Markers of DNA Oxidative Damage

There is increased formation of 8-hydroxyguanine (a marker of oxidative radical damage) in DNA during (short-term) vitamin C depletion (Fraga et al., 1991). In addition, the rate of removal of 8-hydroxyguanine from DNA by excision repair, and thus the urinary excretion of 8-hydroxyguanine, is affected by vitamin C status (Cooke et al., 1998). These results suggest that measurement of 8-hydroxyguanine in DNA, or its urinary excretion, may provide a way of estimating requirements to meet a biomarker of optimum status.

13.6 VITAMIN C REQUIREMENTS AND REFERENCE INTAKES

There have been two major studies of ascorbate requirements in depletion/repletion studies, one in Sheffield during the 1940s (Medical Research Council, 1948) and the other in Iowa during the 1960s (Baker et al., 1969,1971; Hodges etal., 1969,1971). In addition, Kallner and coworkers (1979,1981) have determined the body pool of ascorbate and the fractional rate of turnover under various conditions. Levine and coworkers (1995, 1996, 1999) have measured plasma and leukocyte ascorbate in studies of subjects maintained on more than minimally adequate amounts of vitamin C for relatively prolonged periods of time to determine optimum, rather than minimum, requirements.

Although the minimum requirement for vitamin C is firmly established, there are considerable discrepancies between the reference intakes published by different national and international authorities (see Table 13.3), with figures ranging between 30 to 90 mg per day. This is the result of the use of different criteria of adequacy and reflects differences of opinion as to what represents an adequate intake ofvitamin C. It is possible to produce arguments to support reference intakes of between 30 to 100 mg per day.

Carr and Frei (1999b) reviewed studies of vitamin C intake associated with reduced risks of cancer and cardiovascular disease and suggested that, by this criterion, the average requirement was 90 to 100 mg per day, giving a reference intake of 120 mg per day.

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