Magnesium, zinc and selenium turn up everywhere: in sports drinks, multivitamins, skincare adverts and gym conversations. They are genuinely important, but the body needs them in small amounts, and more is not automatically better. A magnesium, zinc or selenium blood test shows where your level sits at the moment of the sample, which is a more solid starting point than a supplement label.
This guide explains what each mineral does, why each one has two versions of the test, and when checking them tends to be useful.
Magnesium
What it does
The NHS explains that magnesium helps turn the food we eat into energy and helps the parathyroid glands, which produce hormones important for bone health, work normally. LML adds that it supports normal nerve and muscle function and a steady heart rhythm. Good sources include spinach, nuts and wholemeal bread.
Serum or whole blood?
The catalogue has two magnesium tests:
- Magnesium (Serum): the most widely used test. LML notes it is often requested when someone has muscle weakness, tiredness, an irregular heartbeat or tingling, or to monitor supplements.
- Magnesium (Whole blood): measures red cell magnesium.
Most of the body's magnesium sits inside cells and bone, and only a small part circulates in the serum. Reviews of magnesium measurement point out that this limits what a single serum result can say about total body stores (Reddy et al., 2018; Fiorentini et al., 2021). That is why results are best read together with symptoms, diet and other tests.
Too much
The NHS notes that high doses of magnesium supplements, more than 400mg, taken for a short time can cause diarrhoea.
Zinc
What it does
According to the NHS, zinc helps with making new cells and enzymes, processing carbohydrate, fat and protein, and wound healing. LML adds its role in the immune system and in normal growth and development during pregnancy, childhood and adolescence. Meat, shellfish, dairy foods such as cheese, bread and cereal products are good sources.
Serum or red cell?
- Zinc (Serum): LML collects this in a special trace-element tube (dark blue top), which is one reason it suits a collection point or home phlebotomist.
- Red Cell Zinc: measures zinc in whole blood. LML lists low zinc in association with reduced immune function, slower wound healing, hair loss and skin problems.
Zinc deficiency is described as widespread worldwide, and researchers have called for more attention to it at every stage of life (Lowe et al., 2024). LML lists situations linked with low zinc, including poor intake, regular heavy alcohol use, some bowel conditions such as Crohn's disease and coeliac disease, kidney disease and some medicines.
Too much
The NHS warns that high doses of zinc reduce the amount of copper the body can absorb, which can lead to anaemia and weaker bones.
Selenium
What it does
The NHS explains that selenium helps the immune system work properly, plays a part in reproduction and helps protect cells and tissues from damage. LML adds its role in thyroid hormone metabolism. Brazil nuts, fish, meat and eggs are good sources.
Serum or red cell?
- Selenium (Serum): reflects the selenium currently circulating.
- Selenium (Red Cell): LML describes it as a measure of selenium in red blood cells and an indicator of longer-term status.
A narrow window
Selenium is a good example of why more is not always better. Reviews describe a relatively narrow gap between too little and too much (Genchi et al., 2023). The NHS notes that excess selenium causes selenosis, which in its mildest form can lead to loss of hair and nails.
Who tends to check these minerals?
These tests are often used:
- by people on restricted diets, such as some vegan or very low-calorie diets;
- by people taking mineral supplements who want to check their level;
- when there are symptoms that a professional wants to look into, such as cramps or tiredness;
- by athletes and people training hard, as part of a wider look at nutrition.
Many people choose to check once a year as a general guide. That is not a rule, and a professional can tell you whether a single mineral, a combination, or none of these tests makes sense for you.
