Recommended Nutrient Intakes for Vitamin and Minerals

Vitamin A

Background

Vitamin A activity can be obtained from 2 classes of compounds, preformed Vitamin A (retinol and related compounds) and carotenoids (b-carotene and a few other carotenoids).

Carotenoids are pigments in plants and are usually yellow or red, or combine to make an orange colour. There are over 600 carotenoids, but only a few exhibit vitamin A activity. b-carotene is the most potent carotenoid.

Functions

•  Promoting vision
• Promoting cell differentiation (maintains the health of epithelial tissues and skin)
• Supporting the immune system
• Promoting growth and bone remodelling
• Reproduction

Recommended Intakes

Because the body can derive vitamin A either preformed or from b-carotene, its contents in foods and its recommendations are expressed as retinol equivalents (RE).

Table 1: Recommended intake of vitamin A (mg retinal equivalents per day)

Food Sources

The richest sources of preformed vitamin A are from animal sources (e.g. liver, fish liver oils, milk, and milk products, butter and eggs). Plants do not contain preformed vitamin A, but do contain provitamin A carotenoids, the red and yellow pigments of plants. Rich sources of these include some of the orange vegetables such as carrots and squash.

Deficiency

Vitamin A deficiency affects many systems in the body.

Eyes: When deficiency of vitamin A is marginal, individuals become less able to see in dim light because of low levels of vitamin A in the rods of the retina. This has been described as night blindness. With prolonged or severe vitamin A deficiency there are further changes to the eyes which are collectively referred to as xerophthalmia.  Corneal degeneration can cause blindness.

Bones and Teeth: Vitamin A deficiency can cause the cessation of bone growth, painful joints, impaired enamel formation, cracks in teeth, tendency to decay, atrophy of dentin-forming cells.

Blood: Vitamin A deficiency can contribute to a nutritional anaemia.

Skin: Hyperkeratosis is formed which is the plugging of hair follicles with keratin, which forms white lumps.

Digestive System: Vitamin A deficiency causes changes in lining and diarrhoea.

Immune System: Vitamin A deficiency can cause a suppression of immune reactions; frequent respiratory, digestive, bladder, vaginal, and kidney infections.

Nervous/Muscular Systems: The brain and spinal cord can grow too fast for the stunted skull and spine.

Toxicity

Early reports of vitamin A toxicity came from polar explorers who had to resort to eating the livers of polar bears and seals which are extremely rich in Vitamin A. Acute toxicity arises when more than 200mg is ingested by adults or more than half this for children. Symptoms include, vomiting, headache, blurred or double vision, vertigo, unco-ordinated muscle movements, increased cerebrospinal pressure, and skin exfoliation, and even death.

Chronic toxicity can be induced by consuming at least 10x the RDA for a month or more (i.e. 10mg for an adult). A range of symptoms have been reported with chronic vitamin A toxicity, including headache, bone and muscle pain, ataxia, visual impairment, skin disorders, alopecia, liver toxicity and hyperlipidaemia.  The most serious consequences of vitamin A toxicity occur in pregnant women during the first trimester. This can cause spontaneous abortion or fetal abnormalities including those of the cranium, face, heart, kidney, thymus, and central nervous system. Therefore pregnant women, or women who could become pregnant should not be exposed to retinoid therapy such as for skin treatment. Their daily source of vitamin A should not exceed 3mg. Usual sources of high intakes are from supplements. Concern has also been expressed regarding the consumption of liver during pregnancy.