Selenium: Benefits, Side Effects, Best Time to Take It & More

Selenium

(Se)

Evidence: Very High
Potential Benefits: Reasonable
Safety: Reasonable

What is selenium?

Selenium is an essential mineral necessary for proper immune and thyroid function. Selenium supplements are most helpful if you are deficient in this nutrient. If you already get enough from food, you should not take extra selenium as too much can be just as problematic as too little. This article is about the main benefits of selenium as a supplement, its potential side effects, the best time to take it, and more.

What does selenium do?

Selenium is required for nine different enzymes in the antioxidant system. In other words, we need this mineral as protection from oxidative stress. This is the underlying function that leads to most other benefits. For example, selenium is essential for male fertility because it protects sperm from oxidative damage. The more heavy metals, toxins, and pathogens in the body, the more selenium we need for its antioxidant properties.

Selenium helps protect the body from viral infections and when we do catch a virus, this mineral is responsible for minimizing the damage. We often get infected with viruses without noticing because selenium prevents these viruses from morphing into more dangerous strains. If the body doesn’t have sufficient levels of selenium, we are more likely to catch a virus and the virus gets to grow and morph into more dangerous versions.

Another important function of this mineral is supporting thyroid health. To create thyroid hormone, the thyroid gland needs to first make hydrogen peroxide. While hydrogen peroxide is necessary for this process, it can also damage the thyroid gland. The thyroid gland needs selenium to protect itself from this damage. In addition, selenium is needed to create three enzymes used outside of the thyroid gland to activate and deactivate thyroid hormone. This is why selenium supplementation can be effective for Hashimoto’s thyroiditis and Graves’ disease, two autoimmune conditions creating antibodies that attack the thyroid gland.

Potential benefits

It is important to note that all of the potential benefits mentioned below are dose-dependent. Generally, the higher the dose, the more pronounced the effects are. However, they do reach a plateau at a certain point. We will discuss the best dosing protocol later in this article.

Many possible benefits fall under the umbrella of ‘correcting a deficiency’ and are therefore not mentioned in the article. If you are deficient, selenium supplements can affect your cognition, performance, and overall health in a remarkably positive way.

Major benefits

Correcting or preventing a deficiency – Selenium deficiency can be terrible for your health and overall quality of life. Fortunately, a severe deficit is relatively rare. A mild deficiency is much more common and can lead to many problems mentioned later in the article. If you are experiencing symptoms of deficiency or know that your selenium levels are too low, helping you get back to the optimum is the main benefit of selenium supplements.

Minor benefits

Reduced symptoms of Hashimoto’s thyroiditis – Hashimoto’s thyroiditis produces antibodies that attack the thyroid gland. Several studies have shown that selenium supplements decrease antibody production in Hashimoto’s thyroiditis.
Reduced symptoms of Graves’ disease – Graves’ disease generates antibodies that cause the thyroid gland to make too much thyroid hormone. While the connection between selenium and Graves’ disease is not as researched as the effects of this mineral on Hashimoto’s thyroiditis, at least one study suggests an improvement.
Reduced symptoms of HIV – In HIV-infected patients, supplementing with 200 mcg/d decreases hospital admissions and stops the viral load from getting worse. In pregnant women with HIV, selenium supplements reduce the risk of death for the mother and child.

Possible side effects

These side effects are dose-dependent. The risk for them increases (often linearly but sometimes exponentially) as you increase the dose. Some of the side effects only apply to very high doses.

Diarrhea – Diarrhea caused by this supplement is not dangerous (assuming it doesn’t lead to severe dehydration) and usually only occurs if you take very high doses at once.
Nausea – Just like diarrhea, this side effect is not dangerous but can be unpleasant.
Increased risk for diabetes and cancer – Supplementation with 200 mcg/d reduced the risk of cancer in people who had low selenium levels, but it appeared to raise the risk of diabetes and maybe even cancer in people with already high levels of this mineral.
Selenium toxicity – While acute toxicity is rare, taking high amounts of selenium could lead to multiple health problems over time. Toxicity leads to similar problems as a deficiency. For example, those suffering from toxicity also experience their fingernails falling out. Their hair may also fall out, which is rare for those deficient in selenium. Toxicity is best documented in China, where coal pollution has caused some soils to be more than 1000 times higher than the absolute highest amounts found in the United States.
Contamination – Contaminated supplements are uncommon in the USA and other well-regulated countries. However, if you wish to buy products from China, India, or other countries without strict regulations, beware that the supplements may be contaminated. Either way, you need to choose the brand you order from wisely. It is highly recommended to check the certificates or read through some reviews for the specific product before you buy it. The FDA (Food and Drug Administration) has the power to regulate dietary supplements but can only do so after they have been on the market for a while. That’s because supplement companies are not obligated to announce to the FDA when releasing a new product on the market. The FDA has to discover the product and test it for potential impurities, which can take a lot of time. During that time, the supplement can be sold on the market even if it’s impure. Also, supplement companies can change their manufacturers without announcing it to the FDA.

