Exercising for fun may lower the risk of high blood pressure, but heavy lifting on the job does not offer the same benefit, according to a new review of the evidence.
Researchers looking at studies that followed nearly 137,000 people found that recreational exercise for more than four hours a week was linked to a 19 percent lower risk of developing high blood pressure compared to doing little or no leisure-time exercise.
But people with similar levels of physical activity in the course of work had about the same risk as those in less strenuous jobs.
"It makes sense that occupational physical activity is not associated with reduced risk of hypertension," said Dr. Martha Daviglus, a professor of medicine at the University of Illinois in Chicago and a preventive medicine specialist.
People experience more stress at work, and the kinds of physical activity individuals do in the work environment is different, said Daviglus, who was not involved in the study. "When you exercise, you have to be completely relaxed."
Faced with predictions that the total number of adults in the world with high blood pressure will reach 1.56 billion by 2025, researchers are taking a serious look at factors that can lower the risk.
Dr. Wei Ma from Shandong University in Jinan, China, and his colleagues combined the results of 13 studies that followed 136,846 healthy individuals for at least two years and up to 45 years to see what types of physical activity were linked with a person's risk of developing high blood pressure.
The studies included in the review were conducted in North America, Europe and Asia, and 15,607 of the individuals followed eventually developed high blood pressure.
In addition to the heaviest weekly exercisers having a 19 percent lower risk, those who exercised for between one and three hours a week had an 11 percent lower risk than those who did less than an hour's worth of exercise.
People in jobs such as industrial, farm or forestry work with comparable levels of lifting, walking, climbing and other activities had a slightly lower risk of high blood pressure than those in sedentary jobs. But the difference was so small it could have been due to chance, Ma's team reports in the journal Hypertension.
"We found that occupational physical activity could not decrease the risk of hypertension in the present study," he said in an email.
The researchers also looked at the effects of physical activity in the course of commuting to work on foot or by bicycle. Only two studies included data on that kind of exercise, and one found a large benefit, the other found nearly none.
Ma's team speculates that the nature of the activities at work, which tend to consist of "heavy lifting, prolonged standing and highly repetitive work," may explain the lack of apparent health benefit.
Recreational exercise often works large muscle groups in a more varied way that increases the metabolism and cardiac output of the whole body, they write. In addition, individuals who exercise during leisure time can rest when they are tired.
"It's been known for a long time that exercise can help prevent the development of hypertension," said Dr. David Jacobs, an epidemiologist at the School of Public Health at the University of Minnesota.
"This study is good in that it has nailed the relationship down a little better and formalized it in a large body of information. Their data shows that a wide variety of studies produced the same answers," he said.
Jacobs agreed that the kinds of physical exercise individuals perform in a work environment do not actually involve a variety of movement.
People who do leisure time exercise are also different in other ways from people who don't, he pointed out, and a study like this cannot precisely identify the difference.
Although there is strong evidence that physical activity is an important factor to help prevent a range of chronic diseases like hypertension, the missing piece is what kind of activity and the level of physical activity needed, Daviglus said.
"There is no prescription of one kind of exercise for everybody," she said. Rather, the recommendation is to incorporate physical activity into your life.