For immune function, a wealth of preclinical evidence of favorable effects on immunity has received minimal support from clinical studies.

A group of adults with low selenium status exhibited better viral clearance when given a selenium supplement than did those given placebo. A study of patients with allergy-associated asthma had no observable benefits from selenium supplementation. Conversely, HIV patients had a substantial reduction in the risk of mycobacterial infections when given selenium.

A strong rationale exists for a positive effect on immune function, as multiple studies have shown that selenium supplementation has a pronounced immunostimulatory effect involving a variety of immune-related cells.

For HIV and other viral infections, the limited evidence has been conflicting and open to interpretation, although selenium deficiency has been linked to incidence, virulence, and progression of some viral infections.

In the brain, several studies have linked low selenium status to epileptic and febrile seizures in children. Other studies have suggested a role for selenium in coordination, Parkinson’s disease, and cognitive function.

A few small studies have shown an association between selenium supplementation and a reduced frequency of childhood seizures. A large placebo-controlled trial showed no effect of supplementation on mood or quality of life in older adults.

For fertility and reproduction, several studies have shown an association between the selenoprotein GPx4 and male fertility and sperm quality. In women, low selenium status has been linked to an increased risk of miscarriage, preeclampsia, and preterm birth. Data from prospective interventional studies are lacking, Rayman said.

For cardiovascular disease, epidemiologic and laboratory evidence has linked selenium status to the state of various aspects of cardiovascular health, including lipid oxidation, inflammation, and platelet function. However, randomized clinical trials have shown no beneficial effect on cardiovascular disease or mortality.

Finally, for cancer, individual studies and meta-analyses have shown that higher selenium levels are associated with a reduced risk of various types of cancer, including lung, bladder, colorectal, liver, esophageal, gastric, thyroid, and prostate.

Clinical trials have thus far failed to confirm the beneficial effects, most recently in the large Selenium and Vitamin E Cancer Trial (SELECT) that examined the impact of selenium supplementation on prostate cancer risk. Rayman noted that SELECT included few men with baseline selenium levels previously associated with a benefit from supplementation (JAMA 2009; 301: 39-51).

Rayman asserted that future clinical trials should involve only patients with low or relatively low selenium levels. She also called for genotyping of study participants, noting that polymorphisms in selenoproteins affect selenium status and disease risk.

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Rayman disclosed relationships with Wassen International and Pharma Nord.

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Primary source: The Lancet
Source reference: Rayman MP. “Selenium and human health” Lancet 2012; DOI: 10.1016/S0140-6736(11)61452-9.