Vitamin A for good eyesight? Where is it needed everywhere? + Food sources

Vitamin A (axerophthol) is one of the essential micronutrients. The body cannot produce it on its own, so it must be replenished through the diet. What role does it play in our body and how can we find a balance in its intake?

To start with a little history...

The first information on the importance of vitamin A dates back to 1819, when physiologist Francois Magendie observed rapidly developing ulcers on the cornea of the eye in malnourished dogs.

However, it was not known which missing food component was the cause.

It was not until 1913 that this phenomenon was discovered. Researchers Elmer McCollum and Marguerite Davis, while investigating the function of dietary fats, accidentally discovered unknown compounds. They found that restricting the intake of these compounds caused a condition called xerophthalmia (drying of the cornea) in rats.

In the 1920s, these "unknown compounds" were identified as vitamin A. Gradually, knowledge of its effects began to spread. Today, it is known to have several indispensable roles in our bodies.

Vitamin A is the second oldest vitamin after vitamin B1.

What is the role of vitamin A in the body?

Vitamin A enters the body through the diet in two forms:

• from animal food as retinol or retinyl esters.
• from plant foods as vitamin A precursors

The precursors of vitamin A include the carotenoids beta-carotene, alpha-carotene and betacaryptoxanthin. These are converted into vitamin A in the body after dietary intake, depending on the body's actual needs.

The well-known and often mentioned beta-carotene is an antioxidant along with other carotenoids. Therefore, it has very beneficial effects even independently of its conversion to vitamin A.

Retinyl esters and vitamin A precursors are metabolised into retinol (referred to as vitamin A1) after entering the small intestine.

With the help of lipids, bile acids and pancreatic enzymes, retinol is transferred into enterocytes (blood cells). It is then packaged into so-called lipoprotein chylomicron particles. In this form, it is excreted into the lymphatic system and the bloodstream.

Retinol is the main active form of vitamin A. In the body, it is converted by chemical reactions into two other compounds - retinal and retinoic acid.

The most important site of vitamin A metabolism in the body is the liver. Two types of liver cells are important for this process - liver parenchymal cells (hepatocytes) and so-called star cells.

Hepatocytes are centrally involved in the absorption and processing of retinol in the liver.

The role of the star cells is to store retinol in the liver in the form of retinyl esters, which are reversibly degraded into retinol when needed.

Retinol, retinyl esters, retinal and retinoic acid are collectively referred to as retinoids.

Retinoic acid has the highest biological activity of vitamin A.

Why is vitamin A important?

1. It protects vision and eye health

Retinal, a compound converted from retinol, is a key molecule for the formation of the visual pigments rhodopsin and iodopsin.

Rhodopsin ensures that we can recognise objects even in low light or darkness. This pigment is regularly formed in the rods of the retina and is extremely sensitive to light.

Iodopsin is found in the retinal cones and is essential for colour vision in higher light intensities.

The depletion of iodopsin in the photochemical reaction of the eye is approximately 4 times slower than that of rhodopsin. This is also the reason why retinol deficiency in the body is a manifestation of glaucoma. Rhodopsin is then unable to replenish itself in the retinal rods and so-called night vision is limited.

In addition, vitamin A can slow down the age-related deterioration of vision, the so-called macular degeneration.

Macular degeneration is thought to result from cellular damage to the retina attributable to oxidative stress. Vitamin A may be beneficial in this process, particularly as an antioxidant.

2. Improves immune system function

Vitamin A also plays an important role in maintaining our body's natural defences.

An important metabolite of vitamin A in this process is retinoic acid, which promotes the growth, differentiation and distribution of T-cells.

T-cells are a type of white blood cell that protects the body from infection.

3. It promotes healthy growth and reproduction

Vitamin A is also essential for maintaining a healthy reproductive system in both men and women. It plays an important role in the development of sperm and eggs.

It is also essential for ensuring normal growth and development of the embryo during pregnancy. This effect is mediated primarily by retinoic acid.

4. Protects mucous membranes, promotes skin health

The differentiation and growth of epithelial cells throughout the body is mainly influenced by the presence of vitamin A.

Retinoic acid again plays a central role here. When it is deficient, the epithelial cells of the mucous membranes and skin are unable to renew themselves, flattening and accumulating keratin.

The consequence is reduced mucosal secretion, drying and keratosis of the tissues.

Vitamin A acts as an antioxidant in skin cells. It may therefore also be beneficial in slowing ageing and maintaining younger looking skin.

Also in the fight against acne...

Retinoic acid is a widely used molecule in the treatment of moderate to severe forms of acne. In dermatology, it is referred to as isotretinoin (13-cis retinoic acid).

It reduces the size and secretion of sebaceous glands, which in turn reduces the amount of acne-causing bacteria.

Internal treatment with isotretinoin is lengthy, laborious and not suitable for everyone. It is particularly dangerous for women who are planning a pregnancy in the near future or who are pregnant, as it is a drug with teratogenic effects.

Treatment with oral isotretinoin must be carried out under medical supervision!

In the case of acne treatment with retinoids, in addition to internal use, topical application directly on the skin is also used.

In addition to isotretinoin, the modern synthetic retinoid adapalene may also be the active ingredient in external preparations. These preparations are subject to prescription and treatment is indicated by a dermatologist.

Retinyl palmitate is mainly used in commercially available cosmetic preparations. It is broken down into retinol and metabolised in the skin to retinoic acid. Its use is recommended for acne, but also for ageing and maintaining the overall health of the skin.

During skin treatment with retinoids (both oral and topical) it is important to:

• Avoid extreme exposure to the skin (excessive sun exposure, tanning beds, cold wind, sauna, peeling).
• Use products with UV factor (50+).
• Make sure to moisturize the skin
• Make sure to follow a good drinking regime and a healthy lifestyle

Read more about acne and its treatment.

