Selenium is an essential mineral that has a wide range of important functions for human health.
Unfortunately, selenium is also one of the most common deficiencies around the world.
This article looks at the health benefits of selenium, what the mineral does, and why it is so important.
What Is Selenium?
Selenium is one of the essential minerals, and we need to obtain these nutrients from our diet.
The amount of selenium in food depends on the selenium concentration of the soil, and this can vary both regionally and internationally (1).
Since selenium is such a powerful mineral, we only need trace amounts to maintain good health, and excessive amounts can quickly lead to toxicity.
On this note, the reference daily intake (RDI) for selenium is 55 mcg for most adults over the age of 19 (4).
What Are the Health Benefits of Selenium?
Selenium has numerous nutritional benefits for the body.
Although the mineral has a wide range of potential health impacts, the following are supported by stronger levels of evidence.
1) Plays An Important Role In Thyroid Health
The thyroid gland produces two vital hormones called triiodothyronine and thyroxine (T3 and T4 for short).
These hormones perform lots of different functions within the body, including the regulation of temperature, metabolism, and heart rate (5).
Notably, certain nutrients are crucial to the optimal function of the thyroid, and these include iodine and selenium.
2) May Protect Against Mercury Toxicity
Mercury is a metallic chemical element (heavy metal) that we can be exposed to via air, food, soil, and water.
Unfortunately, mercury exposure can cause a range of potentially serious health problems, mainly when the metal accumulates in the body (12).
Although seafood as a whole is an incredibly beneficial food group, certain species of fish and shellfish can be significant sources of mercury (13).
More research is necessary on the precise relationship between mercury and selenium, and some studies suggest that the binding works both ways.
In other words; since mercury binds with selenium, this can cause health problems by reducing the body’s availability of the mineral (16).
The converse is also true, and selenium can help eliminate mercury and reduce its bioavailability in the body (17).
Either way, a higher intake of selenium likely lessens the detrimental effects of mercury.
3) Enhances the Immune System
Selenium has antioxidant properties, and research shows that the mineral may help improve immune response.
In a randomized controlled trial featuring sixty patients with diabetic nephropathy, selenium supplementation significantly increased blood levels of antioxidants such as glutathione (18).
Glutathione is otherwise known as the body’s “master antioxidant,” and it plays a crucial role within the immune system (19).
In particular, it appears that selenium may have beneficial properties for fighting viral infections (20).
On this topic, there is a fair amount of research on the role selenium may play in the fight against HIV.
4) May Play a Role In DNA Repair
DNA is present in all living things, and it is a kind of genetic code that contains information on who we are, our characteristics, and how our body will develop and function.
On the positive side, a variety of studies show that selenium may play a role in repairing damaged DNA. According to the research, selenium enhances the DNA repair response and helps to protect cells from further DNA damage (28, 29, 30).
5) Selenium Is Associated With Improved Symptoms of Depression
Research suggests that selenium may lower the risk of depression.
While it is important to remember that associations are not proof of causation, numerous studies show that higher selenium status has links to better mood;
- In a study featuring 978 young adults, an optimal range of selenium (not too high or too low) was associated with reduced symptoms of depression (31).
- A case-control study based on food questionnaires demonstrated a higher risk of depression when selenium intake is low (32).
In addition to the epidemiological research, randomized controlled trials also show potential benefits for treating depression.
For example, in one clinical trial, selenium supplementation during early pregnancy reduced the risk of post-natal depression (33).
6) May Have Benefits For Male Infertility
Infertility is a growing problem, and estimates say that fertility problems affect 15% of couples around the world (34).
In the case of male infertility, selenium may provide benefits due to its effect on improving sperm motility.
For instance, in sixty-four sub-fertile men with low blood levels of selenium, supplementation improved “sperm motility and the chance of successful conception” (35).
However, despite the promising results on selenium and reproductive health, the existing evidence base is small, and larger intervention trials are necessary (38).
7) Selenium May Lower Markers of Inflammation
Due to its immune-enhancing antioxidant role, evidence suggests that optimal selenium intake may lower oxidative stress and inflammatory markers.
In this regard, here is a summary of research on the topic;
- According to a randomized clinical trial featuring 70 women with gestational diabetes, selenium supplementation decreased oxidative stress and C-reactive protein (CRP – a marker of inflammation) (39).
- In another randomized clinical trial featuring sixty women with polycystic ovary syndrome (PCOS), selenium supplementation significantly increased blood antioxidant capacity. Furthermore, selenium “significantly reduced” CRP levels (40).
- A further randomized clinical trial featured 69 men with no pre-existing health conditions. In this study, supplementation with selenium-enhanced yeast for nine months significantly decreased biomarkers of oxidative stress by around 30% (41).
8) May Protect Against Cognitive Decline
A further potential benefit of selenium is the role it may play in preserving cognitive health, particularly in older adults.
However, the evidence for this is mixed and inconclusive.
Additionally, a population study of 1166 healthy adults with four years of follow-up found an association between low selenium status and cognitive decline. In the study, participants at the highest risk of cognitive decline had low levels of selenium and higher levels of oxidative stress (44).
Interestingly, a systematic review of 14 studies also found that Alzheimer’s patients have low concentrations of selenium in the brain compared to control subjects (45).
Despite this, another systematic review noted that the existing evidence base provides no proof of a preventive role for selenium in Alzheimer’s. However, the authors did suggest that the research “allows speculation on a potential preventive relevance” which large, long-term trials could clarify (46).
9) An Inconclusive But Potential Role In Cancer Prevention
Research on potential cancer-protective benefits from selenium are ongoing, and this is an area of interest due to the immune-enhancing and DNA repair functions of the mineral.
At this stage, the evidence is mixed and inconclusive, and studies in this area are “very complicated” due to the wide variety of different selenium sources (whether supplementary or food-based) used in trials (47).
However, research demonstrates that the mineral may have some potential benefits in this area;
Systematic Reviews and Randomized Controlled Trials
- A systematic review and meta-analysis of randomized controlled trials showed that, in 12 studies featuring 13,254 patients, the risk of prostate cancer decreased with increasing blood levels of selenium up to a certain threshold. The researchers note that further studies are required (48).
- In a systematic review and meta-analysis of studies on selenium and lung cancer, researchers found conflicting evidence. While selenium supplementation appeared to help prevent lung cancer in patients with low selenium levels, those with the highest selenium status had a “non-significantly increased risk” (49).
- An extensive Cochrane systematic review of 83 studies found “no evidence to suggest that increasing selenium intake through diet or supplementation prevents cancer in humans.” However, the researchers noted that more research is necessary to determine whether selenium intake may modify cancer risk in people with certain nutritional status or genetic backgrounds (50).
What Foods Contain Selenium?
A wide range of foods are rich in selenium, and among these, Brazil nuts are by far the most significant source.
Following this, various animal foods are excellent sources – particularly organ meats and shellfish.
For a full guide to the top 20 food sources of selenium, see this article.
Although much of the research on selenium is still ongoing, the mineral has immune-enhancing properties which may protect against various health problems.
However, it is important to note that studies show inadequate and excessive selenium levels as being potentially harmful to health.
As a result, increasing selenium intake for people with already sufficient levels may be detrimental.
On the other hand, the research suggests that increasing selenium intake may have numerous benefits for individuals with low selenium status.
For more on minerals, see this guide to the benefits and functions of zinc.