THURSDAY, May 5, 2016 (HealthDay News) -- Keeping trim throughout your life could help you live longer, while being obese might do the opposite, two new studies show.
In the first study, U.S. scientists found that slim people had the lowest risk of dying over a 15-year period -- 12 percent for women and 20 percent for men. Meanwhile, obese men and women had the highest risk -- 20 percent for women and 24 percent for men.
"People who maintain the leanest body shape have the lowest risk of dying prematurely," said lead researcher Dr. Mingyang Song, a research fellow in the departments of nutrition and epidemiology at Harvard T.H. Chan School of Public Health, in Boston.
Weight management should start early and last throughout life, Song said. "By the time you reach middle age, it's difficult to lose weight," he said. "Obesity should be prevented by managing weight in childhood and adolescence."
Preventing obesity may also prevent its consequences, which include diabetes and heart disease, Song said, though the study did not prove a cause-and-effect link between weight and life span.
For the study, Song and colleagues collected U.S. data on more than 80,000 women who took part in the Nurses' Health Study and more than 36,000 men who were enrolled in the Health Professionals Follow-up Study.
Participants were asked to recall their body shape at ages 5, 10, 20, 30 and 40. They also provided their weight at 50. They were followed from age 60 over an average of 15 to 16 years. In addition, participants completed questionnaires on lifestyle and health every two years and on diet every four years.
In a second study in the same journal, international researchers found that gaining weight over time was linked with higher risks of premature death.
In their analysis of 230 previously published studies that included more than 30 million people and nearly 4 million deaths, they found that among people who never smoked, the leanest lived the longest.
"A BMI [body mass index] in the range of 20 to 23 may be optimal for reducing premature mortality in adulthood," said lead researcher Dagfinn Aune, from the department of epidemiology and biostatistics at the School of Public Health at Imperial College London, in England.
BMI is a measure of weight that takes into account the ratio between height and weight. A BMI of 18.5 to 24.9 is considered normal, while a BMI of 25 to 29.9 is considered overweight. A BMI of 30 or higher is considered obese.
"The findings demonstrate the importance of smoking and health conditions in influencing the association between BMI and mortality, and reinforce concerns about the adverse effects of excess weight," Aune said.
The report was published May 5 in the BMJ.
"Weight management throughout life is important, particularly during middle age," said Dr. Sarah Wild, a professor of epidemiology with the Centre for Population Health Sciences at the University of Edinburgh, in Scotland.
"We need to continue to try and support healthy environments, as well as expecting individuals to resist the temptations of the obesogenic [promoting weight gain] environment," added Wild, who wrote an editorial that accompanied the two studies.
One expert agreed.
"There has also been some argument that fitness matters, while fatness matters less," said Dr. David Katz, director of the Yale University Prevention Research Center, in New Haven, Conn.
However, these new studies show that, "being lean, and staying that way, confers the health benefits long suspected," he said.
Visit the U.S. Centers for Disease Control and Prevention for more on obesity.