Vitamin A is a fat-soluble pigment, found in fish and dairy food in three main, isoforms – retinol, retinal and retinoic acid. It can also be made in the body from carotenoids, which are pro-vitamins of vitamin A. Alpha, beta and gamma-carotenes are found in fruits and vegetables also have direct health benefits of their own. Vitamin A plays a role in a variety of functions throughout the body. In the eye retinal is essential for the formation of the sensory rods and cones which explain the importance of retinal for night vision. Retinal and retinoic acid are important for normal function gene expression and healthy epithelial cell differentiation particularly of the skin and mucous membranes. Vitamin A, and more specifically, retinoic acid, appears to maintain normal skin health by switching on genes and differentiating keratinocytes (immature skin cells) into mature epidermal cells.
Recommended Daily Amounts of vitamin A in adults is between 600-900 micrograms/day with extra needed during pregnancy and lactation. Blood levels are reduced by malabsorption syndromes, and chronic exposure to oxidants, such as cigarette smoke, and chronic alcoholism. Zinc deficiency can impair the absorption and metabolism of vitamin A.
Vitamin A deficiency – Vitamin A deficiency is estimated to affect approximately one third of children under the age of five in developing counties. It is the leading cause of preventable childhood blindness and increases the risk of death from common conditions such as diarrhoea, malaria and HIV.
Vitamin A and cancer: A deficiency of vitamin A is likely to contribute to a lower immunity. In addition,laboratory studies have shown that carotenoids may have further direct anti-cancer effect. For example, prostate cancer cells grown increased apoptosis (programmed cell death) and reduced proliferation (growth) when exposed to synthetic retinoids. Likewise, in genetically susceptible mice, carotenoids reduced the incidence of prostate cancer. In humans, numerous large cohort studies including the prestigious EPIC and HPFS have linked the regular intake of carotenoid rich foods with a lower risk of breast, prostate, bowel, head and neck, oesophagus and lung cancer.
Vitamin A supplements and cancer: Despite the anti-cancer benefits of caroentoids, caution must be taken with extra supplementation. In a laboratory experiment, using rats with cancer, half had their diet supplemented with dried tomato powder and the other half with the carotenoid lycopene. After only a few weeks there was a measurable difference in the growth rate of their tumour in favour of the natural tomato powder but not the pure lycopene. Likewise is humans, although the HPFS and reported that regular intake of tomato rich (lycopene) foods over long periods of time were linked to a lower risk of prostate cancer, three prospective RCT of lycopene supplements, summaries in a Cochrane overview, revealed no benefit. In a landmark European study large group of individuals who had a previous cancer of the throat or were heavy smokers were given beta-carotene and vitamin E (alpha-tocopherol) or placebo (The CARET Study). After several years, involving thousands of participants, the trial was stopped because the results showed an increased risk of both lung and prostate cancer. Another large human dietary prevention study combined beta-carotene with retinol (The ABTC study), and showed complex but fascinating results which provides the best insight on the supplementation story. People who started the trial with naturally low blood levels of beta-carotene had lower levels of prostate cancer after years supplementation. Those people who had normal or high initial levels of beta-carotene at the start, eventually ended up with a higher risk of cancer, particularly prostate. This trial provides a clear take-home message – correcting a natural or acquired deficit is beneficial, but too much of a good thing, as in this case, is harmful.
Food sources of retinol (vitamin A):
This vitamin is mainly found in animal and daily products but ample vitamin can be formed in the body by the pro-vitamins, carotenoids (below) which also have additional direct health benefits. Common food sources include:
- Oily fish
- Cod liver oil
- Other fish oils
- Meat – beef, pork, lamb
- Chicken liver
- Milk, cream, yogurt and cheese.
Food sources of alpha, beta & gamma carotene (pro-vitamin A):
These are red, orange and yellow pigments are found mainly in fruits, vegetables, herbs and spices such as coriander, thyme, parsley, sage, chillies, turmeric and particularly paprika. Carotenoids not destroyed by the cooking process, so although, lycopene for example, are only found in relatively small quantities in tomatoes, higher concentrations can be found in tomato sauces and pastes. Many foods contain different types and quantities of carotenoids but the most common ones, within each category, include:
- Alpha, beta and gamma carotene (sweet potato, carrots, apricots, pumpkin, kale, peppers, mangoes, gac, papaya)
- Lycopene (tomatoes watermelon, pink grapefruit, guava, tomato source, tomato puree)
- Lutein (corn, eggs, kale, spinach, red pepper, paprika, pumpkin, oranges, rhubarb, plum, mango, papaya)
- Zeaxanthin (corn, eggs, kale, spinach, red pepper, pumpkin, oranges)
- Astaxanthin (salmon, shrimp, krill, crab)
Conclusion: Too little and too much vitamin A can be harmful. Aim for a balanced diet rich in healthy carotenoids. If concerned you can measure your micronutrient levels and adjust you diet accordingly