PIP: This volume details the history of vitamin B6, its chemistry and biochemistry, methods for the assessment of vitamin B6 status, and the clinical chemistry of the vitamin. Since its discovery and synthesis over 40 years ago, vitamin B6 has been implicated in a number of disease states. All approaches to the assessment of vitamin B6 status--direct measurement of blood levels, measurement of the excretion rate of the vitamin, measurement of the metabolites or abnormal metabolic products resulting from a deficient state, or measurement of some other process dependent on the concentration of the vitamin in the body--have significant technical or physiological problems. Dietary allowances vary for different age groups and situations. In the US, the National Academy of Sciences has recommended a daily dietary allowance of 2.2 mg for young adult males and 2.0 mg for young adult females. Additional allowances have been suggested for women during pregnancy and lactation, but not for users of oral contraceptives (OCs). Vitamin B6 deficiency can be either exogenous (when intake falls below the recommended dietary allowance) or conditioned (in cases where the physiologic requirement for the vitamin is higher than the dietary allowance). Conditioned deficiency arises in the following situations: defective intestinal absorption, defective cellular and intercellular transport, and impaired oxidtion or phosphorylation mechanisms in vitamin B6 metabolism. Studies aimed at assessing the abnormal tryptophan metabolism observed in some OC users have produced conflicting results. It appears that severe depression and impairment of glucose tolerance are the only important abnormalities encountered in OC users related to vitamin B6 deficiency. Abnormalities of tryptophan metabolism have been noted in patients with rheumatoid arthritis, some malignant diseases, liver disease, diabetes mellitus, atherosclerosis, and hyperkinetic syndromes.