Exercise and heart disease: (Too) much does not help a lot

Numerous studies have shown that physical activity reduces a person’s risk of developing cardiovascular disease. Physical exercise is also recommended for patients who have been diagnosed with heart disease to prevent a worsening of their condition. In the United States, for example, clinical specialist associations recommend from 30 to 60 minutes of moderately intense aerobic activity on five or, even better, seven days per week. These recommendations are based on outcomes from clinical trials that were conducted to examine the effectiveness of cardiac rehabilitation exercise programs, which are supervised and usually have a limited duration of a few months.

Dr. Ute Mons and Professor Hermann Brenner at the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ) aimed to find out whether these recommendations are also valid under patients’ normal living conditions. "We wanted to verify whether there is really a linear association between the frequency of physical activity and a prognosis," Mons explains. "In addition, most prior studies have not taken into account whether or not patients adhere to the prevention recommendations over a period of several years."

To answer these questions, the epidemiologists conducted a 10-year survey involving more than 1,000 patients who had participated in a clinical rehabilitation program for coronary heart disease (myocardial infarction or acute coronary syndrome). Study participants were asked to complete questionnaires on their physical activity at intervals of 1, 3, 6, 8 and 10 years after their hospital stay. The study documented all new cardiovascular complications (infarction, stroke) that occurred over these periods of time.

In their evaluation of the data, the researchers noticed that most patients had not maintained an initial level of activity as the years passed and they grew older. The proportion of participants who exercised daily or at least 5-to-6 times a week continuously declined, while the proportion of those who rarely exercised rose.

The DKFZ epidemiologists defined a reference group of patients who reported that they exercise twice to four times a week, representing the average level of activity. As expected, the risk was highest in patients who rarely or never exercised (this group experienced four times more deaths related to cardiovascular incidents than in the reference group), followed by those who exercised only one to four times a month.

However, the number of severe cardiovascular events among patients who exercised every day also was about twice that of the reference group. This non-linear association (which could be mapped on a reversed "J"-shaped curve) was particularly obvious when the scientists looked only at cases of infarctions and strokes with fatal outcome.

The researchers can only provide speculations about the causes of the rise in the risk for patients in the group that was physically most active. Since the study did not document the exact circumstances of death, they could not determine whether the elevated risk in this group was associated with sudden cardiac death during or immediately after physical exercise.

"Our analysis may overestimate the risk for patients in the physically most inactive group," Brenner states, "because people who are already sick and in a weak condition will not exercise. There is no doubt that moderate physical activity, if possible in a patient’s case, has health benefits. However, our results also suggest that there is a limit beyond which more exercise does not provide any benefits. This should be taken into account in clinical recommendations for the secondary prevention of cardiovascular disease."

Ute Mons, Harry Hahmann and Hermann Brenner: A reverse J-shaped association of leisure-time physical activity with prognosis in patients with stable coronary heart disease: evidence from a large cohort with repeated measurements. Heart 2014, DOI: 10.1136/heartjnl-2013-305242