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Can a Blood Test Detect Vitamin D Deficiency?

Vitamin D is a group of five closely related chemicals that play vital roles in human health. Technically speaking, vitamins D1 – D5 are secosteroids – a type of steroid or biologically active organic compound with a distinctive molecular structure.

Like all vital nutrients, it’s important to ensure that your body is supplied with a sufficient quantity, or your health will suffer.

Why is vitamin D so important?

But let’s start at the beginning. What exactly is a vitamin? They are ‘essential micronutrients’ – in other words, organic chemical compounds required by the body in very small quantities in order to function correctly and maintain health. Crucially however, in all but one instance they cannot be synthesised within the body and so must be sourced from food and drink.

That sole exception is vitamin D3, scientifically known as cholecalciferol. This is produced in the upper layers of the skin (the epidermis) on exposure to sunlight. For this reason some scientists argue that vitamin D3 is not truly a vitamin but is in fact a hormone: an organic compound involved in the transmission of signals from one part of the body to another.

Different vitamins play multiple roles within the body. D vitamins are involved in: 

  • The absorption of the minerals phosphate and calcium: these are fundamental building blocks of bones and teeth.
  • The absorption of the mineral magnesium: another very important nutrient which has multiple functions, including helping nerves and muscles to function correctly. That includes regulation of the heartbeat – the heart is the biggest muscle in the body after all. Magnesium also enables healthy immune system function and works alongside calcium and phosphate to ensure healthy bones (what does healthy mean?).
  • The production of biological energy from our food.
  • The maintenance of healthy levels of glucose and calcium. In the latter case, three way communication between the kidneys, skeleton and stomach takes place.

Vitamins D2 and D3 (learn the difference between vitamin D2 and D3) are the two most important forms for human health. Exposure to sunlight is the primary source of vitamin D for most people, as dietary sources are quite limited: primarily oily fish, egg yolk, fortified foods and a very small amount in some milk and red meat however this depends upon the season and is minimal.

What happens if you’re deficient in vitamin D?

A deficiency in vitamin D can have a serious impact on your health. Depending on the extent of the lack, you may experience such symptoms as:

  • Excessive tiredness
  • Muscle and joint pain
  • Frequent infections and illnesses
  • Slow wound healing
  • Brittle or weak bones

As we saw above, low levels of vitamin D can hinder the absorption of calcium, magnesium and phosphate, making it harder for the body to build healthy bones. In the event of a deficiency, vitamin D already laid down in bones can be diverted for use elsewhere in the body, further weakening the skeleton.

A vitamin D deficiency can manifest in such disorders as osteoporosis – bone weakening, and osteomalacia – bone softening, called rickets when it occurs in children as a result of malnutrition. Unsurprisingly, both conditions increase the risk of fractures and breaks.

Low levels of vitamin D have also been linked to an increased risk of several serious conditions, including asthma, multiple sclerosis, cardiovascular disease, and even some forms of cancer.

How vitamin D blood tests can help

Given these potential problems, vitamin D blood tests provide a quick and easy way to monitor and assess the levels of this vital nutrient in your body.

If your results reveal a low or deficient level of vitamin D, you will then be able to take positive steps to improve your levels by, for example:

  • Increasing your intake of foods naturally high in vitamin D – for example oily fish (salmon, sardines, pilchards, trout, kippers, herring and mackerel) and eggs.
  • Taking a vitamin D supplement.
  • Eating more foods fortified with vitamin D, such as certain breakfast cereal.
  • And last but certainly not least: increasing your exposure to sunlight.

How they work

Vitamin D blood tests work via laboratory analysis of a finger prick sample. They detect levels of an inactive form of vitamins D2 and D3 called 25-hydroxyvitamin D. Also known as 25(OH)D, this is later converted into a biologically active form of vitamin D.

If you’d like to gauge your levels, you can take a vitamin D test at home.

Sian Baker

Medically reviewed by Sian Baker, Dip ION mBANT mCNHC – Written by Beth Giddings.

Updated on 1st December 2021