Introduction to Human Nutrition

220 Introduction to Human Nutrition

The evidence for suboptimal selenium status
increasing the risk of cardiovascular disease is more
fragmentary, but it has been proposed that optimiz-
ing the activity of the seleno-dependent glutathione
peroxidases and, thereby, increasing antioxidant activ-
ity may be a factor. As selenium has well-recognized
antioxidant and anti-infl ammatory roles, other oxi-
dative stress or infl ammatory conditions (e.g., rheu-
matoid arthritis, ulcerative colitis, pancreatitis, and
asthma) may benefi t from selenium supplementation.
In addition, some, but certainly not all, studies have
suggested benefi cial (possibly antioxidant) effects of
selenium on mood and reproduction in humans. The
evidence, however, supporting a role for optimum
selenium status preventing or ameliorating most
infl ammatory conditions is not strong and may be
confounded by other dietary antioxidants, particu-
larly vitamin E, compensating for low selenium
status.

Toxicity

There is a narrow margin, perhaps not much more
than three- or fourfold, between benefi cial and
harmful intakes of selenium. The dose necessary to
cause chronic selenosis in humans is not well defi ned,
but the threshold for toxicity appears to lie some-
where in the range of 850–900 mg/day. Symptoms
of chronic selenium toxicity include brittle hair
and nails, skin lesions with secondary infections, and
garlic odor on the breath, resulting from the expira-
tion of dimethyl selenide. Toxicity depends on the
chemical form of selenium, with most forms having
low toxicity. Data from animal studies indicate that
selenite and selenocysteine are a little more toxic than
selenomethionine and much more toxic than other
organic selenium compounds (dimethyl selenide, tri-
methyselenonium ion, selenoethers, selenobetaine).
Methylation in the body is important for detoxifi ca-
tion of the element.

Genetic diseases

Although no important genetic diseases affecting
selenium status are apparent, polymorphisms in gene
sequences of some selenoenzymes may determine
selenium utilization and metabolic needs, and hence
dietary requirements. These polymorphisms may
explain the signifi -cant variation among individuals
in the extent of the response to supplementation of
selenoenzyme activities.

Assessing status

Plasma or whole blood, hair, and toenail selenium

concentrations can indicate changes in selenium

status in humans. Plasma and serum selenium con-

centrations respond rapidly to changes in selenium

intakes, whereas erythrocyte selenium is an index of

longer term or chronic intake. Dietary intake data,

however, are insuffi cient to determine selenium status

in individuals because of uncertainties about bio-

availability and variations in the content and form of

selenium in foodstuffs. Although plasma (or prefera-

bly platelet) glutathione peroxidase activities have

been used as functional indices to estimate selenium

requirements, it has not been established how these

measurements relate to other biochemical functions

of selenium, such as thyroid metabolism, or immune

function and their health sequelae. For example, at

higher levels of selenium intake, glutathione peroxi-

dase activities plateau but immunoenhancement

may be evident at supplementation levels higher than

those needed to optimize the selenoenzyme activity.

Perhaps the best way forward is to select from a

battery of functional indices, such as selenoenzyme

activity, plasma thyroid hormone concentrations,

and immune measures, according to the function or

disease under investigation.

Requirements and dietary sources

Dietary intakes of selenium vary widely with geo-

graphical spread (Table 9.19). Requirements for

selenium have been estimated at intakes required to

saturate plasma glutathione peroxidase activity

Table 9.19 Dietary selenium intakes worldwide

Country (region) Range (μg/day)

Australia 57–87

Canada 98–224

China (Keshan county) 3–22

China (Enshi county) 3200–6690

Greece 110–220

Mexico 10–223

New Zealand (Dunedin) 6–70

Portugal 10–100

Russia 60–80

UK 30–60

USA 62–216

After Reilly, Selenium in Food and Health, copyright 1996 with kind

permission of Springer Science + Business Media.