The workout vitamin?

New research suggests low levels of vitamin D may be keeping you from seeing results at the gym.

Blame it on sunscreen or dairy-free diets, but vitamin D deficiencies are becoming increasingly common. Since it’s main job is to help the body absorb calcium, bone growth and maintenance are the vitamin’s biggest responsibilities, but now, with about 36 percent of adults proving deficient, scientists are researching what else it might effect. The latest: muscle strength.

Though preliminary, the study, published in Medicine & Science in Sports & Exercise, found that participants with healthy vitamin D levels were stronger (more visibly in the arms than the legs) than those who were lacking. Researchers concluded that supplementation may be an effective way to preserve—and maybe even build—muscle strength.

The study was geared toward older adults, but Dr. Paul Spector, an Equinox Tier 4 coach and educator, says there's no reason why the same wouldn't be true for someone who simply wants to see results in the gym. "The influx of calcium into the muscle cells plays a part in muscle contraction, which is what builds strength, so this is not surprising," Spector says. "I've even seen some research that vitamin D regulates insulin sensitivity, which means it also affects metabolism, and thereby weight loss and gain."

So why not pop a pill? While Spector says supplementation is usually safe, overdoing it with D can lead to calcium deposits, kidney stones, and even liver damage. But with so many of us proving deficient, it's worth a quick blood test to see if you need the boost.