Correspondence 800 www.thelancet.com Vol 379 March 3, 2012 benefits of physical activity reported in this paper. We declare that we have no conflicts of interest. Debra Efroymson, *Peter Berti, Kristie Daniel pberti@healthbridge.ca HealthBridge, Dhaka, Bangladesh (DE); and HealthBridge, Ottawa, ON K1N 7B7, Canada (PB, KD) 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: 1244–53. 2 Besser LM, Dannenberg AL. Walking to public transit: steps to help meet physical activity recommendations. Am J Preventive Med 2005; 29: 273–80. 3 Ewing R, Bartholomew K, Winkelman S, Walters J, Chen D. Growing cooler—the evidence on urban development and climate change. Washington, DC: Urban Land Institute, 2008. 4 Dumbaugh E. Designing communities to enhance the safety and mobility of older adults: a universal approach. J Planning Lit 2008; 23: 17–36. 5 Lake A, Townshead T. Obesogenic environments: exploring the built and food environments. Persp Public Health 2006; 126: 262–67. as prophet. In any event, neither episode occurred in the context of a randomised controlled trial. We contend, therefore, that the risk of mortality for everyone—prophets included—is 1·0 (1·0–1·0). We declare that we have no conflicts of interest. *Ashley M Croft, Joanne V Palmer ashley.croft810@mod.uk Headquarters Surgeon General, Lichfield WS14 9PY, UK 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: 1244–53. have affected the extrapolation made by Veerman and colleagues. James O’Keefe and colleagues raise an interesting question as to whether excessive strenuous exercise can become deleterious. We showed graphically that the benefits of mortality reduction peaked at 50 min with a hazard ratio of 0·60, without showing that it continues beyond 70 min. By 120 min, the hazard ratio for all-cause mortality was around 0·55, with even better hazard ratios for cardiovascular diseases (although less than 0·3% did daily vigorous exercise at this level). The adverse effects of strenuous exercise for incremental efforts for more than an hour a day did not seem to outweigh the benefits. We were not able to identify an upper limit of physical activity, either moderate or vigorous, above which more harm than good will occur in terms of long- term life expectancy benefits—an observation similarly made by the 2008 Physical Activity Guidelines for Americans. 3 We hope, however, that concerns about too much vigorous activity are more academic than practical, with adverse effects more short-term than long-term. Most of the general population is sedentary 3 and our major challenge is how to make the inactive start exercising, and not to give them reasons to avoid exercise. Debra Efroymson and colleagues argue that, of the four domains of physical activity, transportation- related physical activity is the most appropriate for population-wide interventions. We wholeheartedly support walking or cycling to get to daily work destinations, if possible. For a small minority, such activity is achievable. However, when one stretches a three-block walk to a 13-block walk, transportation takes on the nature of leisure-time physical activity (LTPA), because it becomes a voluntary, optional effort, not a daily requirement. Altering the infra- structure of the transportation system is easier said than done. 4 The tipping point for governments is the exercise Chi Pang Wen and colleagues 1 claim that exercising for 15 min per day results in a 14% reduced risk of all-cause mortality (0·86, 95% CI 0·81–0·91). Further, they claim that every additional 15 min of daily exercise beyond the minimum daily amount of 15 min reduces all- cause mortality by an additional 4% (2·5–7·0). This cannot be true, however, since the risk of mortality is an absolute. The risk can be postponed, but it cannot be reduced, as Wen and colleagues claim, and nor can it be eliminated. In other words you can run faster, but you arrive later. We are aware of just two reported exceptions to this otherwise invariable rule and they are Elijah, who while still alive went up by a whirlwind into heaven (Kings 2:11), and Enoch, who was translated that he should not see death (Genesis 5:24). The details of Enoch’s final end are veiled in mystery but Elijah’s heavenward passage in a fiery chariot was witnessed by his servant Elisha, who picked up Elijah’s mantle as it fell to the ground and thereafter assumed his master’s role Authors’ reply We thank Lennert Veerman and colleagues for giving us the oppor- tunity to clarify the relation between summary mortality ratios and extended life expectancy. There are several reasons why these are not totally comparable. Veerman and colleagues applied our adjusted mortality ratios to estimate the differences in life expectancy, whereas in fact they should have used the unadjusted ratios. As a result, they have underestimated the true dif- ferences by about two-fold. Women were more affected by risk factors than men, and the adjustment process seems to have produced a result quite different from the actual data on which the life expectancy calculation was based. Second, in estimating life-lengthen- ing effects from a summary mortality ratio, we need to assume that age- specific mortality ratios are constant across all age groups. 1,2 This was not the case: the age-specific ratios for the age groups 40–44 years, 60–64 years, and 80–84 years were 0·77, 0·78, and 0·82 for men, and 0·99, 0·80, and 0·65 for women, respectively. A third factor to consider is whether the study cohort is comparable with the general population. Our cohort had distinct advantages in life expectancy over the general population, owing mainly to the self-selection factor of those with above-average health. This, too, would