Chemistry Reference
In-Depth Information
immune function (Spielberg et al., 1979; Belver and Donaire, 1983; Rajen-
dran et al., 1994).
Evidence suggests that boron may be under metabolic control, possibly
through the function of a boron transporter localized recently in tissues with
excretory functions (kidney, parotid gland, submandibular gland, pancreas
and liver) (Park et al., 2004). Human blood boron concentrations are insen-
sitive to changes in dietary boron intake (Hunt et al., 1997), and the concen-
tration of boron in milk from mothers of full-term healthy infants is highly
conserved across time despite the fact that dietary intake of boron typically
varies widely with food intake patterns and drinking water sources (Hunt
et al., 2004).
Because no biological function has been identified for boron in humans,
neither an EAR nor an RDA nor an AI has been established for the element.
The DRI UL for boron is 20 mg/d for adults; boron is considered to have a
low order of toxicity (Table 10.2).
Boron is present naturally in all foods (Hunt and Meacham, 2001). Recent
findings indicate that the mean concentration of boron in human milk from full-
term mothers over the first 12 weeks of lactation (28 mg/l) is much less than that
present in ready-to-eat infant formulae (
120 mg B/l) or in bovine whole milk
(280 mg B/l; fluid, 3.3% milkfat) (Hunt and Meacham, 2001). Assuming that
breast-fed full-term healthy infants consume milk at a daily rate of
0.74 l during
the first 4 months of lactation (Butte et al., 1987), their total daily boron intake
during the first 12 weeks of lactation is estimated at 0.022 mg/d. Even so, at a
slightly older age (6-11 months), American infants are estimated to consume
0.55 mg B/d from all dietary sources (Hunt and Meacham, 2001). For males aged
51-70 yr, boron intake was estimated at 1.34 mg/d (1st percentile, 0.39; 99th
percentile, 3.34 mg/d); for lactating females, 1.39 mg/d (1st percentile, 0.38; 99th
percentile, 3.49 mg/d) (Food and Nutrition Board: Institute of Medicine, 2001).
In postmenopausal women, diets that provide 0.36 mg B/2000 kcal (and other-
wise nutritionally replete with minor supplements) result in negative boron
balance (Hunt et al., 1997).
The bioavailability of boron in human milk is likely to be high. It is
known that low amounts of naturally occurring dietary boron (0.36 mg B/d)
as well as supplemental inorganic forms (2.87 mg B/d; as orthoboric acid) are
absorbed almost completely and excreted in the urine (Hunt et al., 1997) of
post-menopausal women. Little is known about the speciation of boron in
natural foodstuffs. However, boron transport molecules in breast milk are
probably associated with the soluble, instead of fat, fraction because the
boron content of bovine whole milk (3.3% milkfat) and skim milk (0.08%
milkfat) are not significantly different: 280 and 310 mg/l, respectively (Hunt
and Meacham, 2001).
