Vitamin C is an important water soluble antioxidant, and it is for this role that is is best known. However, vitamin C also functions as a cofactor to metalloenzymes in a number of reduction reactions involving the formation of neurotransmitters, collagen, carnitine and peptide hormones. The requirement for vitamin C in the synthesis of collagen explains the vitamin C deficiency disease scurvy, which is caused by a deterioration in the ability of the body to manufacture collagen. Such a scenario results in reduced collagen formation in blood vessels, which ultimately causes bleeding and death. Most animals can synthesise vitamin C, but humans do not possess high enough concentrations of the required enzymes L-gulono-gamma-lactone, the terminal enzyme in the biosynthesis pathway. As a result humans are reliant on dietary sources of vitamin C to maintain adequate body pools, and these pools reflect dietary intake. A number of tissues are particularly reliant on adequate intake because of their high requirement of vitamin C.
Although it is the cells of the immune system such as leukocytes that are considered to have the highest requirement for vitamin C, particularly during illness, the requirements of skeletal muscle may outstrip even those of white blood cells. This is because of the large generation of free radicals in skeletal muscle as a consequence of the oxidation of fuels, coupled to the large requirement for the synthesis of collagen to create extensive connective tissue. The ability of dietary vitamin C to raise skeletal muscle concentrations of vitamin C have been investigated in healthy non-smoking men1. The men consumed either 0.5 or 2 kiwi fruit per day for 6 weeks and their skeletal muscle vitamin C levels were measured. At baseline the mean vitamin C concentration in the vastus lateralis muscle was 16 nmol per gram of tissue, but after 6 weeks of kiwi fruit treatment the levels rose to 53 and 61 nmol per gram in the 0.5 and 2 kiwi fruits per day groups, respectively. However, the difference between these two groups was not significantly different.
Therefore increasing the vitamin C concentrations of the diet can increase the skeletal vitamin C concentration around 3.5 fold. This is in contrast to the 1.5 to 2 fold increase in vitamin C that was reported in the same study for the leukocytes. The skeletal muscle concentrations of vitamin C were highly significantly correlated with the plasma concentrations in the 5 to 80 μmol/L range, and highly significantly correlated to vitamin C intake. This suggests that skeletal muscle is highly responsive to dietary vitamin C intake and that low intakes of the vitamin may negatively affect the structure and function of skeletal muscle, due to the risk of ascorbate depletion at inadequate intakes. This study also provides more evidence of the low micronutrient intakes of Western populations, as at baseline the plasma levels of the subjects were borderline deficient (22.7 μmol/L) and the skeletal muscle concentrations were also low. Therefore a free living Western diet does not provide enough vitamin C to saturate either plasma or skeletal muscle with vitamin C.
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