The Importance of Vitamin A
PLEASE NOTE: The
Carrot Museum does not recommend self diagnosis or self medication. The
information contained in this web site has not been verified for correctness.
Some of the information contained herein is hearsay and may not be correct.
Use the information from this page only at your own risk! If in doubt
consult a doctor.
As discussed earlier Carrots are a super source of beta carotene which is converted to Vitamin A in the body as needed This page gives much more detail on the health benefits of Vitamin A. Click on a subject which interests you.
History of Vitamin
In 1906 the British biochemist Frederick Hopkins demonstrated that foods contained necessary "accessory factors" in addition to proteins, carbohydrates, fats, minerals, and water. In 1911 the Polish chemist Casimir Funk discovered that the anti-beriberi substance in unpolished rice was an amine (a type of nitrogen-containing compound), so Funk proposed that it be named vitamine for "vital amine." This term soon came to be applied to the accessory factors in general. It was later discovered that many vitamins do not contain amines at all. Because of its widespread use, Funk's term continued to be applied, but the final letter e was dropped.
In 1912 Hopkins and Funk advanced the vitamin hypothesis of deficiency, a theory that postulates that the absence of sufficient amounts of a particular vitamin in a system may lead to certain diseases. During the early 1900s, through experiments in which animals were deprived of certain types of foods, scientists succeeded in isolating and identifying the various vitamins recognised today.
Elmer V. McCollum and M. Davis discovered vitamin A during 1912–1914. In 1913, Yale researchers, Thomas Osborne and Lafayette Mendel discovered that butter contained a fat-soluble nutrient soon known as vitamin A. Vitamin A was first synthesized in 1947.
In 1912 E V McCollum from Wisconsin University developed an approach which opened the way to the discovery of all nutrients - the use of rats, because they were easy to maintain, inexpensive and could give quick results. Rats fed butter were healthier than those fed lard; butter contains more vitamin A.
Using rats he discovered the first vitamin
- A it was first called "fat soluble A".
So what is VITAMIN A- it is a pale yellow primary alcohol derived from carotene. It affects the formation and maintenance of skin, mucous membranes, bones, and teeth, vision and reproduction. In addition dietary Vitamin A, in the form of beta carotene, an antioxidant, may help reduce the risk of certain cancers. Read more about antioxidants here. Like all the other vitamins which are necessary for normal growth and maintenance of a healthy body, vitamin A must be included in small amounts in the diet. Man and all animals cannot make their own vitamin A, and so must receive the necessary quota from an external source.
We can get all our vitamin A ultimately from plants, which contain not vitamin A as such but vitamin A precursor substances, called carotenes.
Once the fruit or vegetable has been eaten and digested, a simple chemical reaction converts the carotenes into vitamin A proper. This chemical conversion takes place either in the intestinal wall or within the liver, which also acts as a store for about 90 per cent of the total vitamin A in the body. Man therefore gets his vitamin A by eating plants which are rich in carotenes or by eating those parts of domestic animals which are rich in the vitamin itself: liver, butter and cream from milk; egg yolk; and cod-liver oil.
It is highly recommended that vitamin A
be consumed from the diet rather than from supplements (particularly in the
case of beta carotene). Because vitamin A obtained from a varied diet offers
the maximal potential of health benefits that supplements cannot. The richest
sources of preformed vitamin A are liver, fish liver oils, milk, milk products,
butter, and eggs. Liver is an especially rich source because vitamin A is
primarily stored in the liver of animals and humans.
As you can see Vitamin A intake is essential
to human health. If you do not get enough then you could become A-deficient,
this is known as Hypovitaminosis A. One of the first evidences of vitamin
A deficiency may be noted in the difficulty of adjusting your sight in a
dim light, as in entering a dark room, or theatre, from a brightly lit hall.
A more serious symptom is the inability to regain focus of the road after
the glare of passing car headlights. This sight defect is known as "night
blindness" or "glare-blindness" and has been the cause of many serious accidents.
