Following up on my last post about moderate exercise being more health enhancing than heavy exercise,I came across this excellent article on slowtwitch.com by Mark Sisson. This article is well worth reading. He makes the point the ultraendurance athletes who do long distance running, cycling, swimming, etc. are actually harming their chance at a long healthful life. He outlines the steps that the committed endurance athletes can do to minimize the ill-effects from their sport, but warns that it is hard to counter the ill-effects of too much exercise.
Moderate exercise is best, although I don't always follow my own advice. Too much exercise works against longevity and a youthful appearance. The best lifestyle strategy is to pick activities that you love doing and do it moderately. A sensible approach to exercise combined with an excellent eating style, like the Fuhrman nutritarian approach is a great way to maintain a healthy weight and stay healthy over the long haul.
One of the many downsides of "over-exercising" is that the endurance athletes consumes too many unhealthy carbohydrates -- sports bars, gels, simple sugars, and other processed foods. So, not only does too much exercise harm our immune systems, this damage is exacerbated by an unhealthy diet. Sisson explains this vicious circle in his excellent article.