Assessment of vitamin C status.

Vitamin C is L-xyloascorbic acid (ascorbic acid, AA). Its first oxidation product is dehydroascorbic acid (dehydroAA) which also possesses vitamin C activity, possibly because of the ease with which many animal tissues can reduce it back to AA (Grimble & Hughes, 1967). AA and dehydroAA may be referred to as ‘total vitamin C’. T h e determination of AA, or of AA and dehydroAA, is central to all accepted methods for the assessment of vitamin C status. Two main methods (or modifications of them) are in current use. The more sensitive, but less specific, method is the one introduced in 1943 by Roe & Kuether; the AA, after oxidation to dehydroAA, is condensed with 2,4-dinitrophenylhydrazine and the product treated with sulphuric acid to give a red colour, the intensity of which is measured photometrically (Roe & Kuether, 1943 ; Raiha, 1958). This method, unless appropriately modified, gives a value for ‘total vitamin C’ as it includes any preformed dehydroAA. Small amounts of dehydroAA are probably present in plasma; because of its rapid biological reduction to AA and its short half-life at physiological conditions of p H and temperature, it is unlikely, however, to be present in living tissues in measurable amounts. It has been known for some years that certain tissues contain a dinitrophenylhydrazine-reacting substance other than AA or dehydroAA (Hughes, 1956) ; this may introduce an appreciable error into vitamin C lcvels determined by this method, particularly with samples taken during states of depletion or semi-depletion (Hodges, quoted by Brook, 1973). The second method is based on the reduction of an acidic solution of the redox dye ~,6-dichlorophenolindophenol by AA (Harris & Ray, 1935 ; Ressey, 1938). AA (but not dehydroAA (a non-reducing compound) is measured. This method also lacks specificity. However, many of the interfering substances (reducrones, thiosulphates etc.) are unlikely to be present in animal tissues. Furthermore, by appropriately controlling the p H and the reaction time the specificity can be greatly increased and interference from glutathione and other sulphydryl compounds, virtually completely eliminated (Bland, Constable, Harris & Hughes, 195 T ; Hughes, 1956). This method is simpler and probably more specific than the dinitrophenylhydrazine method and is the method of choice for many determinations ; where low concentrations of vitamin C are being measured (as in the determination of leucocyte AA) the more sensitive dinitrophenylhydrazine method must be used. The primary aim of any type of nutritional assessment is to obtain a measure either of the concentration of the nutrient in pre-selected ‘key’ tissues ur of the