Selenium: Benefits, Deficiency Symptoms and the Best Sources

Selenium is a trace mineral that the body needs only in very small amounts, yet without which many essential processes would not function properly. Unlike calcium or magnesium, which you need in amounts of hundreds of milligrams, selenium is measured in micrograms (mcg) - a thousand times less.

Its main role is to be incorporated into selenoproteins, a family of about 25 proteins that contain selenium directly in their structure, in the form of the amino acid selenocysteine. The most important selenoproteins include glutathione peroxidase (a key antioxidant enzyme) and the deiodinases (enzymes that activate thyroid hormone).

Soil is the primary source of selenium for plants, and animals obtain it from the plants they eat. This dependence on soil explains why the selenium content of a plant food varies enormously by region - for example, soils in some parts of China or northern Europe are notoriously poor in selenium, while soils in South America (where Brazil nuts come from) are among the richest in the world.

According to the National Institutes of Health (NIH), selenium plays essential roles in reproduction, DNA synthesis, thyroid hormone metabolism, and protection against oxidative stress and infection.

The recommended daily allowance (RDA) for selenium varies slightly by age and physiological status. For most adults, the requirement is easy to meet through a varied diet, without needing a supplement.

Category	Daily requirement (mcg)
Children 4-8 years	30 mcg
Children 9-13 years	40 mcg
Adults (men and women)	55 mcg
Pregnancy	60 mcg
Breastfeeding	70 mcg
Maximum safe dose (UL) adults	400 mcg

It's important to remember the difference between the RDA (the requirement to avoid deficiency) and the UL (the threshold above which toxicity risk rises). Unlike vitamin C, for example, where the margin between a normal and an excessive dose is large, with selenium the margin is relatively narrow - about 7-fold between the RDA and the UL - which means unsupervised supplementation is riskier than with other micronutrients.

The thyroid is the gland with the highest selenium concentration per gram of tissue in the entire body, which shows how important this mineral is for its function. Selenium's role in thyroid health has two main components.

The first is activating thyroid hormone. The thyroid mainly produces thyroxine (T4), a relatively inactive form, which must be converted into triiodothyronine (T3), the biologically active form, in order to regulate cell metabolism. This conversion is carried out by enzymes called deiodinases, which are all selenoproteins - without enough selenium, the conversion of T4 into T3 slows down.

The second component is antioxidant protection. Thyroid hormone production involves generating hydrogen peroxide, an oxidative byproduct that can damage thyroid cells if not neutralized quickly. Glutathione peroxidase, the selenium-dependent antioxidant enzyme, protects the gland from this self-damage.

In Hashimoto's autoimmune thyroiditis, research shows mixed results: some studies report a reduction in anti-TPO antibodies after selenium supplementation, but the effect on long-term clinical symptoms remains uncertain. Always discuss with an endocrinologist before supplementing selenium for a diagnosed thyroid condition.

Selenium influences both innate immunity (the first line of defense, fast and nonspecific) and adaptive immunity (the specific response, with memory, mediated by lymphocytes). Highly active immune cells, such as macrophages and T lymphocytes, need large amounts of selenium to function optimally, because their intense activity generates a lot of oxidative stress that must be neutralized.

• Reduces excessive inflammation by supporting antioxidant enzymes that limit the damage caused by free radicals released during the immune response.
• Supports lymphocyte proliferation - the cells that recognize and destroy pathogens need selenium for efficient cell division.
• Influences viral virulence - one of the most studied examples is the observation that severe selenium deficiency in the host can allow certain viruses, such as Coxsackievirus, to mutate into more virulent forms, a phenomenon documented in regions affected by Keshan disease.

It's important to stress that these benefits refer to correcting an existing deficiency, not an extra "boost" effect in people who already have normal levels. Excessive supplementation does not strengthen immunity beyond the optimal level - in fact, high doses can be counterproductive.

The body constantly produces reactive oxygen species (free radicals) as a byproduct of normal energy metabolism. In small amounts, these play useful signaling roles, but in excess they contribute to oxidative stress, which damages DNA, proteins, and cell membranes and is involved in aging and many chronic diseases.

Selenium fights this process through glutathione peroxidase, a family of enzymes that neutralize hydrogen peroxide and other lipid peroxides before they cause cellular damage. Without enough selenium, these enzymes cannot form properly, and the body's antioxidant capacity drops.

Selenium works synergistically with vitamin E - both nutrients protect cell membranes from oxidation, but through complementary mechanisms: vitamin E is fat-soluble and acts directly within the cell membrane, while selenium acts through enzymes. This relationship explains why combined deficiency of the two nutrients is associated with more severe effects than deficiency of either one alone.

