We have always known that exercise is good for our health, but did you know that exercise also wards off diabetes by increasing your insulin sensitivity?
According to researchers, exercise helps increase the body's sensitivity to insulin by making reactive oxygen species, or "free radicals," which antioxidants work against. These free radicals are thought to damage cells and speed the ageing process, but they are also used by the body to prevent cell damage after exercising.
"When you exercise you do improve your insulin sensitivity, and if you are at risk for diabetes improving insulin sensitivity is good," said Dr C Ronald Kahn, the Mary K Iacocca Professor at the Joslin Diabetes Centre and Harvard Medical School.
Part of the reason that exercise improves insulin sensitivity is that it causes oxidative stress on the muscles. The muscles respond to this stress by creating free radicals, Kahn said.
Antioxidants may be counterproductive
The beneficial effects of exercise on insulin sensitivity may, however, be blocked by taking antioxidants like vitamins C and E. These vitamins block the creation of free radicals that promote insulin sensitivity.
"If you take antioxidants like vitamins C and E, you block the oxidative stress response, but you also block the beneficial effects of exercise on insulin sensitivity," said Kahn in an online report of the Proceedings of the National Academy of Sciences.
For the study, Kahn's team looked at the benefit of exercise in increasing insulin resistance in 39 young men, roughly half of whom were taking supplemental vitamins C and E. Kahn's group found that men taking vitamin supplements had no change in their insulin resistance, but men not taking vitamins had an increase in free radicals, which increases insulin resistance. A month after stopping the vitamin supplements insulin sensitivity was restored, the researchers noted.
"If you are exercising, in part, to reduce diabetes risk, you shouldn't take vitamin C and E, because you are going to block some of the beneficial effect of the exercise to prevent the diabetes," Kahn said.
“Substantial uncertainty” on use of antioxidants
This study raises doubts about the benefits of taking antioxidant supplements, but not about the value of these vitamins in the foods people eat, according to Dr David L Katz, director of the Prevention Research Centre at Yale University School of Medicine.
"We have long held out hope that antioxidant supplements, among them vitamin C, vitamin E, beta carotene, and more recently lycopene and others, would help prevent diseases from the common cold to cancer, heart disease to diabetes," Katz said. "But to date, virtually all of the best research evidence is contrary to this hope."
This study has a counter-intuitive conclusion, namely that antioxidant supplements may actually interfere with the beneficial effects of exercise on insulin sensitivity, Katz said.
"This is a small and short-term study, and thus cannot tell us definitively that antioxidant supplements are harmful in diabetes or the insulin-resistant state that often precedes it. But that is precisely what the study suggests may be true," Katz said.
For now, there is substantial uncertainty about any health benefits and the potential harms of antioxidants as supplements, Katz said. "But we have no such confusion about the powerful health-promoting effects of wholesome, mostly plant-based diets and regular physical activity." (Steven Reinberg/HealthDay News, May 2009)