Vitamin A

Vitamin A is a micronutrient which is present in some food and is necessary for normal human function. It is present at relatively high levels in most Western diets and can be stored in the liver and intestines for long periods, making vitamin A deficiency rare.

Several studies have shown that vitamin A plays an important role in the immune system. Animal studies have shown that a shortage of vitamin A can cause immune problems and disease, and in areas where vitamin A deficiency is more common, such as parts of India, providing supplements of vitamin A has been shown to decrease childhood mortality from infectious disease.

Studies have suggested that some people with HIV have vitamin A deficiency and that this may be associated with lower CD4 cell counts and an increased risk of mortality. Vitamin A deficiency may be a secondary consequence of malnutrition in people with HIV or could be a consequence of HIV infection itself, since infection and fever are known to cause depletion of the vitamin. In test tube studies, vitamin A increases HIV replication in some cell types and decreases replication in others.

Vitamin A is sometimes referred to as retinoic acid, a substance into which it is converted in the body.^1,2,3,4,5

Vitamin A is naturally present at high levels in meat and liver, tomatoes, apricots, sweet potatoes, broccoli, kale, spinach, red peppers, carrots and lettuce. It can also be purchased as a supplement from health food shops. This supplement is toxic in doses above 25,000IU per day if taken for more than a few weeks. Signs of toxicity include headaches, nausea, blurred vision, sores on the skin and in more serious cases, internal bleeding, spontaneous fractures and bone malformations.

The safest way to supplement vitamin A intake is by taking beta-carotene. This is a substance which is converted into an active form of vitamin A in the body when it is needed, thus preventing overdosing. A study in HIV-negative people suggested that beta-carotene is best absorbed when split into three doses during the day and taken with fatty food. In individuals who have problems absorbing fat, it is possible to take vitamin A in a water-soluble form. Individuals with hypothyroidism, liver dysfunction or diabetes cannot convert beta-carotene into vitamin A.

Beta-carotene is usually taken with 'rest periods', such as taking it for 20 days out of every 30. The rest period is assumed to be necessary to avoid the gradual lessening of the immunomodulatory effects of the vitamin.

Doses of vitamin E is excess of 600IU per day have been observed to interfere with beta-carotene absorption and utilisation.

It is unclear whether taking supplements of vitamin A can reverse deficiency or improve the clinical outcome of people with HIV.

Vitamin A, taken in the form of beta-carotene, was shown to improve symptoms and stabilise CD4 cell counts in a small two-year study conducted amongst HIV-positive people in Italy. However, two other trials comparing high dose beta-carotene supplements to placebo found no evidence of any beneficial effects on immunologic or virologic measurements. For example, in one study 21 people took a high dose of 180mg beta-carotene per day, with no immunological or virological changes after four weeks. Furthermore, people with low baseline vitamin A blood levels were no more likely to have significant changes than people with normal baseline vitamin A levels.

Several studies have explored whether vitamin A supplementation in pregnant HIV-infected women reduces the risk of mother-to-infant HIV transmission. One study has suggested that HIV-positive pregnant women in Africa who have vitamin A deficiency may be more likely to transmit HIV to their children. However, low vitamin A levels did not predict HIV mother-to-infant transmission in American women, suggesting that vitamin A may not be a significant factor in industrialised countries.⁶ Moreover, taking too much vitamin A during pregnancy increases the risk that the child will have birth defects. More recently, a large randomised study of pregnant women in Tanzania found that vitamin A supplementation was associated with an increased risk of HIV transmission. In contrast, multivitamin supplements did improve the health of the babies, reducing rates of diarrhoea and boosting CD4 and CD8 cell counts.^{7 8}

Furthermore, two randomised studies of HIV-infected women in Kenya and Tanzania found that vitamin A supplementation had no impact on viral load in vaginal secretions, or on viral load and CD4 cell counts in the blood, HIV progression or the risk of sexual transmission.^{9 10} A similar study also showed no decrease in herpes simplex virus-2 (HSV2) shedding with vitamin A supplementation.¹¹

In HIV-negative people at risk of developing oral cancer, beta-carotene supplementation has been shown to reduce the size of pre-cancerous leucoplakia lesions.

The only side-effect of high doses of beta-carotene is the acquisition of orange skin tone. When this happens it means that fatty tissues are saturated with the vitamin.

Overlapping metabolic pathways mean that heavy use of alcohol and other drugs may interfere with vitamin A absorption. Furthermore, taking vitamin A supplement in the presence of alcohol and smoking increases a person's chances of liver damage, liver cancer and pulmonary cancer.¹²