The concentration of vitamins in the diet is significantly larger than the total body pool for most vitamins. This is particularly true for the water soluble vitamins that are not stored in body tissues to any great extent. The reason for the discrepancy between the ingested and the stored amounts of vitamins is accounted for by low absorption rates and the continual excretion from faeces and urine. Estimating the total body pool of vitamins is difficult and scientists have tried to investigate body pools of vitamins using tracer studies. Such studies use radiolabelled vitamins to allow detection and estimation of the retained vitamin within the tissues of the body. However these studies are not thought to be reliable indicators of total body pools for a number of reasons, and as a result biopsy samples directly from tissue are favoured for the detection and estimation of the total vitamin pools in humans.
Radiolabel studies of vitamin B6 concentrations in humans have suggested that the total body pool is around 100 to 200 µmol. However, a study published in the American Journal of Clinical Nutrition in 19881 used biopsy sample in men and women to assess the skeletal muscle content directly. The total vitamin B6 content of the subjects was 917 µmol in female subjects and 850 µmol in male subjects following estimation of total muscle mass using creatinine excretion rates. Because skeletal muscle is roughly 40% of the total body mass, and because 80% of the vitamin B6 content in present in skeletal muscle, the researchers were able to estimate the total body pool of vitamin B6 to be 1000 µmol. Animal experiments estimate total body pools of vitamins by sacrificing the animals and directly measuring the vitamin. As this cannot be done in humans, extrapolation biopsies are the best alternative.
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