Vitamin A is a collective term given to a group of organic compounds that includes retinol, retinal and retinoic acid as well as several form of carotenoids. The two main form of vitamin A found in food are retinol (animal sources, yellow, fat soluble) and carotenes (mainly from plant sources).
In 1816 a French physiologist by the name of Francois Magendue noted that dogs deprived of nutrition developed corneal ulcers, became blind and died soon after. Although unknown to him at the time, this was the result of severe vitamin A deficiency, with the exact same symptoms in humans. Almost a century later Hopkins suggested compounds other than carbohydrate, fat and protein were found in milk and necessary for growth in rats. These ‘accessory factors’ were later named ‘vitamin A’ and classified as fat-soluble. Further experiments led to improved understanding of the structure and characteristics of vitamin A. This vitamin is now well-known to scientists, however there is still on going research with regards to optimum dosage and side-effects of toxicity.
Vitamin A function in our body
Proper vision
The retinol form of vitamin A is imperative for vision. In the eye retinol is bound the vital eye protein called opsin, for visual signals to be transmitted to our brain, the retinol is chemically modified to form rhodopsin. Without sufficient vitamin A this process cannot occur and blindness results. Vitamin A deficiency also greatly increases the risk of macular degeneration and cataracts—two leading causes of blindness in older adults.
Vital in pregnancy
Millions of women across the world suffer from vitamin A deficiency during pregnancy. Strongly linked to higher death rates in infants because of a compromised immune system increasing the incidence of infectious diseases such as diarrhoea, measles and respiratory infections. Retinoic acid is a key signalling molecule for the development of tissues and organs in the growing embryo. In cases of severe vitamin A deficiency birth defects and miscarriage can result. The mother’s breast milk is also enriched with vitamin A, and when the mother is deficient the infant will not be properly nourished.
Part of the immune response
Vitamin A has various roles in our immune system. Retinoic acid is needed to induce production of T cells that help to fight infections by responding to foreign bacteria and toxins. Moreover low vitamin A levels result in reduced numbers and activity of important immune cells such as goblet cells in the intestines and conjunctiva of the eye. Goblet cells are pivotal in maintaining the mucus linings of the organs in which they are present. They do this by constantly secreting mucus that not only traps foreign bacteria but also provide lubrication. It’s no wonder that vitamin A deficiency is associated with increased risk of a number of cancers, respiratory diseases and gastrointestinal problems.
Maintaining adequate intake
Vitamin A supplements are taken in the form of retinol that converts to its active form in the body. The recommended intake levels differ for each age group and in pregnancy. Try including a diet rich in vitamin A to prevent deficiency. Food sources of vitamin A include sweet potatoes, spinach, carrots, salmon, eggs and green vegetables such as broccoli.
References
1. http://ajcn.nutrition.org/content/71/5/1325s.full
2. http://cid.oxfordjournals.org/content/19/3/489.short
3. http://www.nature.com/nri/journal/v8/n9/abs/nri2378.html
4. http://www.karger.com/Article/Abstract/267958
5. http://www.nature.com/mi/journal/v8/n4/full/mi201532a.html
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