchers determined which of the participants were still alive and which ones had died. They then compared death rates by weight, using five standard body-mass index (BMI) categories: underweight (BMI less than 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), obese (30 to 34.9) and very obese (35 and above). BMI is a measure of weight in relation to height. (You can calculate your own BMI here.)
Using normal weight as a comparison, the researchers found that people who were underweight or very obese were more likely to have died during the 10-year follow-up period, while those who were overweight or obese were less likely to have died. These relationships remained even after the researchers statistically controlled for smoking, a known monkey wrench in these sorts of studies.
But statistical corrections aren’t enough. To get truly meaningful results about the connection between weight and death, you must remove smokers and early deaths from the analysis, says Dr. Walter C. Willett, professor of epidemiology and nutrition at the Harvard School of Public Health. Why? Smokers tend to be lean, and it is their smoking — not their low BMI’s — that is the hazard. Excluding deaths that occur during the first few years of follow-up minimizes the contribution of deaths from cancer, heart failure, and other diseases that often cause weight loss.
Larger, longer studies that eliminated anyone who ever smoked and didn’t include deaths in the first few years of follow-up come to a much different conclusion than the Canadian study. Dr. Willett and colleagues have shown that as weight goes up in men and women, so does the risk of dying early. A huge study of more than 900,000 people published this spring in The Lancet also shows a strong connection between increasing weight and increasing risk of premature death. People with BMI’s between 30 and 35 died two to four years earlier than those with healthy weights.
Although the connection between weight and longevity hasn’t been entirely pinned down, the weight of evidence, pun intended, does not suggest “That spare tire might make you live longer.” If you’re really serious about living longer and healthier, try eating less. An approach called caloric restriction — eating healthy foods and a third fewer calories than normal — may extend the life span by 10% to 20%. Or maybe it will just make it feel longer.
Patrick J. Skerrett tries to keep his BMI in the healthy range while editing the Harvard Heart Letter.