Vitamin A deficiency is the cause of xerophthalmia which can result in night blindness (difficulty in adapting to darkness), if this is not treated with Vitamin A it becomes irreversible. Other symptoms are excessive skin dryness; lack of mucous membrane secretion, causing susceptibility to bacterial invasion; and dryness of the eyes due to a malfunctioning of the tear glands, a major cause of blindness in children in developing countries. It maintains healthy body tissues and normal glandular functions. Vitamin A is necessary in the diet of expectant mothers to build up and maintain the health of both mother and the developing child. Carrots improve a variety of digestive disorders such as upset stomach, peptic ulcers, gastritis, Crohn's disease and celiac disease which is a defective digestion disorder found in children.
Vitamin A deficiency is quite rare in the western world. Who is likely to deficient? Those individuals who limit their consumption of liver, dairy foods and beta carotene containing vegetables. This map shows where the main areas of the world suffer from Vitamin A deficiency (source FAO/WHO full paper here) Also WHO Vitamin A deficiency page here.
According to the World Health Organization (WHO), "vitamin A deficiency (VAD) is the leading cause of preventable blindness in children." In fact, WHO estimates that 250 million young children, and an unknown but substantial number of pregnant women, are vitamin A deficient worldwide.
Vitamin A is essential in keeping the mucous membranes which line all the cavities of the body in a healthy condition. The mucous membrane may be described as consisting of two layers. The top layer, known as epithelium is composed of billions of cells. Close under the epithelium lies the mucous membrane, a thin layer of involuntary muscle fibres, which are extremely pliable and elastic. The vitality of this lining depends upon an adequate supply of vitamin A. When a deficiency of vitamin A exists the cells harden and degenerate into a horny rough surface, obstructing the normal function of the secretion of the disinfectant mucous. Infections resulting from vitamin A deficiency may affect the bladder or kidneys, the alimentary tract, the mouth, tonsils, sinuses, the tongue, ear canal, eyes or tear ducts.
In addition to those already mentioned, some of the symptoms which have been traced to vitamin A deficiency are: dry, scaly and rough skin, especially on the arms and legs; intestinal disorders and diarrhoea; poor appetite; retarded growth; lack of vigour; loss in weight and physical weakness; atrophy of the glands; poor development of teeth through defective formation of enamel and dentine and poor gum structure, and may also be a causative factor in cases of sterility; nerve degeneration and stones in the kidney and bladder. The vitamin can also ease alcohol withdrawal symptoms, fight bronchitis, reduce the risk of high blood pressure and reduce acne. Its antioxidant properties make it valuable in counteracting the onset of old age.
Vitamin A is soluble in fat but not in water. Consequently, surplus vitamin A is not lost in the body fluids, such as urine, perspiration, etc., but is stored for future use. Approximately 95% of all such surplus vitamin A is stored in the liver, with small amounts in the kidneys, lungs and under the skin. As a matter of interest ancient Chinese and Indian writings indicate eye disease was treated by squeezing the juice of liver on to the eye. Although not known at the time enough vitamin A (stored in liver) got into the eye to cure the disease.
The liver is capable of storing large quantities of vitamin A over a period of years to be called upon when needed. In order to keep this surplus at an adequate point, an ample daily intake should be provided. For example, an 8-ounce glass of fresh carrot juice will provide an average of 50,000 or more units of vitamin A. In this form the daily requirements are easily satisfied and any surplus is accumulated in the system to meet any emergency.
Absorption of vitamin A is dependent on fat from food sources and bile (cholesterol substance produced by your liver and stored by your gall bladder and used to break down fats, seeds and skins). Your body can store about a year's worth of Vitamin A in your liver. If you don't eat enough Vitamin A, it can cause blindness or in milder cases, your ability to see or drive at night. But it does need topping up.
The amount of carotene (which is converted
to vitamin A in the liver) contained in carrots is indicated by the depth
of the colouring matter. The large, firm, dark-yellow carrots, therefore,
should be selected for juicing, rather than the light-yellow ones, because
of their greater carotene content. You get approximately 10mg of Vitamin
from 20 average carrots.
