Two recent studies add to the growing list of reasons it pays to be physically active. And both reinforce the notion that you don’t have to do all that much to reap the benefits of regular exercise.

For a study published Tuesday in the journal PLOS Medicine, researchers at Brigham and Women’s Hospital in Boston, working in collaboration with the National Cancer Institute, set out to quantify the relationship between physical activity and the risk of premature mortality.

Analyzing data for more than 650,000 people, pooled from six existing studies, and tracking them for an average of 10 years (during which time more than 82,000 deaths were recorded), they found that even a little bit of activity seemed to help people live longer. Compared to no physical activity, just 75 minutes of brisk walking per week was associated with an extra 1.8 years of life expectancy after age 40. Bumping that up to 150 minutes a week – the amount currently recommended by the World Health Organization – was associated with 3.4 years of added longevity; walking briskly for 450 minutes a week or more added up to an extra 4.5 years of life. The relationship between weekly physical activity time and longevity began leveling off at about 300 minutes, the study notes.

Those findings applied equally to men and women, blacks and whites – and to people who were of normal weight, overweight or obese. Overall, people of normal weight got the biggest boost from being active, adding 7.2 years of life compared to the life expectancy for those who were inactive and very obese. And people who were of normal weight but not active lived 3.1 fewer years than those who were moderately obese and were active.

The authors note that their study only established associations, not cause and effect relationships, between physical activity and longevity. Still, they write, “This finding may help convince currently inactive persons that a modest physical activity program is ‘worth it’ for health benefits, even if it may not result in weight control.”

Physical activity has also long been known to contribute to mental health. But how much is the optimal amount for that purpose? Reporting in the November issue of Preventive Medicine, researchers at Teachers College, Columbia University looked at self-reported data for 7,674 participants in the U.S. Health Information National Trends 2007 Survey to help answer that question.

They found that those with the best mental health were those who exercised between 2.5 hours and 7.5 hours per week (150 minutes to 450 minutes; the optimal number varied a bit according to age, gender and physical health), as compared to those who exercised less – or more.  In a striking finding, the researchers determined that after the 7.5-hour-per-week mark was reached, symptoms of depression and anxiety started kicking in. 

Does that last bit surprise you? It did me. As one who typically gets way more than 7.5 hours of exercise a week – and who has more than her share of anxiety – I wonder whether I might be overdoing it.