According to the Harvard T.H. Chan School of Public Health, research continues to explore selenium's role in preventing certain chronic conditions linked to oxidative stress, though evidence for supplementation in people without deficiency remains inconclusive.

Mild selenium deficiency is often subtle and nonspecific, which makes it hard to identify without a blood test. Possible signs include:

• Persistent fatigue and lack of energy, even with enough sleep
• Lowered immunity - more frequent or longer-lasting respiratory infections than usual
• Brain fog and difficulty concentrating
• Hair loss and brittle nails
• Symptoms resembling hypothyroidism - dry skin, cold sensitivity, slow metabolism

Severe deficiency, although extremely rare in Western Europe thanks to dietary diversity, has been historically documented in regions with very selenium-poor soil, particularly parts of China and Siberia. The two conditions classically associated with severe deficiency are Keshan disease, a potentially fatal cardiomyopathy that particularly affects children and young women, and Kashin-Beck disease, a degeneration of joint cartilage that appears in childhood and adolescence.

With a diet that typically includes fish, eggs, meat, and grains, severe deficiency is very rare, but people with very restrictive diets or digestive conditions affecting absorption should pay attention to their intake.

Selenium is found in a variety of animal and plant foods, but the concentration varies widely depending on the source and the soil of origin. Here is a general guide to the best sources:

Food	Serving	Selenium (mcg)
Brazil nut	1 nut (~5g)	70-90 mcg
Tuna (canned)	100g	~90 mcg
Sardines	100g	~45 mcg
Turkey	100g	~30 mcg
Beef	100g	~30 mcg
Chicken breast	100g	~22 mcg
Egg	1 large	~15 mcg
Mushrooms	100g	~10 mcg
Cooked brown rice	100g	~10 mcg

Brazil nuts are by far the most concentrated natural source - a single nut can meet or even exceed the daily requirement, depending on the soil the tree grew in, which makes the variance from one nut to another significant. Fish and seafood, especially tuna, offer a steady and predictable source. Values in plant foods (mushrooms, grains) vary the most, since they depend directly on the selenium content of the local soil.

Although severe deficiency is rare in countries with diverse diets, certain groups have a higher risk of insufficient intake:

• People living in regions with selenium-poor soil - some areas of China, Siberia, and parts of Europe (for example, Finland implemented a national soil selenium fertilization program back in 1984 precisely for this reason).
• Strict vegans and vegetarians in these regions, who do not eat fish, eggs, or meat - concentrated and reliable selenium sources.
• People with digestive conditions that affect nutrient absorption, such as Crohn's disease or celiac disease.
• Dialysis patients, in whom the dialysis process can remove selenium from the blood faster than it can be replaced through diet.
• Older adults, who often have a lower and less varied food intake.

If you fall into one of these categories, a blood test for selenium status can clarify whether supplementation is necessary, instead of guessing or supplementing preventively without evidence.

Yes, and this is one of the things that sets selenium apart from many other vitamins and minerals: the margin between a beneficial dose and a toxic one is relatively narrow. Selenosis (chronic toxicity) usually occurs at sustained intakes above 400-800 mcg/day.

Symptoms of selenosis include:

• A characteristic garlic odor on the breath and in sweat
• Hair loss and brittle, ridged, or peeling nails
• Nausea, diarrhea, and irritability
• In severe cases, nerve damage (numbness, tingling)

The risk of toxicity arises almost exclusively from uncontrolled or combined supplements, not from normal diet - it is practically impossible to reach toxic doses from food alone, except for excessive and constant consumption of Brazil nuts (more than 4-5 per day, daily, for a long time). According to the NHS, it is recommended not to exceed the maximum safe dose without medical advice and to read multivitamin labels, which may already contain selenium, to avoid overlapping doses.

For most people, the simplest and safest way to meet the selenium requirement is through a varied diet, not supplements. Here is a practical summary:

• Include fish 2-3 times a week - tuna, sardines, and cod are steady, reliable sources of selenium.
• 1-2 Brazil nuts per week (not daily, in large amounts) are enough to round out your intake, thanks to their extremely high concentration.
• Eggs and lean meat (chicken, turkey, beef) consistently contribute to the daily requirement, alongside quality protein.
• Avoid unsupervised supplementation - check whether the multivitamin you already take contains selenium, so you don't accidentally double the dose by adding a separate supplement.
• Test your selenium level only if you belong to a higher-risk group (strict vegan diet, digestive conditions, dialysis) or have persistent symptoms without a clear cause.

Selenium is a perfect example of the "more is not necessarily better" principle in nutrition - the body needs a precise amount, and both too little and too much can harm health. To track your micronutrient intake alongside protein, calories, and the rest of your goals, FitAzi can generate personalized meal plans that include varied selenium sources, without the risk of overdoing supplements.