Skip that post-workout smoothie....

... 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

Photo: Don't kid yourself. The message here is not "don't exercise."

Credit: Getty Images

Comments

[Duncan]

Hello, Tami,

I suggest you go back and read the details of the study which makes a clear distinction between taking antioxidant supplements which interfere with the benefits of exercise and eating foods rich in antioxidants which go not interfere.

"The advice does not apply to fruits and vegetables, Dr. Ristow said;
even though they are high in antioxidants, the many other substances
they contain presumably outweigh any negative effect."

[Tami Dennis]

You're absolutely right. By "loaded," I meant those extra antioxidant "boosts" you can order in your drinkable meal, not the basic antioxidants available in fruits and vegetable (which research strongly suggests are better for us than the supplement forms). I wasn't clear -- so thanks for pointing out this crucial distinction.

[darya]

Duncan, I was going to say exactly the same thing. Nice work! If anything this study just shows once again that grinding up foods and isolating their elements doesn't do us any good. I agree with Tami that those giant smoothies are a whole different bucket of bad news.

Also, in reading the study they didn't exactly prove that free radicals were the cause of the insulin sensitivity effect. They had a cute little story about what they think could be happening, but it is all speculation at this point.

[Kevin]

"vitamin C and vitamin E (1,000 milligrams and 400 IU, respectively)"

That's a lot. My Costco Kirkland "Daily Multi" vitamin has only 120 mg of VitC and 60 IU of VitE in each pill.

"researchers at Harvard Medical School and elsewhere studied the vitamins' effect on 39 young men."

Only one of the seven authors (listed 6th) is associated with Harvard. This is a German study performed at the University of Leipzig with collaborators at the University of Jena. Their ClinicalTrials.gov information lists only those two universities as "Sponsors and Collaborators." Harvard is not mentioned.

It's an interesting result. There are problems, though. The numbers of study participants is small. Only 39 participants completed the study, and of these, only in 23 participants was the study performed in a double-blind, placebo controlled manner. The other 16 participants were in a preliminary open-label study, and it looks like they combined the data from both the open-label and double-blind/placebo-controlled studies. It's going to be important for this to be replicated by other researchers to be sure that this isn't just a spurious result.

[Joe Hadfield]

Things are rarely black and white when it comes to the science of optimizing nutrition and lifestyle. So much to balance.

[Dr. Charles Martin]

Great article about interesting research. The suggestion about a larger study is only prudent to confirm whether the observed effect is consistent. Other research has shown that antioxidants can reduce oxidative stress that leads to inflammation which, in turn, triggers higher CRP levels that can add fuel to the fire of any smoldering periodontal disease. And, for people who have diabetes, gum disease poses potential issues for blood sugar control. We write extensively about related issues at http://dentistryfordiabetics.com/blog, especially the links between elevated blood sugar and gum disease that can interfere with diabetes control and significantly increase risk of serious health events such as heart attack, stroke and blindness.

- Charles Martin, DDS
Founder, Dentistry For Diabetics