Vitamin A

Figure 1. The chemical structure of Retinol (a form of Vitamin A).

Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. Vitamin A is a group of organic compounds that includes Retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably Beta-Carotene, an orange coloured pigment found in plants such as carrots). In food stuff, Vitamin A occurs naturally in two principal forms. Retinol, either as a free molecule or bound to a fatty acid, are found in animal-sourced foods. Caretonoids, such as alpha-carotene, beta-carotene, gamma-carotene, and the xanthophyll beta-cryptoxanthin, are found in plant-based foods.

Dietary Retinol is absorbed from the digestive tract. Unlike Retinol, Beta-Carotene is absorbed by enterocytes in cells by a membrane transporter protein scavenger receptor. Storage of Retinol is in lipid droplets in the liver. A high capacity for long-term storage of Retinol means that the body can go months on a Vitamin A and Beta-Carotene deficient diet, while maintaining blood levels in the normal range.

Vitamin A has multiple functions in the human body. Vitamin A is essential for embryo development and growth, for maintenance of the immune system (e.g. stimulating the production and activity of white blood cells), and for vision, where it combines with the protein opsin to form rhodopsin (the light-absorbing molecule necessary for both low-light and color vision). Vitamin A also takes part in remodelling bone, helps maintain healthy endothelial cells (those lining the body’s interior surfaces), and regulates cell growth and division such as needed for reproduction.

Vitamin A deficiency is common in developing countries, especially in pre-school-age children and pregnant women. Vitamin A deficiency has resulted in hundreds of thousands of cases of blindness and deaths from childhood diseases because of immune system failure.

Food Sources

Foods highest in Retional (or preformed Vitamin A) include:

• Egg yolks
• Beef liver
• Liverwurst
• Butter
• Cod liver oil
• Chicken liver
• Salmon
• Cheddar cheese
• Liver sausage
• King mackerel
• Trout

Foods high in provitamin A carotenoids (like Beta-Carotene) include:

• Sweet potatoes
• Pumpkin
• Carrots
• Kale
• Spinach
• Dandelion greens
• Collard greens
• Winter squash
• Cantaloupe
• Papaya
• Red peppers

Taken as a Dietary Supplement

As a dietary supplement, Vitamin A is commonly promoted as a strong antioxidant, to help reduce the risk of chronic diseases, to promote vision and eye health, to help protect against certain eye diseases such as age-related macular degeneration, to help reduce the risk of certain cancers, to help improve fertility (i.e. sperm and egg development), and to boost the immune system.   

The Recommended Dietary Allowance of Vitamin A for adults 19 years and older is 900mcg for men and 700mcg for women. The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health, and for preformed Vitamin A (Retinol) this level is 3000mcg. As Retinol is fat soluble, it can be absorbed by the body and stored in fat tissue or in the liver. If too much is stored, it can become toxic and may increase the risk of bone loss, hip fracture, certain birth defects, and interfere with beneficial actions of Vitamin D.

In contrast, provitamin A such as Beta-Carotene is not toxic even at high levels of intake. The body can form vitamin A from Beta-Carotene as needed, and there is no need to monitor intake levels as with Retinol.

Signs of Vitamin A toxicity may include:

• Vision changes such as blurry sight
• Bone pain
• Nausea and vomiting
• Dry skin
• Sensitivity to bright light like sunlight
• Hair loss
• Headache
• Liver damage
• Jaundice
• Decreased appetite
• Confusion
• Itchy skin

Background

Vitamin A is listed in the World Health Organization's List of Essential Medicines.