Skip that post-workout smoothie....

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... if it’s loaded with antioxidants, that is. New research suggests that such vitamins can actually make exercise less beneficial than it otherwise would be, at least in terms of insulin sensitivity and thus the risk of diabetes.

In a study published in Proceedings of the National Academy of Sciences, researchers at Harvard Medical School and elsewhere studied the vitamins’ effect on 39 young men. Half of the participants regularly got more than six hours of exercise per week and were assessed as pretty darn fit; the other half normally got less than two hours of exercise a week, which included the not-too-strenuous demands of daily life.

During the study, some of the men got extra daily doses of vitamin C and vitamin E (1,000 milligrams and 400 IU, respectively). Some of them didn’t. All were put through a four-week regimen of exercise.

And those workouts did increase insulin sensitivity -- as such exercise is supposed to do -- but only among the men not taking the vitamins.

The key here is free radicals -- much-maligned molecules that we’re all happy to fight, regardless of whether we understand their function. Antioxidants, of course, are free-radical fighters -- noble chemicals we’re all happy to purchase and consume, regardless of whether we understand their impact.

In this case, free radicals seem to enhance the body’s sensitivity to insulin. And antioxidants get in the way of that. Even in nutrition, few things are black and white.

Here’s an explainer from the Nutrition Blog on about.com:

‘The increased levels of free-radicals stimulates your body to take certain steps to protect itself -- like increasing insulin sensitivity. Taking those antioxidant vitamins may wipe out enough free-radicals so that threshold isn’t reached. It doesn’t mean the exercise isn’t beneficial for other reasons, but at least in this study, the lack of free radicals appear to have reduced some of the benefit of exercise.’

The Big Money posits that this is further evidence we should avoid vitamin supplements.

And FuturePundit offers some additional context and the conclusion that, really, we just need an all-benefits-of-exercise-with-no-downside pill.

As for the post-workout smoothie, seriously, have you considered the calories in those things? Here’s a look, courtesy of ABC News.

-- Tami Dennis