Tests have shown that three percent of the total beta-carotene content is released from raw carrots when consumed in raw pieces. When homogenized (pulped) 21% was released. Cooking the pulp increased the accessibility to 27%. Addition of cooking oil to the cooked pulp further increased the released amount to 39%. (European Journal of Clinical Nutrition (2002) 56, 425–430- Estimation of carotenoid accessibility from carrots determined by an in vitro digestion method, Hedren et al)
Vitamin A can be found in animal livers, fish liver oils, and green and yellow fruits and vegetables. Foods that contain significant amounts include apricots, asparagus, beet greens, broccoli, cantaloupe, carrots, collards/kale, dandelion greens, dulse (a high fibre snack made from seaweed), fish liver and fish liver oil, garlic, mustard greens, papayas, peaches, pumpkin, red peppers, spirulina (a microalgae 60% all vegetable protein), spinach, sweet potatoes, Swiss chard, turnip greens, watercress, and yellow squash.
Vitamin A is also present in the following herbs: alfalfa, borage leaves, burdock root, cayenne (capsicum), chickweed, eyebright, fennel seed, hops, horsetail, kelp, lemongrass, mullein, nettle, oat straw, paprika, parsley, peppermint, plantain, raspberry leaf, red clover, rose hips, sage ,uva ursi, violet leaves, watercress, and yellow dock.
During the first five months of storage, carrots will actually increase their vitamin A content; and, if protected from heat or light, can hold their nutrient content for another two or three months. Since carrots are rich in beta carotene, steaming them makes this nutrient more readily availability to the body as heat breaks down the tough cellular walls that encase the nutrient. The crisp texture of carrots is the result of the cell walls being stiffened with the indigestible food fibers cellulose, hemicellulose, and lignin.
Pre-formed vitamin A occurs in foods of animal origin such as cod liver oil, beef liver, some seafood, butter, whole milk and egg yolks. It is sometimes added to milk. Main sources are:
Liver, fried 100g 10729 mcg RE
The body obtains vitamin A in two ways. One is by manufacturing it from carotene, a vitamin precursor found in such vegetables as carrots, broccoli, squash, spinach, kale, and sweet potatoes. The other is by absorbing ready-made vitamin A from plant-eating organisms. In animal form, vitamin A is found in milk, butter, cheese, egg yolk, liver, and fish-liver oil. Although one-third of American children are believed to consume less than the recommended allowance of vitamin A, sufficient amounts can be obtained in a normally balanced diet rather than through supplements. Excess vitamin A obtained through supplements can be toxic and can interfere with growth, stop menstruation, damage red blood corpuscles, and cause skin rashes, headaches, nausea, and jaundice.
In the body, proteins, carbohydrates, and
fats combine with other substances to yield energy and build tissues. These chemical reactions
are catalyzed, or accelerated, by enzymes produced from specific vitamins,
and they take place in specific parts of the body. The vitamins needed by
humans are divided into two categories: water-soluble vitamins (the B vitamins
and vitamin C) and fat-soluble vitamins (A, D, E, and K).
Fat-soluble vitamins seem to have highly specialized functions. The intestine absorbs fat-soluble vitamins, and the lymph system carries these vitamins to the different parts of the body. Fat-soluble vitamins are involved in maintaining the structure of cell membranes. It is also believed that fat-soluble vitamins are responsible for the synthesis of certain enzymes. The body can store larger amounts of fat-soluble vitamins than of water-soluble vitamins. The liver provides the chief storage tissue for vitamins A and D, while vitamin E is stored in body fat and to a lesser extent in reproductive organs. Relatively little vitamin K is stored. Excessive intake of fat-soluble vitamins, particularly vitamins A and D, can lead to toxic levels in the body. Many vitamins work together to regulate several processes within the body. A lack of vitamins or a diet that does not provide adequate amounts of certain vitamins can upset the body's internal balance or block one or more metabolic reactions.
Vitamin A is the name given to a group of compounds which have certain actions in the body. One of these compounds is called retinol and it is used as a standard against which the activity of other compounds can be measured. While the vitamin A we obtain from food comes in many different forms, these can be divided into two main types - pre-formed vitamin A and provitamin A.
Pre-formed vitamin A which is often in the form of retinol or retinal, is found in foods of animal origin such as liver and butter. Provitamin A is the name given to around 50 compounds in a group of plant pigments known as carotenes (or carotenoids), with beta carotene being the best known of these. This is because these compounds can be turned into vitamin A in the body.
