Vitamin B6 Requirements Estimated from Depletion Repletion Studies

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Early studies of vitamin B6 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 B6 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 B6 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.

Because of the role of vitamin B6 in amino acid metabolism, it is likely that protein intake will affect requirements. A number of studies have shown that adults maintained on vitamin B6-deficient diets develop abnormalities of tryptophan and methionine metabolism faster, and their blood vitamin B6 falls more rapidly, when their protein intake is relatively high (80 to 160 g per day in various studies) than on low protein intakes (30 to 50 g per day). Similarly, during repletion of deficient subjects, tryptophan and methionine metabolism are normalized faster at low than at high levels of protein intake (Miller and Linkswiler, 1967; Kelsay et al., 1968a, 1968b; Canham et al., 1969; Miller et al., 1985). However, Coburn (1994) noted that the requirement for growth in young animals was the same for carnivorous species, with a high protein intake, as for herbivorous species.

From such studies, the mean requirement for vitamin B6 was estimated at 13 xg per g of dietary protein. Reference intakes (see Table 9.6) were based on 15 to 16 /g per g of dietary protein.

More recent depletion/repletion studies, using more sensitive indices of status in which subjects were repleted with either a constant intake of vitamin B6 and varying amounts of protein, or a constant amount of protein and varying amounts of vitamin B6, have shown average requirements of 15 to 16 /g per g of dietary protein, suggesting a reference intake of 18

Table 9.6 Reference Intakes of Vitamin B6 (mg/day)

U.K. EU U.S./Canada FAO Age 1991 1993 1998 2001

0-6 m

0.2

0.1

0.1

7-9 m

0.3

0.4

0.3

0.3

10-12 m

0.4

0.4

0.3

0.3

1-3 y

0.7

0.7

0.5

0.5

4-6 y

0.9

0.9

0.6

0.6

7-8 y

1.0

1.1

0.6

1.0

Males

9-10 y

1.0

1.1

1.0

1.0

11-13 y

1.2

1.3

1.0

1.3

14-15 y

1.2

1.3

1.3

1.3

16-18 y

1.5

1.5

1.3

1.3

19-30y

1.4

1.5

1.3

1.3

31-50y

1.4

1.5

1.3

1.3

51-70y

1.4

1.5

1.7

1.7

>70 y

1.4

1.5

1.7

1.7

Females

9-10 y

1.0

1.1

1.0

1.0

11-13 y

1.0

1.1

1.0

1.2

14-15 y

1.0

1.1

1.2

1.2

16-18 y

1.2

1.1

1.2

1.2

19-30y

1.2

1.1

13

1.3

31-50y

1.2

1.1

1.3

1.3

51-70y

1.2

1.1

1.5

1.5

>70 y

1.2

1.1

1.5

1.5

Pregnant

1.2

1.3

1.9

1.9

Lactating

1.2

1.4

2.0

2.0

EU, European Union; FAO, Food and Agriculture Organization; WHO, World Health Organization. Sources: Department of Health, 1991; Scientific Committee for Food, 1993; Institute of Medicine 1998; FAO/WHO, 2001.

to 20 ig per g protein (Kretsch et al., 1995; Hansen et al., 1996a, 1996b, 2001).

In 1998, the reference intake in the United States and Canada was reduced from the previous Recommended Daily Allowance of 2 mg per day for men and 1.6 mg per day for women (National Research Council, 1989) to 1.3 mg per day for both (Institute of Medicine, 1998). The report cites six studies that demonstrated that this level of intake would maintain a plasma concentration of pyridoxal phosphate at least 20 nmol per L although, as shown in Table 9.5, the more generally accepted criterion of adequacy is 30 nmol per L.

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