Vitamin C is perhaps the most well known vitamin amongst the general population and vitamin C supplements are widely available, cheap and popular. Evidence from the nutritional literature that vitamin C in gram amounts is beneficial at preventing infection and reducing the severity of the symptoms associated with the common cold have made its use widespread amongst Western populations. The recommended intake of vitamin C is around 60 mg (depending on the country) and this amount is the amount defined as adequate to prevent the deficiency disease scurvy. However, 60 mg has been shown inadequate to provide many of the other health benefits associated with vitamin C. In fact the nobel laureate Linus Pauling suggested intakes should be in the 1 to 4 gram range, based on studies of animals that produce their own vitamin C. Vitamin C is available mainly as 1000 mg tablets, and as such it is one of the most common vitamins to take in ‘mega doses’. This has lead some to question the safety of supplements of vitamin C.
However, vitamin C has an excellent safety record. Toxicological studies of vitamin C shows no toxicity even when taken at intakes of tens of grams per day. This is largely because of its high water solubility that results in it fast elimination from tissues and subsequent excretion in the urine. However, just because such high doses are safe does not mean that they are recommended, as nutrient interactions should always be considered. Vitamins and minerals interact with one another in synergistic and antagonistic ways, and and high doses of single vitamins can cause imbalances in metabolism due to the effects they can have on other essential nutrients. Therefore it is only through knowing the way that vitamin C interacts with other nutrients that a suitable dosage can be recommended for long term use. One such interaction that has been suggested is based on a study that observed an increase in the excretion of 4-pyridoxic acid (4-PA) in the urine of subjects taking vitamin C supplements.
Vitamin B6 (pyridoxine) is metabolised to 4PA before excretion in the urine. The observation that vitamin C can increase urinary excretion of 4-PA has lead some to suggest that vitamin C may increase the rate of metabolism of vitamin B6. To address this hypothesis, one group of researchers1 fed subjects either 500 mg or 1000 mg of vitamin C for 2 and 5 days. respectively, before supplements of 2 mg of pyridoxal hydrochloride were administered. This initial study showed that 4PA excretion did increase when the vitamin B6 supplements were administered when compared to the days of vitamin C supplements in isolation. The researchers then gave subjects vitamin C at a dose of 1000 mg for 4 days, with pyridoxal hydrochloride supplements being administered for the entire time. In this second study, there was no increase in the excretion of 4PA over the entire 4 day period. Therefore intakes of ascorbic acid at 1000 mg per day do not appear to affect vitamin B6 excretion rates over the short-term.
Dr Robert Barrington’s Nutritional Recommendation: Other studies measuring levels of several B vitamins in red blood cells have confirmed that concentrations are not associated with vitamin C plasma levels. Therefore it is likely that vitamin C does not interact with vitamin B6 in humans. Studies suggesting an increased excretion of vitamin B6 in the urine following ascorbic acid supplements may have had methodological flaws. In particular, talking a single urine sample on waking and not determining the vitamin B6 content of the diet call into question the validity of the conclusions drawn from the data.
RdB
