Depletion/repletion studies

Since then, eight more subjects have been studied (Sauberlich et al., 1974 Hodges etal., 1978). From these studies, the reference intake for adult men was set at 1,000 /rg of retinol equivalent, with a minimum physiological requirement of 600 /rg per day. Because the signs of deficiency only resolve slowly, it is possible that depletion/repletion studies overestimate requirements. 

On the basis of depletion/repletion studies, an intake ofO.2 mgper 1,000kcal is required to maintain normal urinary excretion, but an intake of 0.3 mg per 1,000 kcal is required for a normal transketolase activation coefficient. At low levels of energy intake, there will be a requirement for metabolism of endogenous substrates and to maintain nervous system thiamin triphosphate. 

On the basis of depletion/repletion studies, the minimum adult requirement for riboflavin is 0.5 to 0.8 mg per day. In population studies, values of the EGR activation coefficient <1.3 are seen in subjects whose habitual intake of riboflavin is 1.2 to 1.5 mg per day. At intakes between 1.1 to 1.6 mg per day urinary excretion rises sharply, suggesting that tissue reserves are saturated. On the basis of such studies, reference intakes (see Table 7.6) are in the range of 1.2 to 1.6 mg per day (Bates, 1987a, 1987b). 

The depletion/repletion studies of Horwitt et al. (1956) and others have suggested, on the basis of restoration of urinary excretion of -methyl nicotinamide, that the average niacin requirement is 5.5 mg per 1,000 kcal (1.3 mg per MJ). Allowing for individual variation, reference intakes (see Table 8.2) are set at 6.6 mg niacin equivalents (preformed niacin - -1 /60 of the dietary tryptophan) per 1,000 kcal (1.6 mgper MJ). Even when energy intakes are very low, it must be assumed that energy expenditure will not fall below 2,000 kcal, and this is the basis for the calculation of reference intakes for subjects with low energy intakes. 

It is apparent that abnormally increased excretion of kynurenine metabolites after a test dose of tryptophan cannot necessarily be regarded as evidence of vitamin Bg deficiency. This means that the tryptophan load test is unreliable as an index of status in epidemiological studies, although it is (probably) reliable in depletion/repletion studies to determine requirements. 

Vitamin Be requirements have been estimated both by isotopic tracer studies to determine turnover of the body pool (Section 9.6.1) and also by depletion/ repletion studies using a variety of indices of status (Section 9.6.2). These studies have generally been conducted on young adults, and there is inadequate information to determine the requirements of elderly people, because apparent status assessed by a variety of indices declines with increasing age, despite intake as great as in younger people (Bates et al., 1999a). As discussed in Section 9.6.3, there is also inadequate information to estimate the requirements of infants. 

This is considerably lower than the requirements estimated from depletion/ repletion studies (Section 9.6.2) and may reflect dilution of the small pool associated with amino acid metaholism, which has a rapid turnover, by the larger and more stable pool associated with glycogen phosphorylase. 

Early studies of vitamin Be requirements used the development of abnormalities of tryptophan or methionine metabolism during depletion, and normalization during repletion with graded intakes of the vitamin. Although tryptophan and methionine load tests are unreliable as indices of vitamin Be status in epidemiological studies (Section 9.5.4 and Section 9.5.5), under the controlled conditions of depletion/repletion studies they do give a useful indication of the state of vitamin Be nutrition. More recent studies have used more sensitive indices of status, including the plasma concentration of pyridoxal phosphate, urinary excretion of 4-pyridoxic acid, and erythrocyte transaminase activation coefficient. 

More recent depletion/repletion studies, using more sensitive indices of status in which subjects were repleted with either a constant intake of vitamin Be and varying amounts of protein, or a constant amount of protein and varying amounts of vitamin Bg, have shown average requirements of 15 to 16 /xg per g of dietary protein, suggesting a reference intake of 18... 

Urinary excretion of ascorbate fails to undetectably low levels in deficiency therefore, very low excretion will indicate deficiency. However, no guidelines for the interpretation of urinary ascorbate have been established, and basal urinary excretion of ascorbate is rarely used in the assessment of status. During depletion/repletion studies, urinary excretion increases before tissue saturation has been achieved (Sauberlich, 1975). 

The depletion/repletion studies of Horwitt et al. (1956) emd others have suggested, on the basis of restoration of uiineuy excretion of -methyl nicoti-... 

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