| Health Mail Vol.1 Issue 3 | Title : Introduction to Vitamin A | 1/1 |
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Key Notes: Functionality: Sight, Skin, Mucous Membranes, Anti-infective, Protein Synthesis, Bones, Anti-anaemia, Growth, Anti-oxidant (carotenoids) Special Observations: Stored in liver; Has a half life of 200-300 days; Needs vitamins C and E and the mineral zinc; peroxide and free radicals destroys the vitamin Description Retinol and its various chemical variations (e.g. retinal and retinoic acid (5) represents the form of vitamin A the human body can use). The human body gets preformed retinal from animal food sources. Plant food sources can also provide the body with vitamin A. The Carotenoids form a large group of compounds many of which are vitamin A pre-cursors. The group is large; Leonard Mervyn(2) states that there are at least a hundred types of carotenoids; Rosemary Stanton (4) brings the figure to more than 600. She also points out that only 50 members of this large group can be converted into vitamin A (or retinol). The conversion happens in the human body in the intestines or the liver. The carotenoid with the highest conversion rate to retinol is beta-carotene. Other important carotenoids are alpha-carotene, alpha and beta cryptoxanthin, zeaxanthin, lutein and lycopene(4). For many years it was thought that the other carotenoids found in our diet were not important. However, where beta-carotene is the best source of vitamin A, other carotenoids [e.g.] are effective antioxidants. Here our references sources start to disagree. Mervyn (2) and Geddes (3) state that beta-carotene has anti-oxidant powers. Stanton writes that research has shown that beta-carotene taken as a supplement not only is ineffective in humans as an antioxidant but can even have deteriorating effects (4 p.25). The opposing views of different authors appears to be explained by Stanton having had access to the most recent research data, but we are not sure. However, the simplest way to navigate through these contradictions is by using a varied diet to get a variety of carotenoids rather than relying on supplements. This view is supported by an observation of Stanton (4) that just taking beta-carotene supplements may prevent absorption of other carotenoids. Vitamin a is fat-soluble. This means that excess amounts cannot easily be secreted and is only slowly secreted into the intestines via the gall bladder. 90% of the body's supply of vitamin A is stored in the liver, 1.5% in the blood and the rest in other tissues. The half-life of the vitamin is 200-300 days (3). Bioavailability A number of factors are involved in ensuring that vitamin A is absorbed properly. For vitamin A to be released from ingested food, pepsin and lipases are required in the small intestines. A certain amount of fat is needed in the diet to ensure proper uptake of both retinol and beta-carotene. R.Stanton mentions a daily intake of 10gms of fat to ensure a proper absorption (4). The presence of fat stimulates the production of bile, which in turn facilitates the absorption in the small intestines (2). The presence of vitamin E and C help to protect vitamin A. The presence of protein and zinc also improves bioavailabilty - zinc is important for vitamin A during absorption, transport and metabolism. Diseases of the liver (e.g. hepatitis and cirrhosis) and of the intestines (e.g. gastro intestinal infections) decreases the amount of vitamin A absorbed. The cholesterol lowering drug cholestyramine lowers the amount of absorbed vitamin A by 60%. Carotenoids are affected the same way as retinol. However, carotene encounters an additional barrier during absorption; carotenes are partially converted to retinol in the intestines. Mervyn states that this only gives an absorption efficiency of 40-60% with lower intakes and even less with higher intakes (2). Unfortunately, he doesn't quantify the low and higher intakes. See also 'losses in food processing'. Inborn errors in metabolism Mervyn mentions a condition where there is an inability to convert carotene to retinol. The condition is treated by giving preformed vitamin A (any form of retinol) (2). Losses in food processing The destruction of all types of vitamin A and carotenes are accelerated by peroxides and free radicals. These products are created from surrounding fat, particularly the polyunsaturated variety. High temperatures, oxygen, light, traces of iron and the presence of copper all enhance the formation of peroxides and free radicals, especially oxygen. Irradiation also has a negative effect. Mervyn gives an example of vitamin A in margarine:
Antioxidants like vitamin C and E acts as protective agents in the above processes. How does cooking affect vitamin A itself? Well, cooking does not affect retinol levels. Beta carotenes may leach into the water used for boiling but losses are not great. Alpha carotene is lost to a greater extent but beta-carotene and lycopene are made more available (2, 4). In canning processes, green vegetables loose 15-20% and yellow vegetables loose 30-35% of vitamin A activity. The drying of vegetables and fruit under controlled conditions give rise to a 20% loss. Drying using traditional open-air methods destroys all vitamin A activity (2). vitamin/mineral relationship The roles of vitamins C and E and the mineral zinc have already been discussed above. One role of zinc not mentioned is that the mineral is needed to free the vitamin from stores in the liver. Functionality Vitamin A is associated with the following functions - vision, cell division, cell differentiation[define] in embryo, surface layer of cells covering skin, mucous surfaces in organs and tissues, lining of blood vessels, sperm production, immune response, formation of bone, taste, hearing and appetite. Nielsen's book also mentions that the vitamin counteracts allergic reactions, that it activates the immune system (thymus activation and stimulation of lymphocytes)(1) and increases the concentration of oxygen in cells. Finally, carotenoids are anti-oxidants (possibly with the exception of beta-carotene (4)). Usage That vitamin A is an important supplement can be seen from information provided by UNICEF (10, 11). The organisation estimates that vitamin A deficiency (VAD) affects more than 100 million children and is responsible for as many as one out of