Retinol (Vitamin A)

What is retinol (vitamin A)?

Retinol is the biologically active form of vitamin A that circulates in the blood. Vitamin A is a fat-soluble vitamin, which means that the body can store it in the liver and fatty tissue. The liver acts as the main storage reservoir and releases retinol into the bloodstream in a controlled manner, bound to a transport protein called retinol-binding protein.

The body can obtain retinol in two ways. Preformed vitamin A, directly available as retinol, is found in animal products such as liver, fatty fish, eggs, and dairy. Plant-based sources provide carotenoids, such as beta-carotene, which the body converts into retinol itself. This conversion is less efficient, which is why people who eat an exclusively plant-based diet deserve extra attention when assessing their vitamin A status.

In the body, retinol performs functions that extend beyond a single organ. It is essential for the production of rhodopsin, the pigment in the rods of the retina that enables us to see in low light. In addition, retinol regulates cell division and maturation in virtually all tissues, and supports the mucous membranes that serve as a barrier against infections.

Why is retinol measured?

A retinol determination in the blood provides an indication of the available amount of active vitamin A. Doctors order this test when they want to rule out or confirm a deficiency or an overdose. The test is also relevant for conditions that impair the absorption of fat-soluble vitamins, such as chronic bowel diseases, celiac disease, Crohn's disease, or after gastric bypass surgery.

People who use high doses of vitamin A supplements for a long time are at risk of accumulation. For them, measurement can show whether the values ​​exceed a safe level. Conversely, in children with recurring infections or eye complaints, a low retinol level can be an indication that warrants further investigation.

The test is also used in the follow-up of certain liver conditions. Because the liver stores and releases retinol, liver diseases can disrupt retinol metabolism. A blood measurement then helps to make the clinical picture more complete.

What do abnormal values ​​mean?

Retinol levels that are too low

A reduced retinol level can indicate insufficient dietary intake, poor absorption in the intestines, or excessive consumption by the body during, for example, prolonged infections. The first symptoms of a deficiency are often subtle. Many people notice that they see less well in twilight or darkness, a phenomenon known as night blindness. This is one of the earliest and most characteristic signs of vitamin A deficiency.

With a persistent deficiency, the skin can become drier and the mucous membranes of the eyes and respiratory tract may perform their protective function less effectively. The immune system becomes weakened, causing infections to occur more frequently and last longer. In severe cases, the cornea of ​​the eye can be affected, a condition called xerophthalmia which can lead to blindness without treatment.

High retinol levels

Vitamin A poisoning, also known as hypervitaminosis A, is caused almost exclusively by prolonged excessive use of supplements or medications based on vitamin A derivatives. Eating a normal diet rarely leads to dangerously high levels, although regularly consuming large quantities of liver is an exception.

Symptoms associated with elevated levels may include headache, nausea, hair loss, dry or flaky skin, and bone pain. During pregnancy, elevated retinol levels warrant extra attention, as an excess of vitamin A can be harmful to the development of the unborn child. Women who are pregnant or planning to become pregnant are advised to be cautious with high doses of vitamin A supplements and liver products.

When is measuring useful?

Measuring is useful if there is a concrete reason to do so. Symptoms such as reduced vision in twilight, persistent skin problems, frequent infections, or eye irritation may be reasons to check retinol levels. Periodic monitoring is also advisable for people with a malabsorption disorder or after bariatric surgery, as their absorption of fat-soluble vitamins may be structurally reduced.

Anyone taking vitamin A supplements long-term would do well to have their levels checked before symptoms develop. This applies particularly to people who use both a multivitamin and a separate vitamin A supplement, as the combined intake can result in unintentionally high levels.

A blood test is a simple and reliable way to assess vitamin A status. The results provide guidance for adjusting diet or supplementation, always in consultation with a doctor or dietitian who can place the value in the proper context.