It’s taking researchers a while to untangle all the ways body mass, lifestyle choices, diet, and health outcomes interact. It’s a really complicated subject and one without any clear answers. But now we have a new hint suggesting that the trajectory of someone’s body shape may be an important factor to consider — not just whether they are, at a given moment, above a “healthy” weight.
In a study published earlier this week in the British Medical Journal, a team led by Mingyang Song, a research fellow at Harvard’s T.H. Chan School of Public Health, took a big data set consisting of tens of thousands of health workers who were asked about their health and lifestyle every two years. Participants were consistently asked about their weight and BMI, allowing researchers to trace these numbers over the course of the slice of the participants’ lifespans captured in the study.
The researchers determined that there were five basic trajectories that captured how the participants’ bodies changed — or didn’t — between ages 5 and 50:
lean-stable — those who were skinny early in life and stayed skinny over the years
lean-moderate increase — those who started skinny but gained some weight
lean-marked increase — those who started skinny but gained a lot of weight
medium-stable/increase — those who started with a medium body shape and either retained that shape or got heavier
heavy-stable/increase — those who started with a heavy body shape and either retained that shape or got heavier
The researchers then correlated these trajectories with various health outcomes, including mortality. They were particularly interested in examining these relationships among nonsmokers; since smoking is very bad for you, but can also prevent you from gaining weight, it can confound the relationship between body mass (or, in this case, body-mass trajectory) and health seen in nonsmokers.
So among those who had never smoked, holding a bunch of other stuff constant: among women, those in the lean-moderate increase group were eight percent more likely to have died during the study period than those in the lean-stable group; those in the lean-marked increase group were 43 percent more likely to have died; those in the medium-stable/increase group were just as likely to have died; and those in the heavy-stable/increase group were 64 percent more likely to have died. For men, on the other hand, those in the lean-moderate increase and medium-stable/increase were just as likely to have died during the study period as those in the lean-stable group; those in the lean-marked increase group were 11 percent more likely to have died; and those in the heavy-stable/increase group were 19 percent more likely to have died. (Among smokers, these correlations indeed tended to be weaker.)
Overall, then, the results “found that heavy body shape from age 5 up to 50, especially the increase in middle life, was associated with higher mortality. In contrast, people who maintained a stably lean body shape had the lower mortality.” Now, people don’t really have much control over the body type they are born with, so a key insight here is that dietary and lifestyle decisions you make in midlife appear to have important health consequences. All the more reason to watch what you eat and get up from the couch, even if your leanest days — assuming you ever had lean days — are behind you.