We would like to thank Dr Fox for his comments. The studies mentioned are population cohort studies looking at a wide range of individuals with varying activity and fitness levels. The Taiwan study[1] attempted to define the minimal amount of exercise required and looked at all comers in a standard medical screening program. They did demonstrate that higher levels of moderate or vigorous activity conferred no additional health benefits and, thus, more of a reverse J-shaped curve than a U-shaped curve. Given the scope of this review, which focused on excessive endurance exercise, we have focused on those at the extreme end of these mortality curves. The other related articles in the April and May issues of the BCMJ may provide more insight into the specific benefits of exercise, since it is clear that moderate exercise is beneficial. The specific studies mentioned are all observational studies with inherent limitations. There are other similar studies not included in the scope of the review that demonstrate similar U-shaped curves or reverse J-shaped curves, but there appears to be a consistent signal that further benefit and potential harm may lie at the extreme end of exercise. To our knowledge, there are no randomized studies that directly compare differences in exercise modality on cardiovascular morbidity or mortality. Overall, our take-home message is that we know moderate and even high levels of exercise appear to show benefit, but the upper limit at which adverse cardiac effects occur is not known.
—Andrea K.Y. Lee, MD
—Andrew D. Krahn, MD

References

1.    Wen CP, Wai JP, Tsai MK, et al. Minimum amount of physical activity for reduced mortality and extended life expectancy: A prospective cohort study. Lancet 2011;378(9798):1244-1253.