Who should not take selenium?

You should probably avoid taking the supplement if you:

already consume enough selenium from food
experience a severe adverse reaction after taking the supplement
experience signs of toxicity
have blood levels higher than 120-140 mcg/L
are on a tight supplement budget (since there are more cost-effective supplements on the market for most people)

Who will benefit the most?

You should consider taking the supplement if you:

experience symptoms of deficiency, or you know you are deficient based on blood tests (90 mcg/L or less)
don’t get enough from food (while the content in food is heavily based on the soil, the easiest way to predict how much of this nutrient you are getting in your diet is to track your food intake for a while with Cronometer.com)
have a high amount of toxins and pathogens in the system
suffer from iron overload
are deficient in vitamin E (the mineral can replace the antioxidant functions of vitamin E to some extent)
often get sick from viral infections
suffer from Hashimoto’s or Grave’s disease
have a high budget for supplements (assuming that the supplement doesn’t contain impurities, it appears to be safe in most cases and rarely does more harm than good if you stick to regular doses)

Symptoms of selenium deficiency

The most common signs of deficiency include:

Getting sick more often (especially from viral infections)
Osteoarthritis
Faster aging
Thyroid-related problems
Hashimoto’s thyroiditis
Graves’ disease
Infertility (especially in males)
An increased risk of cancer
Fatigue
Brain fog
Fingernails falling out

Keep in mind that these signs are just indications of a deficiency. If you experience some or even most of them, it does not necessarily mean you need more selenium. On the other hand, some people may be deficient even though they are completely asymptomatic.

With that said, if you experience many or most of these symptoms, there is a high chance that you are not getting enough selenium. The more of these symptoms you have and the more severe they are, the more likely you are to have a deficiency.

Beware that selenium toxicity can lead to similar problems as a deficiency. For example, those suffering from toxicity also experience their fingernails falling out. Their hair may fall out as well, which is rare for those deficient in selenium.

Types of selenium

The different types of selenium supplements include:

Selenomethionine – This is the most popular type and the main form in plant foods. It is perhaps the best form to take since it’s very effective at preventing and treating a deficiency.
Selenium L-selenocysteine – Also known as Se-Methyl L-Selenocysteine, this is the worst form to take. Se-Methyl L-Selenocysteine is a detoxification product made by plants that are trying to get rid of selenium they have randomly taken up from the soil.
Sodium selenite – This form is less effective for treating a deficiency because it needs additional processing to get absorbed. Sodium selenite acts as a strong pro-oxidant and should be avoided.
Sodium selenate – This type leads to effects virtually identical to those of sodium selenite.
Selenium-enriched yeast – This type of yeast contains mainly selenomethionine but also other forms.

Of all these forms, selenomethionine is the best one to take.

How much selenium should you take?

The RDA (recommended daily allowance) for this nutrient is 55 mcg/d for both men and women. This amount should be enough for most people to avoid symptoms of deficiency. The RDA for pregnant and lactating women is 60 mcg/d and 70 mcg/d, respectively.

The RDA was set in the year 2000. Since then, more studies have come out suggesting that the RDA should be increased to 75 mcg/d. Cancer studies suggest that 120 mcg/d is the best amount to reduce the risk of cancer without increasing the risk of diabetes.

For most people, 75-120 mcg/d appears to be the best daily target. This amount should provide most of the benefits without meaningful side effects.

Beware that some things increase your need for selenium. These include Hashimoto’s thyroiditis, Graves’ disease, and iron overload.

The upper safety limit is 400 mcg/d. Higher amounts could increase the risk for diabetes and cancer. Taking more than 100-200 mcg daily should not be necessary and is not recommended unless you have a great reason to do so.

Because the optimal range is so strict and the selenium content in food varies so greatly, it is best to get your blood levels checked every once in a while. For plasma or serum selenium, the acceptable range is 90-blood levels checked mcg/L and the sweet spot is 100-120 mcg/L.

If you are unsure of your selenium status and want to be proactive about getting enough, taking 50 mcg/d would be the best choice. This is a good enough maintenance dose to keep you out of deficiency, and when combined with your diet, it will probably bring you close to the optimal target of 120 mcg/d.

Food sources of selenium

More than any other mineral, selenium content in food is greatly dependent on the soil.

Just in the US, soil in one place may contain 20 times more selenium than soil in another place. In China, the possible variation would be 450-fold instead of 20-fold.

Since plants don’t require selenium as an essential nutrient, they take it up randomly from the soil. Because of this, the variation can be as high as 100-fold.

Brazil nuts are by far the richest food source of selenium, but their content also varies greatly. One nut can contain anywhere from 8 mcg to 220 mcg. This is why it’s best to limit brazil nuts intake to 2 nuts a day to prevent the possible side effects of too much selenium.

