Selenium — Sources, Health Benefits, Deficiency, and Side Effects

Antioxidant Defense and Immune System

Selenoproteins exhibit significant antioxidant properties, essential for protecting cells from oxidative damage and maintaining a balanced cellular redox environment. Their antioxidant activity is crucial for various biological functions, including cell signaling, growth, aging, immune response, and metabolism regulation (3).

The effect of selenium can also depend on its intake levels. Recent studies suggest that selenium intake within the range of 40–100µg a day primarily enhances antioxidant activity and strengthens the immune system, while intake between 200–500µg a day is believed to possess specific properties for preventing cancer (6).

Adequate selenium intake affects the transcription process of the gene responsible for immunoglobulin (Ig) synthesis, which is crucial for regulating immune system activity (7).

Experimental animal studies have demonstrated that selenium deficiency leads to weaker immune responses to viruses, tumors, and allergens than animals with adequate selenium levels. High selenium levels may be especially beneficial during HIV-1 infections (8).

Cancer

Selenium's potential role in cancer prevention is attributed to its influence on DNA repair, apoptosis (programmed cell death), the endocrine and immune systems, and its antioxidant properties (5). 

Epidemiological research has indicated that increased selenium levels decrease the likelihood of developing colorectal, prostate, lung, bladder, skin, esophageal, and stomach cancers (5).

A recent study involving 1312 cancer patients who received daily supplementation of nearly 200μg of selenium demonstrated a reduction in tumor size for prostate cancer by up to 63%, lung cancer by 58%, and colon cancer by 46%, indicating promising outcomes (7).

Anti Inflammatory Effects

Several studies have demonstrated that selenium can reduce inflammatory reactions by inhibiting the proinflammatory NF-kB pathway, thus suppressing the production of proinflammatory mediators, such as tumor necrosis factors and interleukins (9).

Research indicates that inflammatory conditions often coincide with decreased selenium levels. Studies have shown that individuals with cystic fibrosis, Crohn’s disease, ulcerative colitis, and acne may exhibit lower selenium levels compared to healthy individuals (7).

Inadequate selenium levels tend to promote the formation of pro-inflammatory M1 macrophages. Conversely, high selenium levels typically lead to the predominance of anti-inflammatory macrophages known as M2 (7).

Thyroid Function

Selenoenzymes play a vital role in the synthesis of thyroid hormones, regulating the metabolism in all body tissues (3). The highest selenium content in the body can be found in the thyroid gland (5).

Research has revealed an inverse correlation between serum selenium levels and thyroid volume, as well as the risk of goiter and thyroid tissue damage among individuals with mild iodine deficiency, particularly in women (5).

Another study found that selenium supplementation resulted in a decrease in thyroid-stimulating hormone (TSH) levels among healthy children (10).

Alzheimer’s and Cognitive Function

Lower serum selenium levels are observed with advancing age. Diminished or insufficient selenium concentrations may correlate with age-related cognitive decline, potentially due to reductions in selenium's antioxidant properties (5).

Animal studies have indicated that changes in the levels of selenium-containing compounds can impact the metabolic activity of neurotransmitters, which is particularly significant in Alzheimer's disease due to the presence of defective neurotransmitters in this condition. However, the primary importance of selenium in Alzheimer’s lies in the antioxidant function of specific seleno-enzymes.

Fertility and Pregnancy

Selenium is crucial for both male and female reproductive health. In males, oxidative damage to sperm cells is a primary factor affecting fertility. Selenium protects sperm cells from this damage, thus serving as a key factor in maintaining male fertility (7).

A diet deficient in selenium has been shown to result in decreased sperm motility and poor-quality semen when compared to diets with increased selenium intake (11).

The daily required selenium amount increases during pregnancy. A study involving expectant mothers found that lower selenium levels were significantly associated with neural tube defects. Selenium deficiencies during pregnancy can contribute to oxidative stress, which may lead to various complications such as miscarriages, premature birth, intrauterine growth restrictions, preeclampsia, bile flow disruption, thyroid disorders, and diabetes mellitus (7).

Cardiovascular Disease

Selenium is believed to play a crucial role as an antioxidant in protecting against atherosclerosis and heart disease. Selenoproteins help prevent the oxidative changes of lipids, thereby reducing inflammation and inhibiting platelet aggregation (5, 7).

There is a strong correlation between selenium deficiency and the occurrence of cardiomyopathy (a disease affecting the heart muscle), particularly in regions with significantly low selenium intake (7).

However, while several studies have suggested a potential inverse relationship between serum selenium levels and the risk of high blood pressure or coronary heart disease, others fail to establish significant links or even indicate that higher selenium levels might increase the risk of cardiovascular disease (5).

Diabetes

Numerous studies have demonstrated the importance of selenium in blood sugar regulation, with evidence suggesting that it can delay the development and progression of diabetes. Selenium has been observed to exhibit insulin-like effects, particularly in the form of selenite (7). 

However, multiple observational studies indicate that high selenium levels might raise the risk of developing type 2 diabetes (12).

Causes

Certain groups are more likely to have insufficient selenium levels due to insufficient dietary intake, poor absorption, or increased demand during certain health conditions.

People from selenium-deficient regions are at a higher risk of deficiency. Even in regions with low selenium levels, intakes in North America typically exceed the recommended dietary allowance (5). Regions with high selenium concentrations in their soils include North America, Ireland, Australia, and Israel. Conversely, areas with low selenium levels include certain provinces of China (covering 72% of the country's landmass), New Zealand, and a significant portion of Europe (13).

However, in recent years, selenium deficiency has decreased in New Zealand due to importing selenium-rich wheat (5).

Research states that individuals following a specific diet due to phenylketonuria are particularly susceptible to the negative consequences of selenium deficiency (14).

Patients undergoing long-term hemodialysis have significantly lower selenium levels than healthy individuals, as hemodialysis procedures can remove this mineral from the bloodstream (5).

People who have undergone chemotherapy or radiation therapy are prone to selenium deficiency. Additionally, people diagnosed with liver cirrhosis, celiac disease, rheumatoid arthritis, and other degenerative conditions also exhibit significantly reduced selenium levels (7).

Symptoms

Selenium deficiency in humans can manifest in various symptoms, including muscle weakness, inflammation, fragile red blood cells, abnormal skin coloration, dysfunction of the heart muscle, increased susceptibility to cancer, as well as the development of Keshan and Kashin-Beck diseases (15). Selenium deficiency can also be one of the main causes of macrosomia and endocrine disorders (16). 

Keshan disease is a type of pediatric cardiomyopathy that primarily affects young women of reproductive age and children between the ages of 2 and 10 and can result from selenium deficiency added to another stress factor, typically a viral infection. It was prevalent in certain regions of China prior to a government-initiated selenium supplementation initiative that commenced in the 1970s. Before this, adults in these areas typically consumed no more than 11 mcg/day of selenium, while intakes of at least 20 mcg/day were found to offer protection against Keshan disease (5, 7).

Selenium deficiency contributes to various other conditions, including asthma, due to reduced glutathione peroxidase activity, circulatory issues, irregular heartbeats, coma, and even sudden infant death syndrome (7).