Exercise correlates with all sorts of better measures of health, but there is some debate and conflicting evidence on whether more is better past the point of moderate regular exercise. This ties in to questions of causation - to what degree are endurance athletes drawn to their activities because they are already more robust than their peers, for example?

Telomeres are the molecular caps on chromosomes. They shorten with each successive cell division and are thus linked to aging. The shortening rate also varies among people. Shorter telomeres have been linked to increased disease risk as well as shortening of lifespan.

Chronic endurance training is at least modestly linked with long lifespan, though there are some controversies about whether it may increase the risk of some heart diseases. In the current study researchers sought to determine if chronic endurance training is associated with telomere length in older aged individuals. To perform the trial they measured the length of telomeres in four groups of individuals: young people and older people who did or did non engage in chronic endurance training. For the endurance training the researchers chose participation in a 58 km cross country ski competition.

They found that indeed the older people who were chronic endurance trainers had significantly longer telomeres than moderately active older controls. There was no difference in telomere length in the younger subjects whether they did endurance training or not. There was also an association in older people between VO2 max and telomere length.

Link: http://extremelongevity.net/2013/01/10/chronic-endurance-training-linked-to-longer-telomeres-in-older-adults/

Source:
http://www.fightaging.org/archives/2013/01/endurance-training-associated-with-longer-telomeres.php