Vitamin C, or ascorbic acid, is a nutrient that can be synthesized by many animals but not by humans. It’s an essential com- ponent of our diets and functions primarily in the formation of connective tissue such as collagen. Vitamin C is also involved in immunity, wound healing, and allergic responses. As an antioxidant, vitamin C keeps free radicals from destroying the outermost layers of cells. When paired with a plant-based iron source, vitamin C enhances the absorption of this hard- to-absorb form of nonheme iron. Adding lemon juice to your spinach can give a better boost to your iron stores.

If you work out regularly or train for ath- letic competition, you know that a cold or respiratory infection can sideline you pretty fast. Fortunately, researchers have found that supplementing with 500 milligrams daily of vitamin C appears to cut the risk of upper respiratory tract infections. This benefit may be due to the antioxidant effect of vitamin C or to its overall immune-boosting capability.

Supplementing with vitamin C will improve your performance but only if you are deficient in this nutrient. Supplementation does not enhance performance if you already eat a healthy, nourishing diet that is high in citrus fruits (which are high in vitamin C) and other fruits and vegetables. In fact, a review of 12 studies showed that in 4 of those studies, daily doses of 1 gram (1000 mg) of vitamin C may lead to decrements in performance. The conclusion of the authors was that consuming vitamin C at levels that are naturally found in foods every day (250 mg) is best for supporting training effects and athletic performance.

My recommendation is that you should first and foremost make plant foods, which are often naturally high in vitamin C, the mainstay of your diet, ensuring a vitamin C–rich diet. During cold and flu season you might experiment with supplementation if you don’t consume at least 500 milligrams of vitamin C in food alone. If you choose to supplement, stay below 1 gram (1000 mg) daily in food and supplements.