Vitamin C reserves are affected by the following factors which influence vitamin C intake (Schorah et al., 1981):
• Social class
There are social class differences in intakes of vitamin C that are probably associated with income.
Where vitamin C intakes are lowest in spring, this is generally attributable to seasonal unavailability and high cost of fresh fruits and vegetables, and possibly to the consumption of stored vegetables with low levels of vitamin C.
Decreased intakes by the elderly is the main factor influencing age-related changes in vitamin C reserves.
There are increased losses of vitamin C during large-scale institutional food preparation (see chapter 'Stability in Foods').
Vitamin C reserves are also affected by the following factors that influence vitamin C metabolism (Schorah et al., 1981):
There is considerable evidence that mean plasma and leucocyte vitamin C concentrations are lower in males than in females and that this difference persists into old age. The reason for these sex differences is not known.
There is general agreement that smoking lowers the level of vitamin C in both plasma and leucocytes even if the precise reason is not known. Some believe that it is due to smokers' reduced ability to absorb the vitamin.
Large differences have been reported around the world for vitamin C reserves; some of the highest values are found in North America and some of the lowest in India, which may be predominantly diet-related. The nomadic tribes of the Sahel and the northern part of Nigeria and Ghana do not normally develop scurvy even on a diet almost devoid of fresh fruits and vegetables. However, Nicol (1958) and Watson (1976) noted the importance of sylvan produce from plant sources for covering vitamin C requirements. Schorah and others (1981) in turn suggested that it might be possible for some populations to synthesize vitamin C, for example in the case of malnourished Ghanaian children who apparently maintained surprisingly high plasma vitamin C concentrations. However, the possibility that this may occur should not influence the decision to intervene.
• Acute disease
Early writers such as Lind noted that scurvy often followed infectious epidemics. In the 1930s, investigations into the effect of infection on vitamin C reserves showed that serum levels in people with adequate vitamin C intake were markedly depressed by acute infection. The reason for such a change in metabolism during acute trauma and disease is unknown.
• Chronic conditions
Most sick people have low vitamin C reserves, for example in the case of gastrointestinal disease, liver disease, alcoholism, asthma and diabetes. A number of studies indicate that plasma vitamin C concentrations also decrease gradually throughout pregnancy. Lactation can lead to significant losses of maternal vitamin C-as much as 32 mg/day (Rajalakshmi et al., 1965; Salmenpera 1984).