Both pre-formed vitamin A and provitamin A are fat soluble. Vitamin A is essential to the human body, either directly or indirectly, for the function of all the organs in your body and is particularly important for growth and development. Despite the fact that vitamin A was the first vitamin to be discovered, its actions in the cells of our bodies are not well understood at a chemical level.
Maintenance of normal vision - Our eyes need vitamin A to function effectively as vitamin A is involved in the production of a chemical called visual purple, which helps us to see in dim light. Growth, repair and cell differentiation Vitamin A is necessary for the growth and repair of many body cells including those of bones, teeth, collagen and cartilage. It is also essential for a process known as cell differentiation in which unspecialised cells are modified so that they can perform specific functions. Thus vitamin A plays a central role in tissue development and maintenance.
Health of epithelial cells - Vitamin A is vital for the formation of healthy epithelial cells. These cells cover the internal and external surfaces of the body and are found in the skin, lungs, developing teeth, inner ear, cornea of the eye, sex organs, glands and their ducts, gums, nose, cervix and other areas. Many epithelial cells produce mucus which is necessary to lubricate body surfaces and protect against invading micro-organisms. For example, the good health of the digestive tract lining is important in protecting against ulcers, and maintenance of the lining of the vagina and uterus is important in fertility.
Pregnancy and fetal development - Because of its vital role in cell development and differentiation, adequate vitamin A helps to ensure that the changes which occur in the cells and tissues during fetal development take place normally. It may be involved in cell to cell communication.
Protection against infection - Known as 'the anti-infective vitamin', vitamin A plays an essential role in protecting your body from infection. It keeps body surfaces healthy so they can act as barriers to invading micro-organisms. Vitamin A stimulates and enhances many immune functions including antibody response and the activity of various white blood cells such as T helper cells and phagocytes. This immune-enhancing function promotes healing of infected tissues and increases resistance to infection.
Other actions - Laboratory experiments have shown vitamin A to have antiviral activity. Vitamin A also has antioxidant activity and has a role in protecting against free radical damage which contributes to many common diseases. Vitamin A is involved in iron metabolism and storage. Read more about antioxidants here.
Absorption and metabolism - The presence of fat and bile in the intestines is necessary for vitamin A absorption. Around 80 to 90 per cent of vitamin A in the diet is absorbed, although this is reduced in older people and those who have trouble absorbing fat, such as pancreatitis, celiac disease and cystic fibrosis sufferers, who may run the risk of vitamin A deficiency. Vitamin A is joined to fatty acids in the intestinal lining, combined with other substances and transported to the liver, which stores 90 per cent of the body's vitamin A.
The World Health Organization estimates that as many as 250 million children worldwide are threatened by vitamin A deficiency. However, it is relatively rare in developed countries and is usually limited to those who have absorption difficulties, liver disease or who drink a lot of alcohol. Vitamin A deficiency is common in alcoholics and contributes to some of the disorders of alcoholism such as night blindness, skin problems, cirrhosis of the liver and susceptibility to infections.
Aspects of deficiency.
Eyes - One of the first symptoms of deficiency is night blindness due to lack of visual purple. Prolonged deficiency leads to xerophthalmia, a condition in which eyes become dry, ulcers appear on the cornea, the eyelids become swollen and sticky, and which eventually leads to blindness. Vitamin A deficiency is the leading preventable cause of blindness in developing countries.
Skin - Prolonged deficiency leads to thickened dry skin which is prone to infections. Small hardened bumps of a protein known as keratin may develop around the hair follicles.
Growth - Deficiency causes growth retardation; weight loss; diarrhoea, thickening of bone shafts; congenital malformations; impaired hearing, taste and smell; wasting of testicles; and reduced sperm count. Inadequate vitamin A intake can lead to improper tooth formation in children and to gum disease.
Immune system - Epithelial surfaces are adversely affected by vitamin A deficiency, causing increased susceptibility to skin and respiratory infections. Immune cells and antibody functions are also affected which may lead to an increase in pre-cancerous cells in the epithelial tissues of the mouth, throat and lungs. Many studies have shown that vitamin A deficiency is associated with increased risk of infection in developing countries. This may also be the case in developed countries. A 1992 study involving 20 children with measles in Long Beach, California found that half of them were vitamin A deficient.
