The EAR based on a plasma PLP of at least 20 nmol/L would be less than 1.3 mg (less than 1.6 mg for food B6) in men and about 0.9 mg (1.1 mg for food B6) in women. Xanthurenate excretion in nearly half the men and all the women was near the baseline value at the equivalent of a food B6 intake of 1.6 mg/day. The four subjects on the lower protein diet (0.8 g/kg of body weight) reached baseline values of plasma PLP at a lower level, 1.33 mg/day of B6, yet xanthurenate excretion did not return to baseline for some of the subjects by the end of the three repletion periods.
Selhub and colleagues (1993) investigated the relationship among plasma homocysteine, dietary B6, and plasma PLP concentration in subjects from the Framingham Heart Study aged 67 to 96 years. The data were adjusted for age, gender, and folate and B12 intakes. No clear cutoff for homocysteine has been established, but about half the subjects whose B6 intakes were 1.4 mg (and who had plasma PLP concentrations of 25 nmol/L) were in the same homocysteine range as subjects consuming much higher intakes. At a plasma PLP concentration of 20 nmol/L, homocysteine concentrations averaged about 13 µmol/L, and this corresponded to a dietary intake of about 1.3 mg B6.
Meydani and coworkers (1991) reported impairments in interleukin-2 and lymphocyte proliferation in eight healthy elderly subjects (four men and four women) placed on a B6-deficient diet of 0.3 µg/ kg body weight/day for up to 20 days (approximately 0.17 mg/day for the men and 0.10 mg/day for the women). Restoration of various parameters of cell-mediated immunity to baseline values (obtained when the subjects had been on self-selected diets) required more than 22.5 µg/kg body weight/day, the second highest repletion level tested, because most of the parameters measured returned to baseline values at the highest repletion level tested in both the men and women, which was 33.75 µg/kg body weight/day or about 2 mg/day of B6 for the women and 2.88 mg/day for the men. Some measures of immunity did not return to baseline during the study period whereas others exceeded baseline values only at very high (50 mg) B6 intakes. No indication was given as to which level of any of these parameters indicated a dysfunction in immune response. Although the changes in these parameters with B6 intake suggest they may be of potential use as indicators of B6 status, it is presently not possible to determine a requirement from these data.
B6EAR and RDA Summary, Ages 51 Years and Older. From the above studies, it appears that the EAR for B6 is higher than the EAR for younger men and women and, unlike that for younger adults, is