Current evidence does not establish selenium deficiency as a direct cause of vitamin B12 deficiency. These nutrients have different roles in the body, and research has not shown that taking selenium reliably raises B12 levels. However, both can be low in people with limited diets or conditions that affect nutrient absorption. If you have symptoms or concerns, a clinician can assess B12 status and look for the underlying cause.
What does research say about selenium and vitamin B12?
Selenium and vitamin B12 are both essential nutrients, but they perform different jobs. Selenium is a mineral used to make proteins that support thyroid hormone metabolism and help protect cells from damage. Vitamin B12 is a vitamin needed for healthy red blood cells, DNA production, and normal nerve function. The National Institutes of Health (NIH) Office of Dietary Supplements describes selenium's key functions, but these roles do not establish that a shortage of selenium causes a shortage of vitamin B12.
Some research has examined selenium status alongside vitamin B12 and other nutrients. A randomized controlled trial involving 501 healthy older adults in the United Kingdom compared selenium supplements with a placebo over six months. The researchers found that selenium supplementation increased selenium levels but did not significantly change the measured vitamin B concentrations. The trial reported on PubMed therefore does not support using selenium supplements to improve vitamin B12 status.
This finding does not prove that the two nutrients can never be related in any clinical situation. It does, however, highlight an important distinction: a relationship between two nutrient levels does not necessarily mean that a deficiency in one causes a deficiency in the other. More evidence would be needed to establish a direct effect.
Why might selenium and vitamin B12 deficiencies occur together?
The two deficiencies can coexist when an underlying problem affects overall nutrition or the body's ability to absorb nutrients. For example, a study of people with gastrointestinal diseases found that low selenium levels were associated with reduced vitamin B12 absorption and other signs of poor nutrient absorption. The researchers concluded that selenium deficiency in this group was mainly related to malabsorption, meaning that the digestive system was not absorbing nutrients effectively. This study published on PubMed supports the possibility of overlapping deficiencies in people with significant digestive disorders, not the conclusion that low selenium directly causes low B12.
Which factors may contribute to low levels of both nutrients?
Several circumstances can affect nutritional status. Some influence both nutrients, while others are more specifically linked to vitamin B12.
- Digestive disorders: Conditions such as Crohn's disease and other intestinal disorders can interfere with nutrient absorption. Surgery involving the stomach or small intestine can also affect vitamin B12 absorption and, in some circumstances, overall micronutrient status.
- Limited food intake: A restrictive or poorly balanced diet can provide insufficient amounts of several nutrients. Vitamin B12 is naturally found mainly in animal-derived foods, although fortified foods and supplements can provide it. Selenium intake depends on food choices and, for many plant foods, the selenium content of the soil in which they are grown.
- Medicines that affect vitamin B12: Long-term use of certain medicines, including metformin and some acid-reducing drugs, can contribute to low B12 levels. These medicines are not, by themselves, evidence of selenium deficiency.
- Conditions that increase nutritional risk: Some chronic illnesses, major digestive problems, and situations involving restricted or medically provided nutrition may warrant closer assessment of more than one nutrient.
The UK National Health Service (NHS) identifies dietary insufficiency, problems affecting the stomach or intestines, and certain medicines among the causes of vitamin B12 deficiency. These factors deserve attention when evaluating low B12, regardless of whether selenium levels are also low.
What symptoms might occur if vitamin B12 is low?
Vitamin B12 deficiency can affect the blood and nervous system. The NIH Office of Dietary Supplements explains that possible effects include fatigue, anemia, and neurological changes. Some people develop symptoms gradually, while others may have low B12 without obvious symptoms at first. Examples of symptoms associated with vitamin B12 deficiency include:
- Persistent tiredness, weakness, or reduced energy.
- Numbness or tingling in the hands or feet.
- Difficulty with balance or coordination.
- A sore or red tongue.
- Changes in memory, concentration, or other aspects of thinking.
These symptoms are not specific to vitamin B12 deficiency, and selenium deficiency does not produce a reliable symptom pattern that can be used to identify low B12. Persistent or worsening symptoms, particularly numbness, balance problems, or changes in thinking, should be assessed by a healthcare professional. Neurological effects of untreated B12 deficiency can sometimes become irreversible.
How do clinicians check vitamin B12 status when selenium is low?
Vitamin B12 status is assessed separately from selenium status. A clinician may recommend a blood test to measure vitamin B12 and a complete blood count to check for changes in red blood cells. If the B12 result is borderline, a blood test for methylmalonic acid (MMA), a substance that can rise when the body lacks B12, may help clarify the result. The NIH Office of Dietary Supplements' vitamin B12 guidance describes these tests and notes that laboratory results must be interpreted in context.
The assessment may also consider diet, digestive symptoms, medical history, previous gastrointestinal surgery, and medicines that can affect B12 absorption. Depending on the circumstances, a clinician may order a separate selenium blood test. A low selenium result alone does not establish that vitamin B12 is low, and a normal B12 result does not rule out a selenium deficiency.
Can selenium supplements improve vitamin B12 levels?
There is no established evidence that selenium supplements reliably correct vitamin B12 deficiency. The clinical trial described above did not find a significant improvement in vitamin B12 concentrations after selenium supplementation. Taking more selenium is therefore not an evidence-based substitute for assessing and treating a confirmed B12 deficiency.
Excess selenium can also be harmful. Supplements may contain substantial amounts of this mineral, and Brazil nuts can provide particularly high quantities. The NIH Office of Dietary Supplements' selenium fact sheet describes the risks of excessive intake, including hair loss, brittle nails, digestive problems, and nervous system effects. For this reason, high-dose selenium supplements should not be started simply because vitamin B12 levels are a concern.
What is the most useful next step?
If you are concerned about either nutrient, focus on identifying the actual nutritional problem rather than assuming that one deficiency is responsible for the other. A balanced diet, including reliable sources of vitamin B12 and selenium, can help support adequate intake. People following vegan or highly restrictive diets may need a specific plan for obtaining vitamin B12, particularly through fortified foods or supplements when appropriate.
If blood tests have shown a deficiency, or if you have persistent fatigue, unexplained weakness, numbness, tingling, or balance problems, discuss the findings with a healthcare professional. Treatment should address the nutrient that is actually low and any underlying cause, such as inadequate intake, a medication effect, or impaired absorption. Correcting selenium alone may not resolve a separate vitamin B12 deficiency.
Key point: Selenium deficiency and vitamin B12 deficiency may occur together, especially when nutrition or nutrient absorption is affected. However, current evidence does not establish that low selenium directly causes low vitamin B12. Each nutrient should be evaluated on its own merits, with attention to the factors that may affect both.