Vitamin C is beneficial to the health, and this may be one of the reasons that high intakes of fruits and vegetables are protective of disease. In particular, vitamin C consumption is associated with a decreased risk of certain Western lifestyle diseases including cardiovascular disease and cancer. Evidence for such associations derive partly from epidemiological studies, but it is very difficult to prove a cause an effect in such studies, due to the inherent way that the studies are designed and performed. It is therefore difficult to state categorically that the vitamin C is the reason that fruit and vegetables are protective of disease, because other factors are present in fruits and vegetables that may contribute to the effects. Clinical trials are able to prove cause and effect, however such trials can only be performed on a relatively small number of individuals, for a relatively short time, and as such can only often measure markers for disease, rather than actual disease rates. Often evidence from both types of studies are combined to provide evidence but this is also problematic.
Researchers have tried to work around these problems by designing studies that provide additional data that, when combined with other evidence, enhances the understanding of vitamin C within fruits and vegetables. For example, in one study1, researchers investigated the effects of raised levels of plasma vitamin C on cardiovascular disease. The authors showed that the risk of ischaemic heart disease and all cause mortality was significantly lower in those with the highest intake of fruits and vegetables compared to those with the lowest intake. This supports previous evidence that shows a beneficial effect for fruits and vegetables against cardiovascular disease and total mortality. The authors then analysed the risk of of ischaemic heart disease and total mortality in subject with genetically raised levels of vitamin C. These individuals had an increased number of the sodium-dependent vitamin C transporter in the enterocytes of the gut, and this raised the absorption rates of vitamin C in the subject above the normal levels, resulting in higher plasma levels.
Those with elevated levels of vitamin C transporters had a corresponding 11 % higher vitamin C level compared to control subjects. In a separate analysis the authors showed that subjects with vitamin C levels 25 % higher than normal had a significantly reduced risk of both ischaemic heart disease and total mortality. The authors then compared the risk of cardiovascular disease between the subjects with the genetically high levels of plasma vitamin C and those with high levels of plasma vitamin C caused by high fruit and vegetable consumption and found significant overlap. Therefore the authors concluded that vitamin C could not be excluded as the causative factor in the benefits of fruits and vegetables towards ischaemic heart disease and total mortality. These results therefore support the contention of Linus Pauling that vitamin C is cardioprotective and that low levels of vitamin C may be a acquisitive factor in the development of cardiovascular disease, and this may in turn increase the risk of total mortality.
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