You already know that exercise is good for you. It's not ground-breaking news that proper exercise has a lowering effect on cholesterol, blood pressure, insulin levels, stress levels, heart disease risk, osteoporosis risk, and of course, weight. Or that it can help maintain vital hormones, increase oxygen utilization, and boost metabolism. You trust that avoiding a sedentary lifestyle will slow down the process of aging. But what if you were to hear that excessive exercise actually has the reverse effect, accelerating visible signs of aging?

As reported in a 2008 Washington Post article, a British health study involving 2,400 twins examined the relationship between their telomere length and their exercise levels over a 10-year period. You can learn more about telomeres here, but essentially they are protective tips on bundles of genes inside cells. When telomeres get too short, their cells can no longer divide, and they thus die off, resulting in wrinkles, weakened muscles, failing organs, etc. The study found that the length of the twins' telomeres was directly correlated to their activity levels. The more exercise they did, the younger their telomeres appeared. Good news for gym rats everywhere.

However, not so fast. Comparing physically active people and couch potatoes on a molecular scale led to a different discovery in South Korea. There, Shin Yun-a, professor of physical education at Seoul National University, released a paper regarding the effects of heavy exercise on the telomeres of middle-aged women. Her study looked at a much smaller sampling: ten women between the ages of 30 and 60 who had not exercised in six months. After doing heavy exercise that required them to use from 60 to 80% of their maximum oxygen uptake, the subjects' telomere length shrank.

The theory behind these results is that heavy exercise caused lipid peroxidation in the women to increase dramatically. In lipid peroxidation, free radicals "steal" electrons from the lipids in cell membranes, ultimately resulting in cell damage. At the same time, strenuous exercise requires excessive oxygen consumption and triggers the deactivation of superoxide dismutase (SOD), an enzyme that expels disproportionate active oxygen from the body by helping neutralize superoxide (free radicals). In other words, inordinate exercise disables a key enzyme that protects cells against aging.

Exercise is a double-edged sword because it both inflicts free radical damage by requiring the body to use more oxygen and, at the same time, it enhances the body's natural defense system, thereby protecting against free radical damage. A fitness study at the University of Florida found that among participants aged 60 to 85 who performed a series of high or low intensity exercises with weights over 6 months, free radical damage increased 13% in the no exercise group and decreased 2% in the low-intensity group. Interestingly, the high-intensity weightlifting group showed a 2% increase in free radical damage. On the other hand, this group also exhibited higher bone mass, aerobic fitness, and muscle strength, as well as lower blood levels of homocysteine, a substance that can increase the risk of heart disease.

Research from the University of Valencia Department of Physiology determined that when exercise is exhaustive, it increases the oxidation of glutathione (a tripeptide) in the blood, thus leading to cellular damage. It also puts an undue strain on many parts of the body, including the skin, which results in wrinkles and sagging. This damage can be prevented, or at least alleviated, by regular uptake of vitamin supplements (A, C, & E). The body needs the right amount of antioxidants to help remove toxins and reduce oxidative damage, which is magnified when worked out to an extreme.

The negative consequences of strenuous exercise might explain why endurance athlete icons like Jim Fixx die in their early fifties. According to a 1982 study published in the New England Journal of Medicine, which compared the number of deaths in three groups differentiated by their degree of physical activity, the highest death rate was found among men who exercised vigorously for the most amount of time per week. Other studies have shown that in men who exercise more than three-quarters of the population (in the 75th percentile), protection against heart disease levels off and even declines slightly.

All of this research runs counter to what we've been taught all along about keeping fit to stay young and healthy. Of course, it shouldn't be interpreted as an excuse to avoid exercise entirely. But perhaps there is a threshold when healthy, moderate exercise spirals toward detrimental, excessive exercise. This limit is unique for each individual, as is the rate of visible aging and physical decline. The best we can do is to respond to our bodies and practice moderation. Even with exercise, you can have too much of a good thing.