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.