Systemic Conditioning Rebound Scurvy

There is some evidence that systemic conditioning (the accelerated metabolism or disposal of ascorbic acid) may occur after prolonged supplementation of high doses of vitamin C. The physiologic relevance of accelerated vitamin C metabolism in otherwise healthy individuals, however, has not been addressed adequately. In human subjects consuming a vitamin C-deficient diet (5 mg/d) in a live-in metabolic unit, abrupt withdraw of vitamin C supplementation (600 mg/d for 3 wk) was associated with a significant reduction in the mean leukocyte vitamin C concentration (16.8 ± 4.37 and 9.4 ± 5.33 ^g vitamin C/108 cells, presupplementation and at 4 wk after withdrawal of supplementation, respectively) (23). Both of these leukocyte vitamin C concentrations were indicative of vitamin C depletion. Yet simply removing vitamin C-rich fruits and vegetables from the diets of healthy free-living adults can lower mean plasma vitamin C concentrations into the depleted or deficient range within 1-3 wk (24). Thus, even in adults not supplementing their diets with vitamin C, removal of vitamin C from the diet (in the form of supplements or foods), results in rapid vitamin C depletion, reflecting how poorly vitamin C status is maintained in the absence of a dietary source.

Well-nourished subjects receiving vitamin C supplements may also display accelerated vitamin C metabolism; however, because indices of vitamin C nutrition remain within normal ranges, the term "rebound scurvy" is misleading. In guinea pigs consuming a standard guinea pig diet containing 0.1% vitamin C (w/w), chronic vitamin C administration [1 g/(kg body.d) intraperitoneally for 4 wk] appeared to be associated with an increased rate of vitamin C turnover. Mean plasma vitamin C concentrations fell significantly below control values at wk 2 and 5 after the abrupt withdrawal of the administered vitamin C (25); however, all mean plasma vitamin C concentrations measured postwithdrawal were well within normal ranges. In human subjects consuming normal diets, the mean plasma vitamin C concentration did not fall significantly below the presupplement value at 7 or 23 d after the abrupt withdrawal of vitamin C supplements (2 g/d for 9 d) (26).

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