Selenium is a nutrient essential to human health. And as an antioxidant, it may help fight off illness.
Some research suggests that a low selenium level might increase your risk of developing certain cancers. Many studies have been conducted to find out if there’s a link between selenium intake and breast cancer risk in particular.
Another area of research is whether selenium from dietary supplements has the same effect as selenium we get from food.
As is the case with many vitamins and minerals, it’s possible to get too much of a good thing.
Keep reading to learn more about how selenium may relate to breast cancer and what you need to know before taking selenium supplements.
Selenium is a natural trace element. Inorganic forms (selenite and selenate) are found in soil. Plants then convert it into organic forms (selenomethionine and selenocysteine).
Selenium is critical to good health, helping with:
- thyroid function
- immune system function
- reproduction
- DNA synthesis
- protection from free radicals and infection
Your recent selenium intake can be measured in blood and urine. Long-term intakes can be measured in hair and nails.
Your body doesn’t make any selenium on its own. You must get it from food, but you only need a small amount.
Your recommended daily allowance (RDA) depends on your age. It’s measured in micrograms.
| Age | RDA | Tolerable upper intake |
| birth to 6 months | 15 mcg | 45 mcg |
| 7 – 12 months | 20 mcg | 60 mcg |
| 1 – 3 years | 20 mcg | 90 mcg |
| 4 – 8 years | 30 mcg | 150 mcg |
| 9 – 13 years | 40 mcg | 280 mcg |
| 14 + | 55 mcg | 400 mcg |
| pregnant women | 60 mcg | 400 mcg |
| lactating women | 70 mcg | 400 mcg |
You can get your RDA from plants grown in soil that contains selenium, as well as from animals that eat those plants.
You’ll find it in foods such as:
- seafood
- meats
- cereals, breads, and other grain products
- poultry, eggs, and dairy products
Brazil nuts are particularly high in selenium. A single Brazil nut can have as much as 68 to 91 micrograms of selenium. However, regular high consumption of Brazil nuts or overall selenium can lead to selenium toxicity.
On the other hand, selenium deficiency can lead to:
- male infertility
- a type of heart disease called Keshan disease
- a type of arthritis called Kashin-Beck disease
In the United States, selenium deficiency is very rare. Most of us can get all we need from our diet because we eat foods from many sources.
Exceptions might be people who:
- are undergoing kidney dialysis
- have HIV
- eat only foods grown in local soil that doesn’t have enough selenium
Some foods have added selenium.
Selenium has some properties that may help prevent certain types of cancer. Exactly how it might do this isn’t clear. It may have something to do with antioxidant properties that reduce damage to DNA.
Some studies have found no association between selenium and breast cancer. Some have been inconclusive, but other studies seem to indicate some kind of association.
Here are a few:
- A 2017 study looked at the impact of low serum selenium on survival of women with breast cancer. The research suggested that a selenium level over 64.4 µg//L (micrograms per liter) might be beneficial for women in treatment for breast cancer. The study noted that selenium supplementation may be helpful, but more research is needed to confirm this.
- A 2016 meta-analysis suggests an inverse relation between selenium and total cancer risk, including breast cancer. The researchers found that selenium supplementation is not associated with cancer risk. They noted that different types of selenium supplementation may have different effects on health.
- A 2014 study indicates that when selenium is attached to a monoclonal antibody used to treat breast cancer, it’s better at destroying cancer cells in women who have developed resistance to chemotherapy.
- A 2010 study found that radiation therapy reduced selenium serum levels in women with breast cancer.
In 2013, the U.S. Food and Drug Administration (FDA) allowed dietary supplements with selenium to state, “some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer… FDA has determined that this evidence is limited and not conclusive.”
The agency notes that more research is needed to confirm the relationship between selenium and cancer risk, as well as whether supplements are helpful in preventing any form of cancer.
If you take a multivitamin with minerals, it may contain selenium. Selenium is also available as a stand-alone supplement.
If you have breast cancer, it’s important to speak with your oncologist before taking selenium or any other dietary supplements. Some can interfere with your treatment or cause other problems.
The FDA does regulate dietary supplements, but as food — not as drugs. That means they don’t go through the same safety and effectiveness requirements as medicines do.
So, if you do decide to take a supplement, be sure to seek out a trusted source. There have been instances where over-the-counter products were improperly marked and had very large amounts of selenium.
An example of this occurred in 2008 when at least 201 people had severe adverse reactions from a selenium dietary supplement. It had more than 200 times the amount shown on the label.
Some of the first signs that you’re taking in too much selenium are a metallic taste in your mouth or a garlic odor in your breath.
Chronic selenium intake is called selenosis. Some of the signs and symptoms are:
- facial flushing
- skin rash
- fatigue
- hair loss
- irritability
- lesions of the skin and nervous system
- lightheadedness
- mottled teeth
- muscle tenderness
- nail brittleness or nail loss
- nausea
- diarrhea
Be cautious if you plan on taking extra selenium in the form of dietary supplements.
Acute selenium toxicity can lead to:
- severe gastrointestinal and neurological symptoms
- respiratory distress syndrome
- myocardial infarction
- nervous system abnormalities, tremors
- kidney failure
- cardiac failure
- death
Selenium can also interact with some medications, such as the chemotherapy drug Cisplatin.
See your doctor right away if you’re experiencing symptoms of selenium overdose.
The research around selenium and breast cancer is intriguing, but many questions remain.
You probably get all the selenium you need from your diet. If you have questions about how much you’re getting or whether you should take supplements, it’s worth a discussion with your doctor or pharmacist.