Vitamin A

Vitamin A can be found in two forms in food: 

• The active form (which the body does not have to convert) includes retinoids and comes from animal-based foods. Retinoids are called retinol and retinal.
• The provitamin form (which the body converts into an active substance) includes carotenoids. They can be found in fruits and vegetables that contain orange, yellow, and dark green pigments. Beta-carotene is the most commonly known carotenoid.

Vitamin A is liposoluble and can therefore be stored in fat. The vitamin A we consume is largely stored in the liver, which releases it into the blood to be delivered to the cells and tissues as needed. Liposoluble vitamins are easier to absorb when taken with food. 

Roles

What does vitamin A do? 

Vitamin A plays a critical role in: 

• Vision, particularly dark adaptation and the perception of shapes and colors;
• The normal growth of teeth and bones;
• Reproduction;
• Healthy skin and mucus membranes (scarring);
• Iron absorption;
• Regulating inflammatory responses;
• Helping the body to resist infections.

Beta-carotene, the precursor of vitamin A, helps slow the natural aging process of cells. Vitamin A also acts at the cellular level as an antioxidant, which means it protects against certain types of cancer. It is also used as the basis for medications called retinoids, which are used to treat severe acne and even some tumors. 

Needs

Vitamin A doses are expressed in micrograms and in retinol activity equivalents (RAE). 1 µg RAE = 1 µg of retinol = 6 µg of beta-carotene. 

Recommended Dietary Allowances for Vitamin A: 

Source: Health Canada, Multi-Vitamin/Mineral Supplement Monograph, October 23, 2007.

*RAE = Retinol activity equivalents

Sources

Vitamin A exists naturally in the form of retinol exclusively in animal-based foods. It can also be found in plant-based foods in its provitamin A form (beta-carotene). A fruit’s or vegetable’s beta-carotene content is proportional to the intensity of its coloration. 

Source: Health Canada, Canadian Nutrient File, versions 2001b and 2005; U.S. Department of Agriculture (USDA), National Nutrient Database for Standard Reference.

*RAE= Retinol activity equivalents.

Deficiency

In many developing countries, vitamin A deficiency is common, particularly in pregnant women and very young children. In the West, this problem is practically nonexistent. Vitamin A deficiency may occur in certain alcoholics or people with diseases that affect intestinal fat absorption, such as: 

• Cystic fibrosis;
• Chronic diarrhea;
• Liver disease and AIDS;
• Ulcerative colitis;
• Crohn’s disease.

People with any of these problems require medical intervention and must not, under any circumstances, attempt to treat themselves by taking a vitamin A supplement. Certain types of intestinal or pancreatic surgery may have the same effects. 

Smokers, people who regularly consume alcohol, and pregnant women have specific vitamin A requirements and must therefore more carefully monitor their intake.¹ 

Signs of vitamin A deficiency: 

• Hair loss;
• Growth problems;
• Dry skin and internal and external mucous membranes;
• Diarrhea;
• Chronic headaches;
• Abdominal, muscular, and joint pain;
• Increase in white blood cells;
• Nausea and loss of appetite;
• Osteoporosis;
• Vision problems that reduce night vision or that may cause corneal ulceration or irreversible blindness.

Adverse effects

Too much vitamin A is possible only through an excess of retinol, because the body transforms carotene into vitamin A only as needed. Doses of vitamin A 10 times higher than the recommended dietary allowance (RDA) are considered toxic. 

Signs of excess vitamin A: 

• Nausea and vomiting;
• Anorexia;
• Enlarged liver and spleen;
• Fatigue;
• Drowsiness;
• Headaches;
• Behavior disorders;
• Hypercalcemia (excess of calcium in the blood), which leads to brittle bones;
• Joint pain;
• Vision disorders.

Cons-indications

• Pregnancy: a daily intake of over 10,000 IU can cause birth defects. As such, Health Canada recommends that pregnant women taking multivitamin supplements refrain from taking more than one tablet a day in order to avoid exceeding the tolerable upper intake level of vitamin A. This warning does not apply to carotenoid supplements, including beta-carotene. Pregnant women should not eat liver regularly because it contains large quantities of directly bioavailable vitamin A.
• Pregnant women should avoid using retinoid-based skin products. This type of medication is reserved for the treatment of various serious skin disorders. Women of childbearing age who receive this treatment must use two reliable forms of contraception simultaneously.

Interactions

Natural health products or vitamin supplements:  

• none;

Medications that may hinder vitamin A absorption:

• Mineral oil;
• Oral contraceptives;
• Certain medications for treating gout, gastric acidity, epilepsy, and cholesterol problems.

Medications that could increase vitamin A absorption:

• Synthetic retinoids and medroxyprogesterone.

Additional information

If you take a vitamin A supplement, take it either one hour before or two hours after taking any of the types of medications mentioned above. 

¹Speak with your pharmacist if you plan to take vitamin A supplements. Your pharmacist can help you choose the solution that’s best for you based on your health and any drugs you take. 

Other names

Vitamin A, Retinol, Retinoic acid, Beta-carotene, ß-carotene