5. It affects iron metabolism

Vitamin A (specifically retinoic acid) is involved in the binding of iron with transferrin. This ensures that iron is further used in the body, for example in the synthesis of haemoglobin.

6. Regulates bone metabolism

Bone strength is influenced by a myriad of factors, including dietary factors. These include, in particular, adequate intake of calcium, vitamin D, vitamin K2 and regular exercise of a reasonable nature.

Until recently, vitamin A has not been associated with the regulation of bone metabolism.

However, there is now evidence that varying levels of retinoic acid in the body can suppress or promote osteoclast and osteoblast activity.

During life, bones are constantly undergoing a process of remodelling, which involves their resorption and subsequent rebuilding. During resorption, osteoclasts attach to the surface of the bone, destroying it and releasing calcium from it. Osteoblasts in the bone thus destroyed subsequently form new bone tissue and ensure its proper density.

Interestingly, both low and very high levels of vitamin A are associated with a risk of fractures. This means that in such cases, osteoclast activity is increased and osteoblast activity is reduced. Bone density is therefore reduced and fractures can occur more easily.

Thus, bone metabolism may be adversely affected by vitamin A both at reduced and increased levels in the body. The exact mechanism by which this occurs is still under investigation.

Therefore, it is also necessary to adhere to the recommended daily dose.

What is the daily dose of vitamin A?

The recommended daily dose of vitamin A is referred to as the so-called retinol activity equivalent (RAE). This is to account for the different bioactivity of carotenoids (provitamin A) and retinol itself in the body.

One mcg (microgram) of RAE is equivalent to:

• 1 mcg of retinol
• 12 mcg of beta-carotene from natural sources
• 24 mcg of alpha-carotene and beta-cryptoxanthin from natural sources
• 2 mcg of synthetic beta-carotene from dietary supplements

0.3 mcg RAE is equivalent to 1 IU retinol.

The table below shows the recommended daily allowance of vitamin A for healthy people.

The upper limit of daily vitamin A intake for an adult is 3000 mcg RAE. Higher doses already have very serious health risks.

Reference values for vitamin A (in the form of retinol) in a blood sample:

• Children aged 1-6 years: 0.7-1.5 µmol/l
• Children aged 7-12 years: 0.91-1.71 µmol/l
• Children aged 13-18 years: 0.91-2.09 µmol/l
• Adults: 1.05-2.09 µmol/l

In what foods is it found?

A varied and healthy diet provides our bodies with sufficient amounts of vitamin A.

The following table lists a selection of foods rich in vitamin A

Carrots with oil only...

Vitamin A is an organic lipophilic compound. It is soluble only in fats, which plays a major role in the way it is consumed.

For optimal absorption of vitamin A in the small intestine, it must be taken into the body together with fat.

Fat is important in facilitating the transfer of all forms of vitamin A from the lumen of the small intestine to the enterocytes.

Tip: It is always beneficial to drizzle a little oil over vegetable salads. For example, extra virgin olive oil is suitable, but you may also like linseed or pumpkin seed oil. If you like salad dressings, reach for those you can make yourself.

Vitamin A deficiency

Vitamin A deficiency is mainly caused by minimal consumption of foods that contain it.

It is commonly found in poor third world countries. However, it also occurs in developed countries.

A deficiency of vitamin A in the body is represented by a blood retinol concentration of less than 0.7 µmol/l. A very severe deficiency is represented by a concentration of less than 0.35 µmol/l.

Who is at risk of vitamin A deficiency?

• Premature babies
• People with cystic fibrosis
• People with poor eating habits or restricted food intake
• People with malabsorption syndrome

Zinc is an important molecule that promotes the absorption, transport and metabolism of vitamin A in the body. Therefore, zinc deficiency can cause subsequent vitamin A deficiency or lack of biological activity.

Clinical manifestations of vitamin A deficiency:

• Xerophthalmia - drying of the cornea, which can lead to blindness.
• Nyctalopia (glaucoma, impaired night vision)
• Irregular spots on the whites of the eyes
• Dryness of the skin, mucous membranes, nails and hair
• Skin defects
• Increased risk of infections
• Anaemia related to low iron-binding capacity
• Fractures

The solution to vitamin A deficiency in the body is primarily increased consumption of foods that are rich in it. At the same time, the underlying cause must be treated.

Dietary supplements containing vitamin A can only be beneficial in cases of severe deficiency. They should never be a substitute for a varied and diverse diet.

Can we have an excess of vitamin A in the body?

Yes. Vitamin A is fat-soluble and therefore takes much longer to be eliminated from the body than water-soluble vitamins.

In addition, if the intake of vitamin A exceeds the body's needs, it accumulates in the tissues and is stored in the liver.

Vitamin A overdose

Excess vitamin A associated with overdose is rare.

It can occur with inadequate intake of vitamin A from animal sources or from dietary supplements and medicines (e.g. oral retinoids).

Vitamin A overdose has never been observed with intake of vitamin A in the form of natural carotenoids from plant food sources.

Excessive intake of carotenoids can cause carotenodermia (harmless orange-yellow discolouration of the skin).

Clinical manifestations of overdose:

• Nausea, vomiting
• Blurred vision
• Headache, increased intracranial pressure
• Abdominal pain
• Hair loss
• Insomnia
• Fractures
• Liver damage

The ideal source of vitamin A for the body is a healthy and varied diet. A healthy and varied diet means a balanced combination of animal and plant foods. In this way, safe levels of vitamin A can be achieved without the health risks associated with deficiency or excess.