Selenium (Se), one of the trace elements, is a nutritionally essential mineral that exert many important biological functions. Se is essential in the synthesis of selenoproteins such as glutathione peroxidase (GSH-Px) (participates in process of detoxification) and thioredoxin reductase, optimal function of the immune system, and optimal neuropsychological function. Se also acts as an antioxidant and as a regulator of metabolism. Selenium plays an important role in the control of thyroid hormone metabolism and proper reproductive performance as well. One of the most researched and studied functions of Se is cancer protection.

Selenium deficiency may cause reduced growth rates owing to a feedback response which lowers triiodothyronine mediated synthesis of growth hormone in the pituitary gland, whereas a combined deficiency of selenium and iodine exacerbates hypothyroidism. Two diseases which are associated with severe endemic Se deficiency in humans including Keshan disease (a disease of the heart muscle that afflicts children and women of child bearing age), and Kaschin-Beck disease (a deforming arthritis) occurs in rural areas of China and Russia (eastern Siberia) are linked to food systems with exceedingly low Se content. For example, Keshan disease has been found in many mountainous provinces of China where Se levels in soil are very low. The incidence of the disease has been dramatically reduced by the prophylactic administration of oral tablets containing Na2SeO3 or selenite-fortified table salt.

Most of the reported cases of human exposure to hazardous levels of Se have involved occupational exposure and accidental oral consumption of various inorganic Se compounds. Signs of toxicity, referred to as ‘selenosis’, include skin and nail changes, tooth decay, and nonspecific gastrointestinal and neurologic abnormalities.

The Recommended Dietary Allowances (RDAs) for selenium are 55 mcg/day for women, men, and adolescents (aged 14 to 18) and 20 to 30 mcg/day for children. For infants, the Adequate Intake (AI) is 15 to 20 mcg/day. The RDA during lactation is 70 mcg/day. Combs Jr GF stated that in order to provide regular adult intakes, at least 40 mcg/day is needed to support the maximal expression of the Se enzymes, and perhaps as much as 300 mcg/day to reduce risks of cancer. 

Human obtains Se from foods, as selenoamino acids (selenomethionine, selenocysteine, and selenocystine). Thus, individuals with low intakes of protein will also have low intakes of Se. Se enters the food chain through plants. The selenium content of plant foods depends on the bioavailable selenium content of the soils in which they are grown. Selenium which is present in cereals and most vegetables (mainly in the form of selenomethionine) has a higher bioavailability than that observed in dairy products or meats. The Se content of fish is high, presenting a relatively high bioavailability and the same is true of seafoods. The top 10 selenium-rich foods are as follows: 

Rank Food item Serving size Selenium content
1 Brazil nuts 1 oz* 543 mcg
2 Rice, white, long grain 1 cup 44 mcg
3 Egg noodles, cooked 1 cup 38 mcg
4 Wheat germ, toasted 1/4 cup 28 mcg
5 Sunflower seeds 1/4 cup 25 mcg
6 Tuna, canned 1 oz 22.79 mcg
7 Oysters, raw 1 oz 18.67 mcg
8 Chicken, breast, baked 1 oz 13 mcg
9 Shrimp, cooked 1 oz 11.23 mcg
10 Bread, whole-wheat, soft 1 slice 10.38 mcg
*1 oz = 28.35 g

As one can see from the table, there are many kinds of foods that are good sources of dietary selenium. In order to maintain a healthy body, a balanced diet is essential.

Note: An educational article about foods and nutrition by Pattamapan Lomarat (B.Sc. in Pharmaceutical Sciences), Department of Food Chemistry. Edited by N. & V. P. of U.K.

1. Acts as a Powerful Antioxidant

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Antioxidants are compounds in foods that prevent cell damage caused by free radicals.

Free radicals are normal byproducts of processes like metabolism that occur in your body on a daily basis.

They often get a bad rap, but free radicals are actually essential for health. They perform important functions, including protecting the body from disease.

However, things like smoking, alcohol use and stress can cause an excess of free radicals. This leads to oxidative stress, which damages healthy cells (1).

Oxidative stress has been linked to chronic diseases like heart disease, Alzheimer’s disease and cancer, as well as premature aging and the risk of stroke (23456).

Antioxidants like selenium help reduce oxidative stress by keeping free radical numbers in check.

