Selenium is an essential mineral in human nutrition. Natural selenium present in the diet of humans is in the form of organic seleno-proteins such as selenomethionine and seleno-cysteine. Foods such as fish and whole grain cereals are especially rich in organic selenium compounds [11
]. Selenium in cereals is primarily in the form of selenomethionine. This naturally occurring amino acid is the most important nutritional form of selenium.
Deficiencies of selenium contribute to the prevalence and severity of iodine deficiency disorders which are the most important and well-known global nutritional problems, primarily in less developed countries [12
]. Iodine deficiency in childhood impairs neuromotor and intellectual development, with an average reduction in the intelligence quotient of 10 points [13
]. Selenium is required in thyroid metabolism, converting inactive thyroid hormone into active thyroid hormone [14
]. It has been shown that in goitrous children who are both Se and iodine deficient, major Se deficiency partially blunts thyroid response to iodine supplementation [15
]. The mean serum Se level for healthy children (age 1–16) observed in this study was 84.2 ± 11 μg/l with no significant difference between sexes (p = 0.659). Table shows the mean serum Se level of Iranian children compared to children from different countries.
Comparison of the mean serum levels of selenium, among children from different countries (as mentioned previously )
The mean serum Se level for adults observed in this study was 100.6 ± 12.8 μg/l, which was similar to the one reported in a survey in Saudi Arabia [16
]. In the Nutritional Prevention of Cancer (NPC) Trial [17
], a Selenium level of 80 ng/mL is considered the minimum level of plasma selenium necessary in the bloodstream for maximum production of selenoproteins (glutathione peroxidases, thioredoxin reductase, etc.).
Our results show that in adults there is a significant difference between men and women (p < 0.005) with a higher concentration of selenium in men. This suggests a sex-linked hormonal influence over serum level of selenium. It has previously been shown that selenium is essential for spermatogenesis [18
]. This trace element is present in the protein of the capsule surrounding the sperm mitochondria and may have a structural function [19
]. Our data also show a positive correlation between a higher concentration of selenium in serum and age in men (P < 0.001).
Table summaries the selenium serum levels in the Iranian population in comparison with different countries. It is higher than levels calculated for Finland and other countries where soil is poor in selenium content. By using the medium correlation factor (1.51) as introduced by Navarro et al., to estimate the daily intake of nutrients, the daily intake of Se was calculated as 62.19 μg in the female and 67 μg in the male populations [20
Comparison of the mean serum levels of selenium, among adults from different countries (as mentioned previously [9, 11])
Considering the American RDA, which recommends a daily Se intake of 50 μg for women and 70 μg for men, it seems that the normal Iranian diet has an adequate content of selenium for both genders.
Tehran, the capital of Iran, is located in the north of the country and has a population of approximately fifteen million people, representing a large proportion of the country's total population, estimated to be seventy five million people. The diversity of nourishment sources, regional variation and different ethnic diets makes it difficult to extend these results to the whole population.