Physical inactivity is a major risk factor for the development of coronary heart disease (CHD). But while scientists agree that exercise reduces the risk of developing CHD, it is unclear how long you should exercise to reap the preventive benefits. Are two 30-minute workout sessions each day just as good for your heart as one hour of continuous exercise? Can walking 1 mile be compared to running 1 mile?

A study in the August 29 issue of Circulation: Journal of the American Heart Association, examined the association between physical activity of different durations and the development of heart disease. The study reveals that if the total energy expended is the same for the different durations of exercise, then there was a similar effect on heart disease risk.

The most recent recommendations from the Surgeon General advise people of all ages to include a minimum of 30 minutes of physical activity of moderate intensity (such as brisk walking) on most, if not all, days of the week. The recommendation allows that the 30 minutes of exercise need not be done all at once, but rather may be accumulated in shorter episodes. Data from the study supports this recommendation indicating that the accumulation of shorter sessions of physical activity has equal benefit with regard to CHD risk as longer sessions, if the total amount of energy spent on the exercise is the same.

Researchers from Harvard’s School of Public Health and Medical School, and Stanford University School of Medicine collected data on subjects who were enrolled in the Harvard Alumni Health Study. The participants were all men (average age 66 years) who had no history of CHD or cancer at the start of the study.

In 1988, a total of 7,307 participants were sent questionnaires asking them to estimate the frequency and duration of their physical activities each week, and to provide information on their health habits and medical histories. In 1993, a second questionnaire was sent to the subjects and included questions about whether a physician had ever diagnosed chest pain or heart attack, and if the patient had undergone any medical procedures related to heart disease, such as cardiac bypass surgery.

The investigators estimated the total calories burned by physical activity for each subject. For example, walking one block daily was considered to burn 56 calories, whereas climbing up and down one flight of stairs used 14 calories.