They work by neutralizing excess free radicals and protecting cells from damage caused by oxidative stress.

2. May Reduce the Risk of Certain Cancers

In addition to decreasing oxidative stress in the body, selenium may help lower the risk of certain cancers.

This has been attributed to selenium's ability to reduce DNA damage and oxidative stress, boost the immune system and destroy cancer cells (7).

A review of 69 studies that included over 350,000 people found that having a high blood level of selenium defended against certain types of cancer, including breast, lung, colon and prostate cancers (8).

It’s important to note that this effect was only associated with selenium obtained through foods, not supplements.

However, some research suggests that supplementing with selenium may reduce side effects in people undergoing radiation therapy.

For example, one study found that oral selenium supplements improved overall quality of life and reduced radiation-induced diarrhea in women with cervical and uterine cancer (9).

3. May Protect Against Heart Disease

A diet rich in selenium may help keep your heart healthy, as low selenium levels have been linked to an increased risk of coronary artery disease.

In an analysis of 25 observational studies, a 50% increase in blood selenium levels was associated with a 24% reduction in coronary artery disease (10).

Selenium may also lower markers of inflammation in the body, one of the main risk factors for heart disease.

For example, a review of 16 controlled studies including over 433,000 people with coronary heart disease showed that taking selenium supplements decreased levels of the inflammatory marker CRP.

Additionally, it increased levels of glutathione peroxidase, a powerful antioxidant (11).

This indicates that selenium may help lower heart disease risk by reducing inflammation and oxidative stress in the body. Oxidative stress and inflammation have been linked to atherosclerosis, or the buildup of plaque in the arteries.

Atherosclerosis can lead to dangerous health problems like strokes, heart attacks and coronary artery disease (12).

Incorporating selenium-rich foods into your diet is a great way to keep levels of oxidative stress and inflammation to a minimum.

SUMMARYSelenium may help keep your heart healthy by keeping oxidative stress in check and reducing the risk of coronary artery disease.

4. Helps Prevent Mental Decline

Alzheimer’s disease is a devastating condition that causes memory loss and negatively affects thinking and behavior. It is the sixth leading cause of death in the United States.

The number of people with Alzheimer’s disease is growing. Thus, finding ways to prevent this degenerative disease is imperative.

Oxidative stress is believed to be involved in both the onset and the progression of neurological diseases like Parkinson’s, multiple sclerosis and Alzheimer’s disease (13).

Several studies have shown that patients with Alzheimer’s disease have lower blood levels of selenium (1415).

Additionally, some studies have shown that selenium in both food and supplements may improve memory in patients with Alzheimer’s disease (16).

One small study found that supplementing with one selenium-rich Brazil nut per day improved verbal fluency and other mental functions in patients with mild cognitive impairment (17).

What’s more, the Mediterranean diet, which is rich in high-selenium foods like seafood and nuts, has been associated with a lower risk of developing Alzheimer’s disease (1819).

SUMMARYA diet rich in selenium may help prevent mental decline and improve memory loss in people with Alzheimer’s disease.

Selenium is important for the proper functioning of the thyroid gland. In fact, thyroid tissue contains a higher amount of selenium than any other organ in the human body (20).

This powerful mineral helps protect the thyroid against oxidative damage and also plays an essential role in the production of thyroid hormones.

A healthy thyroid gland is important because it regulates metabolism and controls growth and development in the body (21).

Selenium deficiency has been associated with thyroid conditions like Hashimoto’s thyroiditis, a type of hypothyroidism in which the immune system attacks the thyroid gland.

An observational study of over 6,000 people found that low serum levels of selenium were associated with an increased risk of autoimmune thyroiditis and hypothyroidism (22).

Additionally, some studies have shown that selenium supplements may benefit people diagnosed with Hashimoto’s disease.

One review found that taking selenium supplements daily for three months resulted in lower thyroid antibodies. It also led to improvements in mood and general well-being in patients with Hashimoto’s disease (23).

However, more research is needed before selenium supplements can be recommended for those with Hashimoto’s disease.

SUMMARYSelenium protects the thyroid gland from oxidative stress and is necessary for thyroid hormone production. Selenium may be helpful for people with Hashimoto’s disease and other types of thyroid disease, but more research is needed.