Exercising at a moderate intensity for 300 minutes a week is associated with a 20% reduction in CHD risk. Halving this amount of physical activity reduced CHD risk by 14% and a significant risk reduction was also achieved with lower levels of exercise. A meta-analysis was carried out on 33 prospective cohort studies looking at physical activity and primary prevention of CHD, published since 1995. Nine of the studies allowed quantitative estimates of leisure-time physical activity.The researchers found that people who reported the equivalent of 150 minutes per week of moderate-intensity activity had a 14% lower CHD risk, relative risk (RR)=0.86 (95% CI: 0.77-0.96) compared with those reporting no exercise. Those who undertook 300 minutes per week of moderate-intensity activity had a 20% lower risk, RR=0.80 (95% CI: 0.74-0.88). Increasing exercise above 300 minutes per week only produced a modest increase in benefit.