From Science Daily
If you take regular exercise that causes your heartbeat to go up, then the rest of the day your heart rate is lower. That's logical: a trained heart doesn't have to work so hard when resting. An advantage of a well-functioning heart is that it probably protects against cancer, going by a study published in PloS One by researchers at the University Paris Descartes.
Scores of studies have shown that a slow heartbeat is a sign of cardiovascular health, and that people with a lower heart rate are less likely to die from cardiovascular disease. This is a known fact. That a lower heart rate protects against cancer is not.
That we've never heard anything about this is because scientists are not so sure what to do with phenomena that they can't explain. If they're confident, they publish these findings, but they often just ignore these troublesome facts. The Americans did publish, but explained the correlation away. They came up with the theory that people who have cancer but don't know it have a higher heartbeat. [Am J Epidemiol. 1981 Oct;114(4):477-87.]
The explanation didn't entirely hold water. In a follow-up study many years later, the same group of Americans dug up the relationship. [Am J Epidemiol. 1999 May 1; 149(9): 853-62.] You'd have expected that all hitherto unknown forms of cancer had been uncovered in the meantime. Indeed, researchers in France [J Clin Epidemiol. 2001 Jul; 54(7): 735-40.] and Israel [Eur Heart J. 2000 Jan; 21(2): 116-24.] had in the meantime also come across the cancer inhibiting effect of a lower heart rate.
The researchers at the University Paris Descartes used data from over six thousand men aged between 42 and 53. The data were gathered during the first Paris Prospective Study, in which Parisian men were followed for twenty-five years.
This study also showed that a lower heart rate extended life expectancy. The researchers divided the men into 4 groups according to their heart rate. Of the group with the lowest resting heart rate, almost 90 percent were still alive after 25 years, as can be seen in the first figure below. The survival rate of the group with the highest heart rate was 70 percent.
The second figure shows that a slow heartbeat during exertion reduces the risk of dying from a cardiovascular cause.
The most interesting figure is the one above. The slower the heartbeat during exertion, the lower the risk of dying from cancer.
The researchers found a similar relationship between resting heart rate and fatal cancer. In the group with the slowest heart rate, the chance of developing cancer was 2.4 times less than in the group with the highest heart rate.
The researchers put forward the theory that people with a higher heart rate have an overactive sympathetic nervous system. The sympathetic nervous system is the part of the central nervous system that makes organs work harder. [Another part of the CNS, the parasympathetic nervous system, slows down organs' metabolisme.] "Subjects with pre-existing disturbance of their autonomic system may have a lower immunity defense system and an increased risk of dying if the individual develops cancer", the researchers write.
If the theory holds water, then people can probably reduce their chance of developing cancer by becoming fitter.
PLoS One. 2011;6(8):e21310.