every four child deaths the problem exists. The organisation also stresses that VAD causes not just blindness (the main complication associated with A) but that it also improves children's resistance to disease, and is key to improving their survival, growth and development. Also, Retinol palmitate is one of 6 vitamins featuring in the 1997 edition of the WHO's essential drug list (6). vitamin deficiency (1,3) Symptoms of deficiency include - increased susceptibility to infections e.g. viral and respiratory infections; scaly skin; inflammation of the skin; scaly and itchy scalp; weak nails; poor hair quality, poor sight, sensitivity to light, burning and itching eyes, pain in the eyeballs, dry eyes, eye ulceration Illnesses associated with vitamin A deficiency - night blindness, xerophthalmia, kidney stones, skin complaints, inflamed mucous membranes. vitamin toxicity (1,3) Loss of appetite, dry itchy skin, loss of hair, headaches, nausea and vomiting, soreness of arms and legs, red discolouration of gums. vitamin therapy (1,3) Skin cancer, Gastric ulcers, acne, eczema, psoriasis, night blindness Other uses (8) E160 - E161 - Because carotenoids are plant pigments they are used for red and yellow colouring. Examples: E160(a) (alpha-carotene, beta-carotene, gamma-carotene); E160(d) (Lycopene); E161(b) (Lutein). Dosage Units of vitamin A tend no longer to be measured in International units (IU) as the standard was ambiguous. 1 IU is equal to 0.3 microgram of retinol if the source is of animal origin and 0.1 microgram of retinol if the source is from a vegetable or fruit (7). Going from micrograms of retinol to IU, 1 microgram of retinol is equal to 3.3 IU of vitamin A activity from retinol and 10 IU of vitamin A activity from beta carotene. Measurements are instead done in micrograms of retinol equivalents. Table 1 shows the ratios between retinol equivalent and 3 sources of vitamin A.
The recommended daily intake varies from country to country and from age group to age group. The table below list values set by different countries:
Dosage covers requirements for 95% of population and ignores special requirements It is worth noting that these values are rough guides as they cover requirements for 95% of the population. Furthermore special requirements are ignored. Each country or organisation also produces a more detailed list.
The values are in micrograms of retinol equivalents. The values for Denmark have been converted from IU values. PRI EC = Population reference intakes European Community and lists the mean daily requirement plus 2 standard deviation levels Such variations will seem confusing, but it can be taken to mean that it is important to consider individual needs. In terms of trends for the RDI (also called RDA Recommended Daily Allowance in some countries) R.Stanton states that levels are decreasing as more research is done (4). Therapeutic dosages are where large amounts of the vitamin are given for a short period of time. Because the dosages are high, there is a risk of toxicity being introduced and high dosage products tend to be prescription only. Many believers in complementary medicine dislike the restrictions. Whichever way one looks at it, vitamin A does not readily leave the body, and it is therefore important to be careful. Nielsen (1) suggested a therapeutic dosage range of 50000-200000 IU (15000-60000 micrograms of retinol equivalents - assuming Nielsen referred to retinol activity). He further stated that toxic dosages only happened when taking more than 200000 IU for a prolonged period. Geddes' book (3) mentions UNICEF projects, acne and psoriasis. In the UNICEF projects, mothers are given 2000000 IU and children 100000 IU a week against Xerophthalmia and blindness. Acne has been treated with 100000-200000 IU a day for a few weeks. Psoriasis has been treated with dosages as high as 300000 IU per day (because of the high risk of toxicity, a new derivative of vitamin A has been made - isotretinoid). Sources The long list of food sources containing vitamin A compounds include - fish and fish oils, liver, dairy products, egg, carrots, sweet potatoes, tomatoes, spinach, celery, maize, bananas apricot and maize. Rosemary Stanton provides a more exhaustive list (4). Below we have listed some of the more interesting measurements. For dairy products it is worth observing that the skimming process of cow's milk cause a loss in the amount of retinal available. The variation between animal source (e.g. cow, sheep, goat) and production method is also interesting (e.g. Greek yoghurt and natural yoghurt). Soy beverage, is shown for comparison.
In eggs the difference between duck and chicken eggs is surprising. The variation in the quantities of vitamin A in liver is also interesting.
With regard to fish we were surprised at the variation in retinol between young and older eels. The differences between salmon types and type of preparation are also worth noting.
For fruit and vegetables the biggest surprise was the variation in lettuce. Iceberg has long been the standard lettuce to purchase. It is also ironic that the bits of lettuce with the most vitamin A are the bits that get thrown away (this act may be justified, but that is a separate matter).
Readers must remember that information on source may be useful but it is only a guideline. Apart from variations in soil types, climate and use of fertilisers, the press in the UK has from time to time mentioned claims that intensive farming methods (organic or non-organic) yield products containing lower amounts of essential nutrients like vitamin A. References:
Further reading: chemical structure: http://web.indstate.edu/thcme/mwking/vitamins.html#a To look at vitamin A biochemically: http://numbat.murdoch.edu.au/nutrition/vitA/vitaminA.html#what http://arbl.cvmbs.colostate.edu/hbooks/pathphys/misc_topics/vitamina.html A brief description of vitam A with some nice structural diagrams: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/misc_topics/vitamina.html A good review of vitamin A with lots of information: http://numbat.murdoch.edu.au/nutrition/vitA/vitaminA.html 3D Model of vitamin A: http://www.chem.ox.ac.uk/mom/vitamins/vitamina.html The pharmaceutical company Roche's on-line information on vitamin A: http://www.rocheconsumer.co.za/Vitaguide/vitamin_a.htm |
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