WASHINGTON (Reuters) - Adults should aim to get in 2 1/2 hours of exercise a week and children should run and play for at least an hour a day, according to new exercise guidelines issued by the U.S. government on Tuesday.

The guidelines, meant to be akin to the "Food Pyramid" nutritional advice, are based on studies that show clear health benefits from regular, moderate exercise.

"More than 59 percent of adults don't get enough physical activity and a quarter of adults aren't active at all in their leisure time," Health and Human Services Secretary Michael Leavitt said in a telephone interview.

"I think these are the first comprehensive federal guidelines on physical activity."

Many studies have shown that regular moderate exercise lowers the risk of heart disease, many cancers, osteoporosis, diabetes, Alzheimer's disease, depression and other diseases.

"In children and adolescents, physical activity induces cardiovascular and respiratory fitness and also contributes to bone health," Leavitt said.

The latest figures released in July from the U.S. Centers for Disease Control and Prevention show 25.6 percent of U.S. adults are medically obese. Both poor diet and a lack of exercise are blamed.

In 2002, the U.S. Institute of Medicine said Americans need to exercise at least one full hour a day to maintain weight and health.

The new HHS guidelines go a bit easier and recommend an average of half an hour a day, five days a week -- or more.

People should get exercise any way they can, said Melissa Johnson, executive director of the President's Council on Physical Fitness and Sports. "Brisk walking is a phenomenal example," she said.

Children need 60 minutes a day of physical activity, said Rear Adm. Penelope Royall, deputy assistant secretary for health. "We want to encourage muscle-strengthening activity as well," Royall said in the telephone interview

"Just imagine a kid playing on the playground and climbing on the monkey bars, or playing tug-of-war," Royall said.

"We don't want the kids in the gym lifting weights."

The guidelines are available on the Internet at www.health.gov/paguidelines.