Bioavailability of vitamin C
Bates, C.J.
European Journal of Clinical Nutrition 51(Suppl 1): S28-S33
1997
ISSN/ISBN: 0954-3007 PMID: 9023477 Document Number: 2807
Mechanisms of ascorbate and dehydroascorbate absorption, pool size and turnover have been studied in animal models, in vitro systems, and in human subjects. so that the basis of ascorbate economy is fairly well understood, but there remains a lack of knowledge about the critical factors which determine the variations in requirements between groups of subjects. There is also a lack of knowledge about the factors which determine critical subcellular concentrations of ascorbate, needed to achieve optimum tissue protection against free radical damage, and about the significance of ascorbate turnover rates (degradation) in relation to the requirements for tissue protection. Studies of ascorbate bioavailability in human subjects have tended to rely on crude kinetic models, as applied to changes in plasma or urine concentrations of ascorbate, following single pharmacological doses of the vitamin. These studies have yielded some useful data, but are limited in what they can achieve, and they have clearly resulted in disagreements and uncertainties about key questions. They have tended to focus on large, non-physiological doses, usually of synthetic ascorbate. Newer techniques, especially those using stable isotope-labelled ascorbate, and the depletion-repletion technique used by Mangels et al, (1993) should help to give some more definitive answers. Studies of high risk groups of human subjects clearly need to be developed and extended, to clarify the nature and mechanism of risk factors. Reasons for inter-individual variations in apparent bioavailability need to be explored, and it may be necessary to define human intake requirements on an individual basis, for example by use of measurable indices such as absorption efficiency, turnover rates, and risk/benefit assessment, (iron status, oxidant free radical suppression, etc). It will be necessary to define those factors which determine the bioavailability and biological efficacy of pharmacological supplements of ascorbate, and to determine which categories of subjects have high or low risk: benefit ratios, for supplement usage. We need to know whether appropriate advice can be given, either on a population or an individual basis, on ascorbate intakes for optimum health.
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