Studies show athletic men and women tend to be deficient in vitamin D, protein, and iron, and may require extra antioxidants to counteract the cell-damaging free radicals generated during exercise. Meanwhile, squeezing in workouts on top of career and family life can stress the adrenal glands, leading to fatigue. “Nutritional supplements can go a long way to optimize performance and replenish deficiencies,” says Kelly Parcell, ND, whose Boulder, Colorado, clinic specializes in sports nutrition. In addition to taking a daily multivitamin with iron (which helps deliver oxygen to lungs and muscles), consider these research-backed supplements.

D-ribose

This simple sugar, which drives energy production inside cells’ mitochondria, is a promising alternative to perhaps the best-documented workout enhancer, caffeine. Studies show small amounts of caffeine—a half-cup of coffee for a 160-pound man—taken one hour before a workout can boost muscle power and lower perceived exertion levels. But caffeine can also dehydrate the body, overtax adrenals, and lead to energy crashes—and the sugar in many energy drinks only exaggerates this effect. “D-ribose can help prevent muscle cramping during a workout and boost how long and strong you can go,” says Parcell.

Dose: about 5 grams of D-ribose powder; mix into a 16-ounce water bottle and drink throughout a run or ride.

Quercetin

Another energy booster catching on among athletes—including cyclist Lance Armstrong—is quercetin, an antioxidant found in apples, onions, and berries. In addition to quelling inflammation and mopping up free radicals, “it gives you a very smooth energy lift,” says Rikki Keen, RD, who teaches sports nutrition at University of Alaska Anchorage. Men who took 1,000 mg quercetin daily for two weeks went 3 percent farther during treadmill time trials than those taking a placebo, in a 2010 study.

Dose: 650­–1,000 mg in two to three doses daily, one taken one hour before a workout. Take with vitamin C to enhance absorption.

Branched-chain amino acids (BCAAs)

The “superstars” of amino acids, BCAAs promote tissue repair and help build lean muscle mass after exercise. Research shows they also can decrease sarcopenia, or age-related muscle loss. “For anyone who works out to the point they’re sore, BCAAs are critical for recovery,” says Marie Spano, RD, coeditor of NSCA’s Guide to Sport and Exercise Nutrition (National Strength and Conditioning Association, 2011).

Dose: 25–30 grams whey protein (which includes 5–7 grams BCAAs), add to a beverage and drink within one hour after working out. Or take 1.5–5 grams BCAAs daily in capsule or powder form.

Vitamin D

Mounting research shows active adults with good vitamin D blood levels tend to have better muscle strength, speed, and endurance—and they get sick less, even as workout schedules ramp up. In a recent study, gymnasts with lower D levels couldn’t jump as high.

Dose: 2,000–4,000 IU D3 daily, depending on frequency of sun exposure

Adaptogenic herbs

“Many people are burning the candle at both ends trying to squeeze in a workout,” says Parcell. That can take a toll on the adrenal glands, which produce energy-boosting hormones like adrenaline and cortisol. To prevent fatigue, she recommends an adrenal-support formula containing adaptogenic herbs like rhodiola, ginseng, black licorice, ashwagandha, and cordyceps mushrooms. With the exception of ginseng (which has been shown to boost immunity and fight fatigue), research on such herbs for sports performance is fairly young and inconclusive. In a 2010 study, male cyclists who took rhodiola had less lactic-acid buildup and muscle fatigue after workouts.

Dose: Follow label directions. Take before 3 p.m. to support the adrenal glands when they are most active.