Vitamin B6 requirements
Most studies of vitamin B6 requirements have followed the development of abnormalities of tryptophan and methionine metabolism during depletion and normalization during repletion with graded intakes of the vitamin. Although the tryptophan load test is unreliable as an index of vitamin B6 nutritional status in field studies, under the controlled conditions of depletion/repletion studies it gives a useful indica-tion of the state of vitamin B6 nutrition.
Since the major role of vitamin B6 is in amino acid metabolism it is likely that protein intake will affect vitamin B6 requirements. Adults maintained on vitamin B6-deficient diets develop abnormalities of tryptophan and methionine metabolism more quickly, and their blood vitamin B6 falls more rapidly, when their protein intake is relatively high (80–160 g/day in various studies) than on low protein intakes (30–50 g/ day). Similarly, during repletion of deficient subjects, tryptophan and methionine metabolism and blood vitamin B6 are normalized more rapidly at low than at high levels of protein intake.
From such studies the average requirement for vitamin B6 is estimated to be 13 μg/g dietary protein, and reference intakes are based on 15–16 μg/g dietary protein.
Estimation of the vitamin B6 requirements of infants presents a problem, and there is a clear need for further research. Human milk, which must be assumed to be adequate for infant nutrition, provides only some 2.5–3 μg of vitamin B6/g protein. This is very much lower than the requirement for adults, although there is no reason why infants should have a lower requirement.
Based on the body content of 3.7 μg (15 nmol) of vitamin B6/g body weight, and the rate of weight gain, a minimum requirement for infants over the first 6 months of life is 100 μg/day to establish tissue reserves, and an additional 20% to allow for metabolic turnover. Even if the mother receives daily supplements of 2.5 mg of vitamin B6 through-out lactation, thus more than doubling her normal intake, the infant’s intake ranges from 100 μg/day to 300 μg/day over the first 6 months of life. At 1 month this is only 8.5 μg/g protein, rising to 15 μg/g by 2 months.