Chemistry Reference
In-Depth Information
randomized trial with patients with a history of skin cancer. In a separate
study, a probability sample of the US population also indicated that high
serum selenium levels (
>
130 ng/ml) were positively associated with the pre-
valence of diabetes (Bleys et al., 2007). The NHANES III survey found that
the mean serum selenium level for all Americans aged
>
4 yr is 125 ng/ml
(Food and Nutrition Board: Institute of Medicine, 2000). Increased risk for
diabetes suggests asymptomatic changes related to chronic selenium toxicity
that could take place at intake levels lower than the currently defined UL
(400 mg/d for adults). There are some contrary data; a substudy of the Health
Professionals Follow-up Study found an inverse association between toenail
selenium levels and the prevalence of diabetes at baseline (Rajpathak et al.,
2005).
Additional findings from the supranutritional selenium supplementa-
tion study discussed above (Stranges et al., 2007) indicated that selenium
supplementation (200 mg/d) increased the risk of squamous cell carcinoma
(25%) and total non-melanoma skin cancer (17%) (Duffield-Lillico et al.,
2003) despite reductions in total cancer incidence and mortality (Duffield-
Lillico et al., 2002). Also, participants with baseline plasma selenium concen-
trations in the lowest two tertiles (
<
121.6 ng/ml) experienced reductions in
total cancer incidence, whereas those in the highest tertile (
>
121.6 ng/ml)
showed a possible small increase in total cancer risk.
The FNB found no functional criteria on which to base an RDA for
selenium for infants. Thus, they set AIs for selenium (Table 10.2) that
reflected the mean intake of infants fed principally human milk. In the age
group 7-12 months, an amount was added for the contribution obtained from
weaning foods. For children and adolescents aged 1-18 yr and all adults aged
19-50 yr, the selenium EARs were based on selenium intakes expected to
maximize plasma extracellular glutathione peroxidase (GSHPx-3) activity.
Because the aging process does not appear to impair selenium absorption or
utilization, selenium EARs for adults aged
>
50 yr are also based on maximal
GSHPx-3 activity. Data from the available human studies are generally
compatible with the possibility that intakes of selenium above those needed
to maximize selenoproteins have an anticancer effect in humans. However,
confounding factors, small sample sizes and the potential for excess selenium
supplementation to raise cancer risk collectively do not allow use of these
studies as the basis for determining dietary selenium requirements at this time
(Food and Nutrition Board: Institute of Medicine, 2000).
Dietary selenium is consumed principally in the form of selenomethio-
nine (plant, animal and supplements sources), selenocysteine (mainly animal
sources) or selenate and selenite (from supplements). Significant sources of
dietary selenium vary widely because the wide range in soil selenium content
is reflected in the selenium content in foods of plant origin as well as that of
