It’s not news that obesity has become a worldwide public health crisis; the condition increases risk for heart disease, diabetes, and premature death. But, while scientists have long suspected that adults with high scores on the body mass index (BMI) die earlier than most, little work has been done to determine if there's an association between how much a person weighs in their lifetime and time of death. So two recent studies aimed to do just that, and they both arrived at a similar conclusion: being lean all your life can help you live longer.

For the first study, a group of researchers from the U.S. analyzed data from thousands of men and women enrolled in the Nurses’ Health Study. Participants recalled their body shape at ages 5, 10, 20, 30, and so on until age 60, after which researchers kept tabs on them around 15-16 years until death. Participants also filled out detailed questionnaires on medical information, diet, and lifestyle. The team found that those who remained stably lean throughout life had the lowest mortality rate — the 15-year risk of death was 11.8 percent in women and 20.3 percent in men. Participants who reported being heavy in childhood and who remained so through middle age had the highest mortality, with a 15-year risk of death of 19.7 percent for women and 24.1 percent for men.

"Our findings provide further scientific rationale for recommendations of weight management, especially avoidance of weight gain in middle life, for long-term health benefit," the authors wrote.

The second study was an international effort to determine if there was an optimal BMI for lower risk of death. The researchers believed a higher BMI would be associated with higher mortality risk, but the largest study previously conducted on the matter didn't necessarily find this to be true; only high levels of obesity were linked to increased risk of death. To clarify this particular finding, the current team analyzed 230 prospective studies involving 3.74 million deaths and more than 30.3 million participants. Results showed those who had a BMI in the range of 20 to 22 had the lowest mortality rate. This did not include smokers, a measure researchers took to rule out the negative effects.

The authors wrote that their results reinforced “concerns about the adverse effects of excess weight.”

In an accompanying editorial, Sarah Wild and Christopher Byrne, from the Universities of Edinburch and Southampton, said that the studies are addressing important questions about obesity and mortality. They emphasized the need to remain at a healthy weight, but acknowledged that there are "major challenges in finding effective ways to prevent weight gain, support weight loss, and prevent weight re-gain, in both individuals and populations."

Source: Song M, Hu F, Wu K, Must A, Chan A, Willet W, et al. Trajectory of Body Shape in Early and Middle Life and All Cause and Cause Specific Mortality: Results from Two Prospective US Cohort Studies. The BMJ. 2016.