(Roman Sigaev/Alamy)

Milk is good for your bones. Except when it’s not. Taking Vitamin C may help protect against colds. Except when it doesn’t.

Keeping up with the latest headlines on nutrition research is enough to give the average person whiplash. With each new study comes new understanding about how what we eat affects our bodies, but often the results can create confusion about how to modify our own diets.

The latest wrinkle has to do with red wine.

Resveratrol, found in the skin of red grapes, has long been revered as a magical compound in red wine that is believed to have numerous health effects such as lowering blood pressure and helping prevent heart attacks. There has been so much enthusiasm for the substance in recent years that some companies have been selling resveratrol supplements in health stores as a way to enhance the effects of exercise.

But a new study, published in the journal Applied Physiology, Nutrition and Metabolism, raises questions about that theory. Researchers subjected two groups of patients to high-intensity interval training for four weeks, with one group taking resveratrol while the other was given a placebo. They found that those who took the supplement did not see as many benefits from the physical activity as those who had a placebo.

While the study was small — there were 16 participants — researcher Brendon Gurd, a professor in the School of Kinesiology and Health Studies at Queens University in Canada, noted in a news release that it's clear that more research needs to be done on this phenomenon. He said his team found that resveratrol somehow appears to “inhibit the body’s normal training response.”

“RSV supplementation doesn’t augment training,” he said, “but may impair the effect it has on the body.”