HIV - Vitamin A deficiency is often seen in HIV-positive people and this may be due to metabolic changes associated with HIV infection. A 1995 study done on HIV-infected drug users in the US found that there was a higher risk of death in those with vitamin A deficiency. Vitamin A deficiency is often seen in HIV-positive pregnant women and severe deficiency increases infant mortality and the risk of mother-to-child transmission of HIV.
Thyroid gland - A deficiency of vitamin A can contribute to lower levels of active thyroid hormone with symptoms of low body temperature, depression, difficulty with weight loss, headaches and lethargy.
Cancer - Many studies suggest that high blood levels of vitamin A can help prevent certain forms of cancer, particularly cancers of epithelial tissue. This may be due to the importance of vitamin A in maintaining healthy epithelial cells, strengthening the immune system and stimulating the response to abnormal cells. Several population studies suggest links between low vitamin A intakes and various types of cancer, particularly those of the lungs, head and neck. Vitamin A deficiency may also increase the risk of breast cancer. In a study published in 1997, researchers at Harvard School of Public Health compared the concentrations of various forms of vitamin A in the breast fat tissue from 46 cancer patients and 63 women with benign breast lumps. They found an increased risk of disease in those with low levels of vitamin A.
cancer - Results
of a US study reported in the Journal of the National Cancer Institute in
1997 suggest that the development of lung cancer may be due to a decreased
ability of cells to respond to vitamin A-related compounds known as retinoids.
When researchers at the University of Texas looked at the lung tissue from
79 patients with lung cancer and 17 without lung cancer they found that all
the healthy cells carried receptors that bound retinoids. However, only 42
to 76 per cent of the cancerous cells had this ability. Of the six different
types of retinoid receptors, three were found at lower levels in cancer
Lung function - Vitamin A supplements may be useful in treating chronic obstructive pulmonary disease. Researchers involved in a Brazilian study published in 1996 examined vitamin A levels in healthy non-smokers, healthy smokers, those with mild chronic obstructive pulmonary disease, and those with more severe symptoms. Patients in the last group had low vitamin A levels. Supplements of 1000 mcg RE (3330 IU) per day improved lung function.
Heart Disease - Low blood levels of vitamin A may be associated with the development of heart disease. Researchers involved in a 1997 study done in Madrid, Spain looked at vitamin A levels in 62 heart attack patients and compared these with levels in 62 people free of heart disease. The results showed that vitamin A levels in heart attack patients were almost 25 per cent lower.
Arthritis - Vitamin A levels have also been found to be low in rheumatoid arthritis and systemic lupus erythematosus sufferers.
What happens if you take in too much Vitamin A. you could develop vitamin A toxicity, known as Hypervitaminosis A. Excessive intake of Vitamin A in the form of beta carotene is not harmful, but will cause the skin to turn yellow. This can be reversed by consuming less beta carotene.
In contrast large doses of supplements of Vitamin A as retinol (10 to 15 times the RDA) is harmful and could result in fatty liver (hepatomegally), dry skin, nausea, vomiting, fatigue, weakness, headaches, anorexia and possibly an increase in birth defects among pregnant women.
This is reasonably common with as many as 5 per cent of people taking vitamin A suffering from the toxicity symptoms. Stopping the large doses usually reverses the symptoms with no lasting damage, although in children damage can be permanent.
Too much carrot juice and vitamin A supplements can kill you!
Though the yellowing of the skin from indulging in a heavy dose of carrots is seldom serious and will disappear in a few days, continued carrot gorging can cause medical problems. In 1974 one unfortunate English health advocate named Basil Brown consumed 10 gallons of carrot juice and took 10,000 times the recommended RDA of vitamin A in a period of 10 days. Those 10 days were the unfortunate man's undoing, his skin turned bright yellow and he died of severe liver damage.
