Two in three Americans are clinically obese or overweight. Now a new study has identified a key reason one: About 36 percent of U.S. adults do not engage in any type of exercise or leisure-time activity.

That’s the conclusion of a new analysis released by Healthy People 2020, a federal program designed to improve the nation’s public health.

The study, published online in the American Journal of Medicine, also found even patients who have had a heart attack and who undergo cardiac rehabilitation aren’t motivated to up their activity levels. The researchers estimated that fewer than 15 percent or heart patients actually participate in cardiac rehabilitation following discharge.

The researchers noted many studies have found that regular physical activity reduces the risks of heart attacks and strokes, developing diabetes, hypertension, and certain types of cancer. Exercise also enhances mental health and promotes healthy muscles, bones, and joints.

Current federal health guidelines recommend getting at least 150 minutes of moderately vigorous exercise each week — a minimum of 20-25 minutes every day or 30 minutes most days of the week.

"Lack of physical activity accounts for 22 percent of coronary heart disease, 22 percent of colon cancer, 18 percent of osteoporotic fractures, 12 percent of diabetes and hypertension, and 5 percent of breast cancer," said Charles H. Hennekens, M.D., a professor at Florida Atlantic University’s College of Medicine who co-authored the new report.

"Furthermore, physical inactivity accounts for about 2.4 percent of U.S. healthcare expenditures or approximately $24 billion a year."

The researchers noted simply taking a brisk walk every day for only 20 minutes can cut the risk of a heart attack by up to 40 percent.

"There's a lot more that we can do to address this national epidemic among people of all ages," said Steven Lewis, a visiting FAU professor. "For example, clinicians should screen and refer obese patients to programs that offer intensive counseling for weight control and physical activity. This simple, straightforward and easily achievable objective may be the first necessary step to lower rates of obesity and physical inactivity in the U.S. today."

Too often, Americans reach for pills to treat their ills, rather than turning to lifestyle changes that can boost their health, Hennekens said.

"In general, any pharmacologic intervention should be an adjunct, not alternative, to therapeutic lifestyle changes such as increasing levels of physical activity,” he added. “Based on the current totality of evidence, when compared with most pharmacologic therapies, exercise is more readily available at a low cost and relatively free of adverse effect."