There is something that appears as a contradiction to a newbie like me.
Sport is widely recognized as healthy and recommended by medical experts.
Metabolic rate is inversely correlated with lifespan.
To me, it makes sense that the more energy passes through your system, the more it gets damaged.
Sport goes against this desirable frugality, as it makes you consume more energy, which requires increasing your energy input to compensate (you eat more food). Also, growing a greater muscular mass increases the amount of energy needed for moving, for maintaining body temperature at rest; having more cells mean larger amounts of oxygen are needed, more DNA-copying errors are made, more free radicals are produced, etc.
At the same time, I've heard that sport is good because it lowers heartbeat at rest, it makes special immune-system agents enter cells more effectively, and other things...
I'm obviously missing something, but I'm just having a hard time understanding how something that increases the entropy of your system can be beneficial for your health in the long run.