And for those with high heart risk by reason of diabetes, high blood pressure or smoking, the reduction in risk is 45 per cent.
The findings from the US suggest that efforts to get middle-aged people to exercise should pay special attention to those who have risk factors for cardiovascular disease or a history of heart attack or stroke.
It's the first prospective, nationally representative study to show that cardiovascular risk doesn't lessen exercise's impact on mortality risk. But it did confirm that those who have a high heart risk are much more likely to be sedentary, perhaps out of fear that exercising could overtax them.
'Other studies in smaller or less representative groups have shown the long-term benefits of exercise, even light exercise, but this study allowed us to look across different population groups, and different levels of cardiovascular risk, and see who got the most 'punch' out of exercise,' says lead author Caroline Richardson.
'We found that across all ranges of cardiovascular risk, everybody got a benefit from regular activity, but the biggest absolute benefit, the biggest reduction in deaths, was among high risk people,' she adds.
'If we exclude high risk folks from studies or discourage them from being active, then we have lost the chance to make a big impact,' says Richardson. 'We need to prevent people from becoming part of that high risk group, by encouraging exercise as a way to lower their weight, blood sugar and blood pressure. But at the same time we need to address those at high risk now.'
Of the 9,611 people studied, 15.2 per cent led sedentary lives, engaging in light activity such as walking less than once a month. Housework was not included as an activity. Another 13 per cent were classed as regularly active, taking part in moderate to vigorous exercise activities such as aerobics or bicycling at least three times a week. The rest were grouped as occasionally active.
The researchers also placed participants in one of three cardiovascular disease risk groups based on five characteristics: smoking, diabetes, high blood pressure, a history of coronary artery disease including heart attack and angina, and a history of stroke. Individually, each of these factors is known to increase the risk of death, but the risk is higher for those with more than one risk factor.
For the study, any participant with two or more risk factors was classed as having high cardiovascular risk - almost 22 per cent of participants met this description. Those with one risk factor were considered moderate-risk, and those with no risk factors were considered low-risk.
Although it's impossible to tell which came first, the heart risk or the lack of exercise, Richardson notes they go hand in hand - especially among people with existing cardiovascular illnesses who cling to outdated beliefs that they shouldn't exercise because it might set off a heart attack. In fact, she says, the benefit of physical activity for most high cardiovascular disease-risk individuals probably outweighs the risks.
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