Our selenium deficiency

by Jennifer Bowden / 16 June, 2012
Some 45% of adults have an inadequate selenium intake, a national survey has shown.




Question: You have written previously that broccoli contains certain minerals, such as selenium. As New Zealand soil is defi cient in this mineral, can broccoli take up much if there are only traces in the soil? What other plants take up selenium? I eat a good balanced diet, but am I wasting my money taking a 100mcg selenium tablet each day?

Answer: Selenium intake varies tremendously around the world, from regions in China where severe deficiencies cause Keshan disease, through to places where selenium levels are so high that toxic concentrations accumulate in the population, causing garlic breath, hair and nail loss, disorders of the nervous system and skin, poor dental health and paralysis. New Zealand soil is low in selenium, so crops grown here have lower levels than their overseas equivalents, which leads to lower levels of this mineral throughout our food system and a population with a marginal selenium status.

The 2008/09 New Zealand National Nutrition Survey found that although selenium intakes had increased since the previous survey in 1997, 45% of adults still have an inadequate intake. Those most likely to have a low selenium intake were adults 70 years and over, and girls aged 15-18, of whom 70% had an inadequate intake. These are worrying statistics, as low selenium status is associated with an increased risk of mortality, poor immune function and cognitive decline, according to a recent review in the Lancet.

Selenium has many important roles in our body, from antioxidant and anti-inflammatory effects to production of thyroid hormones. Higher selenium status or selenium supplementation has antiviral effects, reduces the risk of autoimmune thyroid disease and is required for successful reproduction. Observational studies have also noted that higher selenium status is associated with a reduced risk of prostate, lung, colorectal and bladder cancers. Clinical trials of selenium supplementation have not demonstrated such a result, leading researchers to conclude that supplements may only benefit people with inadequate selenium status.

Indeed, evidence suggests that if people with an already adequate selenium status take supplements, they may increase their risk of developing Type 2 diabetes. As with many nutrients, there is a U-shaped relationship between selenium intake and human health; that is, there is a higher risk of adverse health effects with both a low selenium status and a higher status. So the crucial point is that although supplements may benefi t people with low selenium status, those supplements may harm people who have an adequate-to-high status, according to the 2012 Lancet review.

The best advice, as always, is to meet all our nutritional needs through diet. Fish, other seafood, poultry, meat, eggs and bread are rich sources of selenium, with bread the largest single contributor to our diet. The recent introduction of high-selenium Australian wheat and other imported wheat into North Island bread has significantly improved our selenium status. Although the selenium content in plant foods depends on soil levels, some plants, including garlic, onion, broccoli and wild leek, are able to accumulate selenium from soil. Hence New Zealand-grown broccoli typically contains 0.5mcg of selenium per cup, whereas the same quantity of cabbage or brussels sprouts contains just 0.1mcg.

Brazil nuts – the richest known source of selenium – also accumulate it from the soil. Selenium supplements aren’t recommended unless taken under a doctor’s supervision, because high intakes can be toxic and harm your health. A safe and effective way to improve your selenium level is to add a couple of brazil nuts to your daily diet. A 2008 New Zealand study, published in the American Journal of Clinical Nutrition, found that eating two brazil nuts daily was as effective at increasing selenium status as a 100mcg supplement. What’s more, they’re a tasty solution.

Email: nutrition@listener.co.nz, or write to “Nutrition”, c/o Listener, PO Box 90783, Victoria St West, Auckland 1142.

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