Just like everything in life, if you obtain too much of something that is typically good for you, an excess of that will cause the reverse effect. The simplest analogy is water. Without water, you would not last more than a few days. But if you drank a few gallons in an hour, you would rapidly die, as your heart could not sustain its electrical activity due to sodium imbalances. This actually happens to a surprisingly large number of endurance athletes who simply drink too much during a race.

Well, just like water, you can overdo it with exercise. I have been running for nearly 40 years and have long admired Grete Waitz for her amazing athletic abilities. We are both about the same age, so I admired her when I was also competing, although nowhere near her level.

Exercise is absolutely necessary for high-level wellness, but if you exercise too much it is clearly harmful and cancer seems to be a possible adverse outcome. My guess is that one's antioxidant threshold is exceeded or micronutrients are consumed at a level that cannot be easily replaced. Either way, they both add up to a major problem.

Most of us will not have to struggle with exercising too much but for those who are tempted to overdo it, please don't. I stopped racing in 1990 when I did the Chicago Marathon. Although, this past weekend I did jump into a local neighborhood 5K race just for the heck of it and won my age group (actually beat the number 2 finisher by over eight minutes and also beat the number 1 person in the younger age group). I hadn't raced in over 15 years, and now I remember why I stopped. It just hurts too darn much.

I don't think I'll be racing for at least another 15 years.

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