Plasma 4-pyridoxic acid concentrations are markedly elevated in renal insufficiency. Plasma PA:PL can distinguish between increases in 4-pyridoxic acid concentrations due to increased dietary intake and those due to renal insufficiency.

Previous estimates of total vitamin B-6 pools in humans based on extrapolations from tracer studies yielded values of 107-190 mumol when the tracer was administered orally and 345-725 mumol when the tracer was administered intravenously. To obtain a more direct estimate of vitamin B-6 pools, muscle biopsies from five female and seven male adults were analyzed by cation-exchange chromatography. Total muscle mass was estimated from creatinine excretion and the assumption that muscle is 40% of the body weight. The total muscle vitamin B-6 pool was estimated to be 917 +/- 319 mumol in the females and 850 +/- 216 mumol in the males. Because muscle accounts for approximately 80% of the vitamin B-6 in the body, the total body pool of vitamin B-6 in adult humans is probably approximately 1000 mumol.

Previous reports indicated that in growing rats the vitamin B-6 pool in muscle was relatively stable during deficiency but increased in response to increased vitamin B-6 intake. To determine whether human muscle would show a similar response 10 college-aged males received a low vitamin B-6 diet (1.76 mumol/d) for 6 wk followed by 6 wk on a self-selected diet supplemented with 0.98 mmol pyridoxine HCl/d. During depletion, excretion of pyridoxic acid rapidly adjusted to approximate the intake. Plasma pyridoxal phosphate concentrations at the end of the baseline, depletion, and supplementation periods were 81 +/- 51, 9 +/- 3, and 455 +/- 129 nmol/L, respectively, whereas muscle concentrations were 21 +/- 9, 20 +/- 4, and 25 +/- 7 nmol/g, respectively and total vitamin B-6 in muscle was 28 +/- 10, 27 +/- 4, and 35 +/- 10 nmol/g, respectively. These data provide further confirmation that the vitamin B-6 pools in skeletal muscle are resistant to depletion. They also demonstrate that in humans with constant body weight, vitamin B-6 supplementation is not associated with marked increases in vitamin B-6 in muscle.