One randomised, double-blind trial carried out in South Africa investigated the effectiveness of vitamin A in treating young children who had measles complicated by pneumonia, diarrhoea or croup (a disease of infants and young children characterised by harsh coughing, hoarseness, fever and breathing difficulties). The researchers found that the children who received vitamin A supplements recovered more quickly from the pneumonia and diarrhoea, and had fewer symptoms of the croup than those taking a placebo. Of the 12 children who died, 10 were taking a placebo (N Engl J Med, 1990; 323: 160-4).
Another study involving young children looked at vitamin A as a potential weapon against malaria. Scientists at the Johns Hopkins Hospital in Baltimore, Maryland, gave children living in areas of malaria epidemics in Papua New Guinea either high doses of vitamin A or a placebo, every three months for more than a year. The results showed that the vitamin-A group had 30 per cent fewer cases of malaria than the placebo (control) group, leading the study’s researchers to conclude that the vitamin could help children boost their immunity after exposure to the malaria-inducing pathogen Plasmodium falciparum (Lancet, 1999; 354: 203-9).
Animal studies have demonstrated the role of vitamin-A deficiency in increasing susceptibility to more serious diseases such as cholera and Staphylococcus aureus-induced arthritis (Infect Immun, 1993; 61: 3952-7; Infect Immun, 1996; 64: 209-14).
The benefits of vitamin A also stretch to cancer prevention. Studies investigating nutrition and cancer have almost invariably demonstrated a relationship between a deficiency of the vitamin and development of the disease. It has been suggested that the role of vitamin A in regulating cell division, cell differentiation and cell death works not only to prevent the proliferation and spread of abnormal, cancerous cells, but also by ‘apoptosis’ – medicalese for ‘programmed cell destruction’ (Expert Rev Mol Med, 2004; 6: 1-23).
ATRA (all-trans retinoic acid) is a metabolic form of vitamin A that has built itself quite a solid reputation in helping to treat acute promyelocytic leukaemia (APL) (Blood, 2004; 104: 3490-3; Zhonghua Zhong Liu Za Zhi, 1993; 15: 125-9). Recently, a ‘drug’ known as Lipo-ATRA (essentially ATRA wrapped in fat) has been licensed for use in this form of leukaemia after scientists found that it successfully reversed the condition in 74 of 84 patients (88 per cent) – without a need for chemotherapy.
In addition, there is a raft of studies that highlights the efficacy of vitamin A in hindering or reversing the development of cancer in a wide variety of tissues, such as from the breast, ovaries, cervix, mouth, colon and lungs (Anticancer Res, 2004; 24: 807-9; 1779-83; J Exp Clin Cancer Res, 2004; 23: 309-16; Cell Mol Life Sci, 2004; 61: 1475-84; Med Oral, 2001; 6: 114-23; J Nutr, 1993; 123: 634-41).
Meat, oily fish, cheese, whole milk and eggs are among the best sources of retinol, the active form of vitamin A, which can be metabolised to retinal (used by the eyes) and retinoic acid. Liver is a particularly rich source of this nutrient as it is where this vitamin is stored in animals as well as in humans.
Vitamin A is not found in plants, but certain fruits and vegetables do contain yellow/orange pigments called ‘carotenoids’. These compounds are converted to vitamin A by the liver when its store of retinol is depleted. Beta-carotene is one such carotenoid, and is found in a wide array of foods, including carrots, mangoes, sweet potatoes, spinach, cantaloupe melons, broccoli, oatmeal, tomatoes, papayas, oranges and carrots. In addition to serving as a vitamin A ‘stand-by’ for when the liver needs it, beta-carotene also acts as an antioxidant to protect cells from damage by harmful free radicals.