Vitamin C is water-soluble, and probably the most famous of all the vitamins. Even before its discovery in 1932, physicians recognised that there must be a compound in citrus fruits preventing scurvy, a disease that killed as many as 2 million sailors between 1500 and 1800. Later researchers discovered that man, other primates and the guinea pig depend on external sources to cover their Vitamin C requirements. Most other animals are able to synthesise Vitamin C from glucose and galactose in their body. The most prominent role of Vitamin C is its immune stimulating effect, which is important for the defence against infections such as common colds. It also acts as an inhibitor of histamine, a compound that is released during allergic reactions. As a powerful antioxidant it can neutralise harmful free radicals and aids in neutralising pollutants and toxins.

Harrison FE. A critical review of Vitamin C for the prevention of age-related cognitive decline and Alzheimer's disease. 1. J Alzheimers Dis. 2012;29(4):711-26.

Antioxidants in the diet have long been thought to confer some level of protection against the oxidative damage that is involved in the pathology of Alzheimer's disease as well as general cognitive decline in normal aging. Nevertheless, support for this hypothesis in the literature is equivocal. In the case of Vitamin C (ascorbic acid) in particular, lack of consideration of some of the specific features of Vitamin C metabolism has led to studies in which classification of participants according to Vitamin C status is inaccurate, and the absence of critical information precludes the drawing of appropriate conclusions. Vitamin C levels in plasma are not always reported, and estimated daily intake from food diaries may not be accurate or reflect actual plasma values. The ability to transport ingested Vitamin C from the intestines into blood is limited by the saturable sodium-dependent Vitamin C transporter (SVCT1) and thus very high intakes and the use of supplements are often erroneously considered to be of greater benefit that they really are. The current review documents differences among the studies in terms of Vitamin C status of participants. Overall, there is a large body of evidence that maintaining healthy Vitamin C levels can have a protective function against age-related cognitive decline and Alzheimer's disease, but avoiding Vitamin C deficiency is likely to be more beneficial than taking supplements on top of a normal, healthy diet.