It's a surprising, subversive and very, very popular idea.
Over the past few years, several studies – including a 2009 analysis of Statistics Canada data – have suggested that being a bit chubby as you get older, far from being a health risk, may actually help you live longer. The extra weight, the thinking goes, could help cushion you from the inevitable slings, arrows and hip fractures of old age.
But the newly published results of a three-decade-long study of clean-living Seventh-Day Adventists in California suggest that you might want to go easy on those early-bird specials after all. When confounding factors such as skinny smokers were removed, the effects of extra weight were clear – and bad – even for those older than 75.
"The message from this cohort is that elderly adults who maintain a lower BMI [body mass index]by following a healthy lifestyle pattern – lower meat consumption, higher consumption of plant foods, higher physical activity levels – will live longer," says Pramil Singh, an epidemiologist at Loma Linda University and lead author of the new study, which was published in the Journal of the American Geriatrics Society.
Most studies investigating the long-term effects of being overweight take a simple approach: Weigh a whole bunch of people, then wait for years or decades to see which ones die earlier. The problem with these studies, Dr. Singh says, is that they only take a single snapshot of their subjects' weight at the beginning of the study – even though people who are relatively thin in that initial measurement might have already lost weight due to as-yet-undiagnosed illnesses.
The new study is unique in several respects. First, because Seventh-Day Adventists generally don't smoke or drink and are encouraged not to eat meat, the cohort was remarkably healthy. From an initial pool of 8,386, the researchers were able to exclude all current or past smokers and anyone with a prior history of heart disease or cancer and still have 6,030 subjects left.
Second, the subjects were weighed twice, in 1960 and 1976, then monitored until 1988. The two weight measurements meant that "weight-stable" subjects – those who neither gained nor lost more than five kilograms between the two dates – could be analyzed separately to rule out the effects of disease-related weight loss or gain.
The results paint a dramatically different picture compared with earlier studies: Older men with a BMI greater than 22.3 died 3.7 years earlier on average, and older women with a BMI above 27.3 died 2.1 years earlier. A BMI (calculated by dividing your weight in kilograms by the square of your height in metres) of greater than 25 is considered overweight, while greater than 30 is considered obese.
The reason for the difference between men and women isn't clear, but Dr. Singh speculates that older women may benefit from having a bit more fat because that's where estrogen is produced after menopause. Higher estrogen levels can help reduce the risk of hip fracture and other ailments.
The new results don't disprove the notion that it's possible to be "metabolically healthy" and obese. The recently introduced Edmonton Obesity Staging System suggests that people who are overweight, but have only sub-clinical signs of problems such as high blood pressure and elevated fasting insulin levels, have no greater risk of dying than people of normal weight. For these people, some researchers argue, trying to lose weight can do more harm than good because it can trigger a cycle of yo-yo weight loss and gain.
Using BMI to assess whether subjects are overweight also has significant limitations. People with muscular builds, for example, may have a high BMI despite low body fat. And fat in certain locations, such as the abdominal cavity, is far more likely to lead to health problems than fat on the hips or under the skin. BMI is unable to distinguish between them. Diet and lifestyle changes over the past 30 years have altered typical fat distributions, according to a forthcoming Queen's University study: At any given BMI level, Canadians now seem to have more abdominal fat and less muscle than in 1981.
In the end, large-scale epidemiological research can only deal with averages and probabilities, not individual differences. What the Loma Linda study tells us is that, contrary to the seductive promise of earlier studies, avoiding weight gain tilts the odds in your favour no matter how old you get.
Alex Hutchinson blogs about research on exercise at sweatscience.com. His new book – Which Comes First, Cardio or Weights? – is now available.