Chemistry Reference
In-Depth Information
grazing livestock. Accordingly, handbook values for selenium in foods
should be considered unreliable unless confirmed by actual analysis. Drink-
ing water usually has negligible selenium content (Sunde, 2006). In the United
Kingdom, meat, poultry and fish make up the largest contribution to sele-
nium intake. In North America, wheat-derived foods make a substantial
contribution to selenium intake (Rayman, 2000). Average selenium intake
ranges from
11 mg/d (in areas of endemic Keshan disease in China), to 25 mg/
d (in New Zealand and Finland with no reported occurrence of Keshan
disease), to 79-104 mg/d (across the United States), to 113-220 mg/d (across
Canada)
Selenomethionine is a less available metabolic source of selenium than
selenite or selenate but is more effective at increasing apparent selenium
status because of its non-specific incorporation and retention in body pro-
teins in place of methionine, a characteristic that increases its toxicity during
long-term consumption. More than 90% of selenomethionine, the major
dietary form of the element, is absorbed. Selenate and selenite are inorganic
forms of selenium commonly used to fortify foods. Selenate is absorbed
almost completely but a significant fraction is lost in the urine before tissue
incorporation. Selenite has a more variable absorption (generally
>
50%) but
once absorbed is retained better than selenate (Food and Nutrition Board:
Institute of Medicine, 2000).
Human milk selenium apparently is not under homeostatic control
because the concentration of selenium in breast milk is strongly dependent
on the maternal intake of selenium (McGuire et al., 1993; Bratter et al.,
1997; Lonnerdal, 1997). For example, the mean concentration of selenium
in breast milk varies from
10 mg/l (collected in Finland with low soil
selenium) (Kumpulainen et al., 1985), to
22 mg/l (collected in St John's,
Canada) (Hunt et al., 2004), to 112 mg/l (collected in a seleniferous region in
Venezuela; Bratter et al., 1997). Selenium concentration in human milk
typically ranges from
12 to 30 mg/l (Arnaud et al., 1993; al-Saleh et al.,
1997; Bianchi et al., 1999; Hunt et al., 2004; Yamawaki et al., 2005). Ready-
to-feed infant formulae manufactured by American companies have
imputed selenium concentrations that range from 13 to 18 mg/l (US Depart-
ment of Agriculture Agricultural Research Service, 2006). Compared to
infants fed humanized formulae not supplemented with selenium, infants
fed human milk or humanized formula supplemented with selenium have
increased plasma and serum selenium concentrations and higher plasma
glutathione peroxidase activity (Carver, 2003). The concentration of sele-
nium in human milk decreases duringthefirst12weeksoflactation
(Levander et al., 1987; Hunt et al., 2004). Selenium speciation studies
suggest that there is a minimum of five selenium-binding compounds in
human milk (Bratter et al., 1998). In bovine milk whey, most selenium is
