The arrival of middle age doesn't necessarily have to bring extra pounds, a new study shows.

Over the course of the 20-year study, about 10 percent of participants — those who performed high levels of physical activity — nearly maintained their weight through their young adult years and middle age, researchers said.

By starting young, people can reduce their risk of putting on pounds during middle age "when the highest risk of weight gain occurs," the researchers wrote.

The findings are the first to show the long-term benefits of steady physical activity on curbing weight gain later in life, the researchers said.

The research is published today (Dec. 14) in the Journal of the American Medical Association (JAMA).

Waists and weights

The researchers found that highly active people gained the least amount of weight over the twenty-year study. Men who were least active were almost 7 pounds (3 kilograms) heavier at the end of the study, on average, than those who were most active.

In women, the effects of remaining physically active were even more pronounced, with about a 13-pound (6-kg) difference in weight gain between the most and least active groups.

In a separate comparison of body mass index (BMI) between the most active (in this case, defined as performing about 150 minutes of exercise weekly) and least active subjects, Hankinson found that BMI increased by 0.15 in highly active men, while the BMI of less active men increased by 0.20 over the course of the study. The BMI of less-active women increased by twice as much as that of highly active women.

Previous research had shown that may not always be the case. A study led by I-Min Lee, of Brigham and Women's Hospital in Boston, also published in JAMA this year, showed a close relationship between BMI and the impact of exercise on weight gain . In a sample of more than 34,000 women, Lee and his team found that overweight women did not benefit from high levels of activity as much as those with a BMI in the normal range.

"We were happy to see that baseline BMI had no effect on the benefits of activity for preventing future weight gain," said Dr. Arlene Hankinson, an instructor at Northwestern University, who led the new study. "This is great news."

Hankinson and her colleagues looked at changes in BMI and waist circumference in 3,554 men and women who were between the ages of 18 and 30 at the study's start. The researchers obtained the data from an ongoing study including participants living in Birmingham, Ala., Chicago, Minneapolis, and Oakland, Calif.

Hankinson took initial measurements in 1985 and again at six time points over a 20-year period. At the study's start, participants’ general fitness level was also tested in a treadmill exercise. Three times during the study, researchers interviewed participants about their diet. At all six time points, the researchers gave the participants a questionnaire to evaluate the types of activity (such as sports, exercise and housework) they engaged in, as well as the amount of activity per week. Each person's height, weight and waist size were also recorded.

Although all participants gained some weight over the course of the study, the researchers found that highly active individuals gained the least.

How much is enough?

Interestingly, Hankinson found that the middle tier of study participants, those who engaged in moderate physical activity, did not reap more benefits than those with who reported the lowest levels of activity. The researchers found the amount of weight gained was similar in both of these groups regardless of sex, education level, race and other lifestyle factors, such as alcohol use.

Exactly how much exercise is enough is difficult to pinpoint, the researchers said. The new study indicates that at the very least, 30 minutes of activity a day — the current recommended amount by the Department of Health and Human Services — is a beneficial amount.

Hankinson also pointed out that all of her study subjects, regardless of activity level, gained some weight over the study. Societal influences and a natural, age-related decline in metabolic rate may make it impossible for a middle-aged individual to wear exactly the same jeans' size she wore several decades earlier.

Still, some people might come pretty close.

"The most important thing is to find activities that you like, that you can do, and that you can stick to over time," Hankinson told MyHealthNewsDaily. "Maintaining activity is really the key."

Pass it on: Staying active may minimize the weight gain that comes with age.