Vitamin C is a popular dietary supplement and often taken in doses above the recommended 60 mg per day. Vitamin C intake is controversial because evidence in the nutritional literature supports an argument for higher doses, particularly in athletes and those under stress. Vitamin C is an important antioxidant in humans and interacts with both vitamin E in the plasma membranes of cells and with glutathione in the cytosol. This makes an optimal vitamin C intake pivotal in maintaining the antioxidant defences required to protect cells from oxidative damage. During times of stress, such as illness or intense physical training, ascorbic acid levels can fall significantly in plasma. This results from an increased uptake of vitamin C to white blood cells such as leukocytes and neutrophils, through activation of inflammatory pathways of the immune system. Inadequate vitamin C intake may therefore lower immunity and promote inflammation under times of stress and lead to delayed healing or an increased risk of infection.
A higher vitamin C Intake may therefore have health benefits. However, the dose of vitamin C required to cause optimal beneficial effects is controversial because differing methodologies in studies make comparisons difficult. Even a very high intake of vitamin C is safe (many grams per day), and so many increase intakes prophylactically, particularly athletes. This is generally good advice, because plasma and white blood cell levels tend to rise significantly as vitamin C intake increases above the recommended levels. For example in one study1, researchers investigated the effects of vitamin C intake on healthy subjects. An initial baseline diet containing 60 mg of vitamin C was consumed for 2 weeks, followed by a 4 week vitamin C depletion diet of <5 mg per day. The researchers then increased vitamin C intake to 600 mg per day for 3 weeks, before another 4 week depletion phase of <5 mg per day. Finally the subjects were returned to a diet containing the recommended vitamin C intake (60 mg per day).
The initial 60 mg per day intake of vitamin C did not produce plasma levels significantly different from the baseline measurement (0.70 versus 0.69 mg/dL, respectively). This suggests that the subjects likely had roughly 60 mg of vitamin C in their self-selected diets. Following the first depletion phase, plasma levels of vitamin C fell to 0.13 mg/dL, but rose to 1.33 mg/dL in the 600 mg per day vitamin C intake period. During the second depletion phase, plasma levels again fell below baseline levels to 0.22 mg/dL, but rose once again to 0.88 mg/dL as vitamin C intake was increased to 60 mg per day. Therefore plasma levels of vitamin C reflect the vitamin C intake and high (600mg per day) doses can increase plasma vitamin C levels to a greater extent than recommended intakes. The leukocyte concentrations of vitamin C reflected the vitamin C intake of the subjects, such that during the first depletion phase leukocyte vitamin C concentrations fell 44.6 % and in the second depletion phase they fell 77.1 %.
This study is also interesting because loss of plasma vitamin C was more rapid following the higher 600 mg per day dose, compared to the lower 60 mg per day dose. This suggest that a high vitamin C intake can upregulate vitamin C turnover, as has been reported elsewhere. Rapid withdrawal of a high vitamin C intake may therefore cause plasma and leukocyte levels to drop and the turnover rate may require time to adjust to a new lower level. Another study2 investigating the optimal vitamin C intake suggested that the recommended intake was too low and should be increased to 200 mg per day. This study reported a sigmoidal increase in vitamin C between 60 and 200 mg per day, with 100 mg per day being on the steepest part of the slope. At a vitamin C intake of 200 mg per day plasma and leukocyte levels of vitamin C were significantly above those at 60 mg per day of vitamin C intake. A very high vitamin C intake increased plasma levels further, but neutrophil levels were saturated at the 100 mg per day dose.
How much supplemental vitamin C is beneficial is difficult to establish. Certainly, good evidence supports the contention that the recommended 60 mg per day is too low. Increasing vitamin C intakes above 100 mg per day seems prudent, as plasma levels continue to rise rapidly at this dose. The upper level of vitamin C intake is more difficult to define because the dose will depend on many factors. A vitamin C intake above 1000 mg per day probably does not increase white blood cell vitamin C levels to a greater extent that 100 mg per day in a sedentary individual, but may prevent deterioration of these levels during intense exercise3. Athletes may therefore benefit from a higher vitamin C intake based on evidence that post-race infections are reduced with intakes of vitamin C of 600 mg per day (here). Further many studies fail to find effect for higher vitamin C intakes because they do not measure appropriate end point. Based on this a dose of 600 mg per day and 1000 mg per day seems reasonable for sedentary and athletic individuals, respectively.
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