A new study published on Monday surprisingly found that consuming antioxidants like vitamins C and E could actually reverse the beneficial effects of exercise.

Physical exercise has a range of health benefits, including on the body’s resistance to insulin. On the other hand, exercise also leads to the formation of free radicals, a form of very active oxygen molecules which can chemically attack the body.

To combat this, people have long turned to antioxidant supplements which are thought to improve health and longevity. However, the new study suggests that antioxidants like vitamin C and E supplements may actually be harmful, at least in regards to diabetes risk and glucose metabolism.

According to this study, performed by researchers in Germany and Boston, these vitamins short-circuit the body’s natural response to free radicals. Dr. Michael Ristow, lead-author of the study says that “short-term doses of free radicals may act like a vaccine, helping the body to defend itself from chronic stressors more efficiently by inducing a long-term adaptive response”.

While the study is by no means conclusive, experts agree that it was well designed and should give people pause before popping antioxidant supplements if they exercise.

If you exercise regularly, you may want to think twice about taking antioxidants like vitamins C and E.