Background: Vitamin C (ascorbate) is likely to be essential for skeletal muscle structure and function via its role as an enzyme cofactor for collagen and carnitine biosynthesis. Vitamin C may also protect these metabolically active cells from oxidative stress.
Objective: We investigated the bioavailability of vitamin C to human skeletal muscle in relation to dietary intake and plasma concentrations and compared this relation with ascorbate uptake by leukocytes.
Design: Thirty-six nonsmoking men were randomly assigned to receive 6 wk of 0.5 or 2 kiwifruit/d, an outstanding dietary source of vitamin C. Fasting blood samples were drawn weekly, and 24-h urine and leukocyte samples were collected before intervention, after intervention, and after washout. Needle biopsies of skeletal muscle (vastus lateralis) were carried out before and after intervention.
Results: Baseline vastus lateralis ascorbate concentrations were ∼16 nmol/g tissue. After intervention with 0.5 or 2 kiwifruit/d, these concentrations increased ∼3.5-fold to 53 and 61 nmol/g, respectively. There was no significant difference between the responses of the 2 groups. Mononuclear cell and neutrophil ascorbate concentrations increased only ∼1.5- and ∼2-fold, respectively. Muscle ascorbate concentrations were highly correlated (P < 0.001) with dietary intake (R = 0.61) and plasma concentrations (R = 0.75) in the range from 5 to 80 μmol/L. Conclusions: Human skeletal muscle is highly responsive to vitamin C intake and plasma concentrations and exhibits a greater relative uptake of ascorbate than leukocytes. Thus, muscle appears to comprise a relatively labile pool of ascorbate and is likely to be prone to ascorbate depletion with inadequate dietary intake.
Note that muscle had as much ascorbate on both interventions, so it seems that a relatively small amount of vitamin C will replete muscle.
Someone on Twitter yesterday mentioned the various reports that vitamin C inhibits or even abolishes the physiological response to exercise training, and that the fact that muscle is avid for vitamin C doesn’t mean that athletes should take it. I agree, but would point out that the studies on antioxidants and exercise training are far from definitive, with one side (groups associated with Ristow and Gomez-Cabrera) saying that antioxidants such as C abolish training benefits such as mitochondrial biogenesis, and the other (Holloszy and others) saying that they can find no such effect.
One line of reasoning beyond the evidence was interesting, this from Holloszy, is that athletes including elite ones have been taking antioxidants, including gram amounts of vitamin C, for decades and that such a profound effect as Ristow found would have been noticed; i.e. someone training for the Olympics who took vitamin C would lose. It seems very unlikely that thousands of athletes and coaches would have failed to notice this. Holloszy also criticized Ristow’s methodology.
Here’s Ristow’s paper that got the ball rolling: Antioxidants prevent health-promoting effects of physical exercise in humans. (If you want to delve deeper into the controversy, a good way to find related studies is to go to Google Scholar (http://scholar.google.com/), type in say Ristow’s paper, then click on either “cited by” or “related articles”.)
Bottom line: antioxidants probably provide neither benefit or harm to exercise training, but it’s best to be cautious, and if you do heavy exercise, don’t take vitamin C (or E) in large amounts. I don’t. But also note that the amount of vitamin C in the first study was ~200 mg (2 kiwi fruit) a day. Not that much, but still greater than the RDA.