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Impact on health

However, beyond a certain point, negative consequences might occur. Older male runners (45-55) who run more than 40 miles (64 kilometers) per week face reduced testosterone levels, although they are still in the normal range. Running a marathon lowers testosterone levels by 50% in men and more than doubles cortisol levels for 24 hours. Low testosterone is thought to be a physiological adaptation to the sport, as excess muscle caused may be shed through lower testosterone, yielding a more efficient runner. Veteran, lifelong endurance athletes have been found to have more heart scarring than control groups, but replication studies and larger studies should be done to firmly establish the link, which may or may not be causal. Some studies find that running more than 20 miles (32 kilometers) per week yields no lower risk for all-cause mortality than non-runners, although these studies are in conflict with large studies that show longer lifespans for any increase in exercise volume.