In Shakespeare's "As You Like It," Rosalind asks, "Can one desire too much of a good thing?"

That's just what investigators wanted to know when they spent 11 weeks monitoring 54 young men and women in an endurance training program. They gave half the participants 1,000 mg vitamin C and 235 mg vitamin E daily. The other half received sugar pills (placebos).

In the end, guess who had signs of greater energy and muscle strength (more mitochondrial proteins, vital for building new muscle and fueling cells)? Only the folks who took the placebo!

Turns out a daily dose of vitamin C equivalent to eating as many as 28 oranges and taking 15 times the dietary reference intake of E may be counterproductive.

So if you're working out, we suggest you look at your multivitamin: Make sure it doesn't give you doses much above 100 percent of most nutrients' recommended intake. (You're dividing your multi in half to take once in the morning and once in the evening, aren't you?)

And you can ask your doctor for a blood test to assess your nutrient levels. Then you can see how much of which vitamins or minerals you may need to take. In the meantime, you can get powerful benefits from food-based nutrients by eating nine servings of fruits and vegetables a day, 100 percent whole grains, lean poultry and fatty fish like salmon and ocean trout. One daily supplement we recommend to protect the heart and brain: 900 mg of DHA omega-3 fatty acid.

© 2014 Michael Roizen, M.D. and Mehmet Oz, M.D.
Distributed by King Features Syndicate, Inc.