There has been much interest in the role of selenium in recent years. As intakes have drifted downwards in Europe, concern has been expressed that marginal deficiencies may be putting us at greater risk of developing chronic diseases like cancer and heart disease. But is this the case and what should be done about it?

Vital role in the body

In the body, selenium is incorporated into a number of vital proteins called selenoproteins. Twenty five selenoproteins have been identified so far, including i) peroxidases that have important anti-inflammatory properties and protect cell membranes from damage by free radicals, ii) deiodinases involved in the production of active thyroid hormone and iii) proteins involved in reproduction and DNA repair.

Sources of selenium

Selenium is found in soil and rocks, it accumulates in plants and this is how it enters the food chain. Selenium is present in most foods, with nuts (especially Brazil nuts), fish and seafood, offal (kidney, liver) and meat being very good sources. Cereals, vegetables and other plant foods contain selenium, but the amount varies according to the soil they grow in. European soil is relatively low in selenium, compared to areas in for example America, Canada and China.

Lower intakes

Selenium intakes in Northern Europe have generally dropped considerably over recent decades. In the 1970s intakes were around 60-70 μg per day whereas now intakes are estimated at only 30 μg for women and 40 μg per day for men¹, which is about half the current recommended amount.² It is believed that the reason may be a greater reliance on homegrown instead of imported wheat from Canada and America, which contains up to 50 times more selenium than the European equivalent. Interestingly Finland introduced selenium into fertiliser in the 1980s and levels have increased correspondingly (see table).

Weakened defences

Although selenium intakes are on the low side, obvious signs of deficiency are not seen in Europe. But suboptimal selenium intakes may reduce the production of selenoproteins, which in turn might affect DNA repair, impair the immune and anti-inflammatory responses, and reduce protection against diseases like cancer and heart disease.

Selenium and cancer

The role of selenium in the development of cancer was reviewed extensively recently.³ Although there are plausible mechanisms by which selenoproteins could reduce the likelihood of developing cancer (for example by mopping up free radicals that cause DNA damage), it was concluded that there was limited evidence that foods containing selenium protect against the risk of any form of cancer, except for prostate cancer. Selenoproteins may lower prostate cancer risk because they are involved in the production of testosterone, which is an important regulator of both normal and abnormal prostate growth.

Heart disease

A meta-analysis of thirteen observational studies examining the role of selenium in heart disease found a moderate inverse relationship between markers of selenium status and the risk of heart disease. However, studies in selenium replete populations find no evidence of cardiovascular protection and supplementation studies are largely inconclusive.⁴

Safety margin

In Europe, the safe upper limit of selenium intake has been set at 300 μg/day for adults, gradually decreasing to 60 μg/day for children aged 1-3 years.² Exceeding these levels may, in mild cases, cause skin lesions and loss of hair and nails, whereas large long-term doses (over 900 μg/day) can result in neurological changes including numbness, convulsions and even paralysis. Excessive selenium intake from food sources is very rare, but cases of toxicity have been reported in Swedish children who accidentally overconsumed selenium tablets.⁵

Moderate intakes are best

Although selenium intakes in Europe have fallen, they are still sufficient to prevent any obvious signs of deficiency and are similar to many other areas of the world (see table).¹ Levels of selenoproteins appear to reach their maximal levels relatively easily at normal dietary intakes and do not increase further with selenium supplementation.³ This is in line with a recent review on antioxidant supplements that attributed no clear benefits to consuming additional selenium from non-food sources.⁶ By eating a varied diet, most people will get enough selenium from their food.

Geographic differences in the selenium intakes (µg/day) of adults¹

Region or country	Selenium intake (Mean ± standard error or range)
New Zealand, low-selenium area	11 ± 3
China, disease-free area	13 ± 3
China, seleniferous area	1338
South Sweden	40 ± 4
Finland, before selenium in fertiliser	26
Finland, after selenium in fertiliser	56
Slovakia	27 ± 8
United Kingdom, 1974	60
United Kingdom, 1995	33
Italy	41
Germany	38 - 48
France	47
United States	80 ± 37
Canada	98 - 224
Venezuela	80 - 500

References

1. FAO/WHO (2002). Human Vitamin and Mineral Requirements. Report of an expert consultation, Bangkok Thailand. Chapter 15 Selenium. Available at: http://www.fao.org/DOCREP/004/Y2809E/y2809e0l.htm#bm21.1
2. Scientific Committee on Food (2000) Opinion of the Scientific Committee on Food on the Tolerable Upper Intake Level of Selenium. Available at: http://ec.europa.eu/food/fs/sc/scf/out80g_en.pdf
3. WCRF/AICR (2007). Food, Nutrition, Physical Activity and the Prevention of Cancer – a Global Perspective. Washington DC. Available at: www.dietandcancerreport.org
4. Navas-Acien A, Bleys J & Guallar E (2008). Selenium intake and cardiovascular risk: what is new?. Current Opinions in Lipidology 19:43-49
5. Johansson L, Åkesson B and Alexander J (1997). Availability of selenium from soils in relation to human nutritional requirements in Sweden – Is there a need for supplementation. Report. Swedish Environmental Protection Agency, Stockholm, 104 pp.
6. Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C (2008). Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews 2008, Issue 2. Art. No.: CD007176. DOI: 10.1002/14651858.CD007176.

FOOD TODAY 05/2008