Among plant foods, legumes, grains, nuts, and seeds tend to be good sources of selenium. Fruits, vegetables, spices, and mushrooms are usually low in this mineral. While mushrooms do contain a good amount of selenium, we only absorb around 5% of it.

In animal food, the variation is only 2-fold to 5-fold. Kidneys and other organ meats are the best animal sources, followed by cheese and eggs. 100-300 grams of these foods should provide enough selenium for the day. Other types of meat and dairy are usually poor sources of this mineral.

Seafood is also a good source but we only absorb around half of the selenium from seafood. The other half is bound to mercury.

The easiest way to find out how much selenium you are getting in your diet is with Cronometer.com – this free app allows you to track all vitamins, minerals, and more.

Best time to take selenium

The nutrient is fat-soluble, which means you should take it with food to absorb it well. Taking the supplement with food is also better if it causes stomach upset when you take it on an empty stomach.

Whether you take the supplement in the morning or the evening doesn’t matter. It does not interrupt your sleep in any way.

Interactions with other supplements

Vitamin B6
Vitamin B6 is needed to incorporate selenium into proteins.
Vitamin B12, folate, and choline
Methylation, which depends mainly on folate, B12, and choline, is required in two ways to support selenium status. On the one hand, methylation is needed to make a protein that delivers selenium to the brain and male reproductive organs and to prevent its loss in the urine. On the other hand, methylation is needed to get rid of excess selenium and prevent toxicity. So, we need methylation to make selenium both safe and effective.
Iodine
These two minerals act together to support thyroid hormone metabolism.

Where to buy selenium

Amazon seems to be the best option for ordering selenomethionine supplements in most countries. They offer some very affordable products backed by many positive reviews. Also, you can choose from a wide range of brands without having to search through other markets on the internet.

Beware some brands display the dosage per serving instead of per pill or capsule. Therefore, you may accidentally buy something less potent than you intended. Do not fall for this marketing trick.

FAQ

You can take the supplement daily and do not need to cycle it. However, it is certainly not a problem if you don’t take it daily. Not taking the supplement every once in a while could lead to better absorption.

All of these options are fine. The two most important things to consider are the price and dosing. Powders are almost always the cheapest form. However, you may need a highly accurate scale (preferably 0.001g) to dose them correctly. You can get one for as little as $20 from Amazon. Dosing the powders also takes some time that can add up over months or years. The disadvantage of capsules is that they often contain bovine gelatine, which rarely comes from an ethical source.

Yes. The nutrient doesn’t impact your sleep in any negative way in the short term.

The supplements should not become harmful once they exceed the expiration date, but they can lose potency over time.

Keep the supplements in a cold, dark, and dry place, and they should remain just as potent for many months or even years.

The supplement doesn’t directly reduce acne, unfortunately.

Yes. While acute toxicity is rare, taking high amounts of selenium could lead to multiple health problems over time. Toxicity leads to similar problems as a deficiency. For example, those suffering from toxicity also experience their fingernails falling out. Their hair may also fall out, which is rare for those deficient in selenium. Toxicity is best documented in China, where coal pollution has caused some soils to be more than 1000 times higher than the absolute highest amounts found in the United States.

Of all the supplemental forms, selenomethionine is the best one to take. This is the most popular type and the main form in plant foods. It is the best supplemental form since it’s very effective at preventing and treating a deficiency.

The most common reason people become deficient is that they don’t get enough of this nutrient from their diet. However, many things can hurt your selenium status and increase your need for this nutrient. These include Hashimoto’s and Graves’ disease.

You may need more of the nutrient if you:
– experience symptoms of deficiency, or you know you are deficient based on blood tests (90 mcg/L or less)
– don’t get enough from food (while the content in food is heavily based on the soil, the easiest way to predict how much of this nutrient you are getting in your diet is to track your food intake for a while with Cronometer.com)
– have a high amount of toxins and pathogens in the system
– are deficient in vitamin E (the mineral can replace the antioxidant functions of vitamin E to some extent)
– suffer from Hashimoto’s or Grave’s disease
– suffer from iron overload
– often get sick from viral infections

Most selenium you consume stays in your system for about 24 hours.

In the long term, it typically takes about a month of taking the supplement to begin noticing the benefits.

Most selenium supplements on the market are vegan-friendly but always check the label to be sure. If you buy the mineral in a capsulated form, beware that the capsules often contain animal-based gelatine. Instead, find a brand that puts its products into cellulose capsules.

The nutrient is fat-soluble, which means you should take it with food to absorb it well.

No, it doesn’t. The supplement does not interfere with fasting in any significant way. However, taking it with a meal is still better because dietary fat increases its absorption.

References

Most of the information provided in this guide is supported by scientific research that can be found and verified in the PubMed medical library. We highly encourage you to use the library to verify anything said in this article. We excluded from consideration studies that are either confounded or have a high conflict of interest.

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