Tuesday, July 3, 2007

Bioavailability of Vitamin B6

Losses of vitamin B6 content caused by thermal instability
occur during food processing, but the remaining
vitamin B6 does not necessarily exhibit incomplete

The bioavailability of vitamin B6 in foods is highly
variable, owing largely to the presence of poorly utilized
PN-glucoside in plant tissues. As expected, vitamin
B6 generally has a lower availability from plantderived
foods than from animal tissues (Nguyen &
Gregory, 1983). Based on plasma PLP levels in male
human subjects, the bioavailability of the vitamin in
an average American diet ranged from 61% to 81%,
with a mean of 71% (Tarr et al., 1981).
Gregory et al. (1991) determined the bioavailability
of PN-glucoside in humans through the use
of a stable-isotope method. The utilization of orally
administered deuterated PN-glucoside was 58 ± 13%
(mean ± SEM) relative to that of deuterated PN.
Intravenously administered PN-glucoside underwent
approximately half the metabolic utilization
of oral PN-glucoside, which suggested a role of β-
glucosidase(s) of the intestinal mucosa, microfl ora, or
both, in the release of free PN from dietary PN-glucoside.
Stable isotope methodology provided evidence
that PN-glucoside weakly retards the metabolic utilization
of non-glycosylated forms of vitamin B6 in humans
(Gilbert et al., 1991). Despite the relatively high
consumption of glycosylated vitamin B6, vegetarian
women did not demonstrate any signifi cant difference
in vitamin B6 status compared with non-vegetarian
women (Shultz & Leklem, 1987; Löwik et al.,
1990). In addition, the intake of glycosylated vitamin
B6 had little, if any, effect upon maternal plasma PLP
concentration and maternal urinary excretion of total
vitamin B6 and 4-pyridoxic acid in lactating women
(Andon et al., 1989). These observations suggest that
there may be little practical signifi cance to the human
consumption of glycosylated vitamin B6.

14.4 Absorption, transport and metabolism
Humans cannot synthesize vitamin B6 and thus must
obtain the vitamin from exogenous sources via intestinal
absorption. The intestine is exposed to vitamin
B6 from two sources: (1) the diet and (2) the bacterially
synthesized vitamin B6 in the large intestine. Whether
the latter source of vitamin B6 is available to the host
tissues (apart from the colonic epithelial cells) in nutritionally
signifi cant amounts is unknown.

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