Extract from "The Times" - Times 15 February 1974
Therapeutic uses of supplements
Immunity - Adequate vitamin A intake, either from diet or supplements, is very important in boosting immunity and preventing sickness and death in children. Many studies have found that vitamin A supplementation reduces the risk of infectious diseases in areas where vitamin A deficiency is widespread. A recent research review analyzing the results of several studies found that adequate vitamin A intake in children resulted in a 30 per cent decrease in deaths from all causes. Children in developing countries are often at high risk of vitamin A deficiency. In developed countries, ensuring adequate vitamin A intake is particularly important in those with life threatening infections such as measles and in those at risk of relative deficiency, such as premature infants.
Leukoplakia - Vitamin A has also been shown to exert protective effects against leukoplakia, a pre-cancerous change in mucous membranes. It often occurs in the mouth and throat and is related to smoking. In a study done in 1997 researchers tested the effects of the retinyl palmitate form of vitamin A on leukoplakia of the larynx. The treatment period was five weeks and the doses used ranged from 90 000 mcg RE per day (300 000 IU) to 270 000 mcg (1 500 000 IU) per day. Complete remission was observed in 15 out of 20 patients and partial response was seen in the remaining five patients.
Skin disorders - Because of its important role in the formation of healthy skin, vitamin A is used to treat skin disorders including rashes, ulcers and wounds. Acne Vitamin A may have some benefit in the treatment of acne although most of the studies that support the effectiveness of vitamin A have involved the use of very large doses. Synthetic vitamin A derivative drugs known as retinoids are used to treat cases of severe acne which have not responded to other treatment or which have only shown partial response to antibiotic therapy. These drugs, including tretinoin, isotretinoin and etretinate are prescription medications which are used under the supervision of a doctor. Side effects are relatively common and are similar to the symptoms of vitamin A toxicity. These drugs can cause birth defects if taken during pregnancy and should also be avoided by breastfeeding women. A 1995 study by National Cancer Institute researchers suggests that long-term therapy with etretinate may increase the risk of osteoporosis.
Psoriasis - Vitamin A derivative drugs are also used to treat psoriasis, particularly pustular psoriasis. These drugs help to normalize skin development by reducing the increased growth, turnover and keratinization of skin which occurs in the disorder. Similar cautions apply to the use of these drugs in the treatment of psoriasis. Skin aging According to a 1997 report in the New England Journal of Medicine, creams that contain the vitamin A derivative tretinoin, may help to combat premature skin aging. Researchers studied the activity of enzymes known as metalloproteinases which break down collagen and found that exposure to ultraviolet light increased the activity of these enzymes. Even a small amount of ultraviolet light, although not sufficient to cause redness, was enough to increase enzyme activity. This suggests that exposure to a few minutes of sunlight periodically over several years may lead to premature skin aging. This increase in enzyme activity was blocked by treatment with tretinoin before radiation. The researchers conclude that tretinoin may be useful in treating patients with signs of premature skin aging but note that careful monitoring of tretinoin use is essential as over-treatment can cause irritation and reddening of skin. The results of this study may lead to the development of new sunscreens and anti-aging creams containing vitamin A derivatives.
Other uses - Vitamin A eye drops have been used to treat eye problems including blurred vision, cataracts, glaucoma, conjunctivitis and dry eyes. Other disorders for which vitamin A has been tried include asthma, sebaceous cysts, fibrocystic breast disease and premenstrual syndrome. Vitamin A may also be useful in the prevention of ulcers. In a study involving almost 48 000 men reported in 1997 in the American Journal of Epidemiology researchers found a lower risk of ulcer in those with high intakes of vitamin A, either from food or supplements.
Recommended dietary allowances - Vitamin A is the name for a group of compounds which have the biological activity of retinol. Vitamin A is measured in retinol equivalents (RE) which allows the different forms of vitamin A to be compared. One retinol equivalent equals 1 mcg of retinol or 6 mcg of beta carotene. Vitamin A is also measured in international units (IU) with 1 mcg RE equivalent to 3.33 IU.
So what is your carotene
need? Meeting your vitamin A requirement from beta carotene is easy:
Eat a handful of baby carrots and you've done it! Six ounces of carrot juice
(made from two medium-sized carrots) supplies a whopping 28 mg. of beta
*RDA = Recommended Daily Amount (The RDA of Vitamin A is 800 micrograms for women and 1,000 micrograms for men.) For more information about Vitamin A click here