The intensity of physical activity needed to reduce the risk of heart disease depends on individual fitness levels, according to a study in today's rapid access issue of Circulation: Journal of the American Heart Association.
"In older men, there is an inverse relation between relative intensity of physical activity – an individual's perceived level of exertion – and the risk of developing heart disease," says lead author I-Min Lee, M.B.B.S., Sc.D., an associate professor at Harvard School of Public Health and Harvard Medical School and an epidemiologist at Brigham and Women's Hospital in Boston. "The harder one exercises, based on his or her perception, the lower the risk of heart disease, even if the activity does not meet current recommendations for physical activity."
It has long been shown that physical activity is associated with decreased risk of chronic diseases, including heart disease. But the level of intensity required is less clear. To reap health benefits from physical activity, the majority of recommendations prescribe the level of activity in absolute terms, referring to the actual rate of energy expended, which is often expressed in metabolic equivalents, known as METs.
"The findings can be helpful for older persons who may be unable or unwilling to follow current recommendations," Lee says. "With the high prevalence of individuals not meeting current activity recommendations, there is a need to clarify the thresholds of physical activity for specific health benefits."
Lee emphasizes that everyone should be physically active. "Don't worry about whether your exertion level corresponds to current recommendations. If you persist at your own rate and become more physically fit, you should then ratchet up your intensity to continue to receive health benefits."
Vigorous exercise like jogging, for example, requires greater than 6 METs and brisk walking requires 3 or greater. Current guidelines from the Centers for Disease Control and the American College of Sports Medicine recommend expending 3–6 METs, or 3–6 times the resting metabolic rate, the researchers write. This range corresponds to moderate relative intensity for most young to middle-aged adults. The alternate approach is to measure relative intensity, referring to the energy required during exercise based on individual fitness levels.
Lee and her colleagues postulated that using a relative scale to gauge the intensity of activity is more appropriate than the absolute scale. The absolute scale is based on the fitness of healthy, younger adults. But, researchers thought it would be important to consider how relative intensity for a certain activity changes with age and in less physically fit people.
"Brisk walking, at 3–4 mph, would not require much effort for a young, physically fit man, relative to his fitness because he is highly fit," Lee says. "He might perceive this as 'light' exercise. However, for an old, unfit woman, the same level of activity might require almost all of her energy. She might perceive this as 'very vigorous' exercise. Lee also says no published data so far examines the relationship of the relative intensity of physical activity and the risk of developing heart disease.
The researchers used data from men in the Harvard Alumni Health Study, an ongoing study of men who attended Harvard University between 1916-1950. Since 1962, alumni have periodically returned mailed questionnaires on their health habits and health status. For this report, the researchers followed 7,337 men, average age 66, from 1988-1995.
The researchers gathered information on the men's perceived exertion during exercise as well as the frequency and duration of specific activities. Perceived exertion was self reported using the Borg Scale, a valid measure of relative intensity often used in exercise stress testing. In 1988, the survey inquired about cardiovascular risk factors, such as smoking, diet, cholesterol and diabetes. In 1993, the survey queried whether a physician had diagnosed a heart attack or angina and whether the men had undergone bypass surgery or angioplasty. Of those who had undergone one of these procedures, the researchers randomly selected 49 who lived in Massachusetts and contacted their physicians to verify the accuracy of the self-reported data, which was 96 percent accurate. In addition, death certificates were obtained for men who died through 1995 and determined whether they developed heart disease.
During the follow-up period, 551 men developed heart disease. After making adjustments for other health factors, the researchers found that the relative risk of heart disease among men who perceived their exercise exertion as "moderate" was 14 percent lower than those who rated their exercise as "weak" or less intense. Men who perceived their intensity as "somewhat strong" had a 31 percent lower risk, and those ranking their level as "strong" or more intense had a 28 percent lower rate. The researchers write that, "This inverse association extended to men not fulfilling current recommendations, i.e., expending less than 1000 kcal/week in physical activity, or not engaging in activities of greater than or equal to 3 METs."
The researchers emphasize that recommendations for "moderate" intensity exercise need to match the physical fitness of the individual. But, they note that their findings do not negate current activity recommendations. In fact, data from the study supports the recommendations, showing that men who expended more than 1000 calories (kcal) per week or who exercised at vigorous absolute intensity had lower heart disease rates compared to those who did not.