[Biochemical markers of vitamin supply]

The biochemical indicators to assess the nutritional status of individuals may be divided into 4 categories: methods which measure body stores e.g. plasma vitamin C or urinary excretion of thiamine, methods related to the rate of conversion of vitamins to metabolites e.g. thiamine pyrophosphate content of blood or urinary excretion of pyridoxic acid, methods based on the coenzymatic or hormonal function of vitamin metabolites, e.g., erythrocyte transketolase in the case of vitamin B1 or plasma concentration of 1,25 dihydroxycholecalciferol in the case of vitamin D. Finally, methods derived from true biochemical impairment of the metabolism, e.g., elevated blood pyruvate in borderline vitamin B1 deficiency or prolonged blood clotting time in the case of vitamin K deficiency. The results of all of these methods are for a certain vitamin related to each other and also to the daily intake of the vitamin in question as long as the daily vitamin intake remains stationary and no confounding variables such as disease, smoking or drinking habits interfere. Otherwise the estimation of vitamin intake by measuring biochemical parameters becomes questionable, since biochemical parameters are indicators of nutritional status and not so much of nutrient intake.