Precursors of Vitamin A
Vitamin A can be synthesized by the body in several forms. Retinol, also called preformed Vitamin A, is probably the most usable form of Vitamin A and is found in animal foods such as eggs, liver, whole milk and fortified foods like fat-reduced milk, cereals and breads. Provitamin A carotenoids, which can be converted to Vitamin A, are the dark-colored pigments found in plant foods like fruits and vegetables (especially dark green leafy ones) and include spinach, cantaloupe, carrots, sweet potatoes and squash. Studies indicate that in the US, 26 % of the female diet is derived from these carotenoids, in comparison to 34 % carotenoid intake in the male diet.1
The most common carotenoids are beta-carotene, cryptoxanthin, lutein, zeaxanthin and lycopene. To date, 563 carotenoids have been identified but of those, less than 10 % have the ability to turn transform into Vitamin A. Of all the carotenoids, beta-carotene is the most efficient precursor to Vitamin A. Although both alpha-carotene and cryptoxanthin are converted to Vitamin A, they are only half as efficient as beta-carotene. As well, lutein, zeaxanthin and lycopene do not have the ability to convert to Vitamin A, but they have many other useful properties.
The most common carotenoids are beta-carotene, cryptoxanthin, lutein, zeaxanthin and lycopene. To date, 563 carotenoids have been identified but of those, less than 10 % have the ability to turn transform into Vitamin A. Of all the carotenoids, beta-carotene is the most efficient precursor to Vitamin A. Although both alpha-carotene and cryptoxanthin are converted to Vitamin A, they are only half as efficient as beta-carotene. As well, lutein, zeaxanthin and lycopene do not have the ability to convert to Vitamin A, but they have many other useful properties.
Dietary uptake of Vitamin A
According to the 2000 National Health and Nutrition Examination Survey (NHANES), most Americans are getting an adequate supply of Vitamin A in their diets. It is estimated that the average person is consuming approximately 3,300 IU per day whether it be from retinol or provitamin A carotenoids sources. There is no current RDA for beta-carotene or other carotenoids, but it is suggested that a daily diet containing 3 to 6 mg of beta-carotene (833-1,667 IU vitamin A) is sufficient to lower the risk of some chronic diseases. A vitamin A rich diet should include at least 5 or 6 servings of fruits and vegetables including the dark leafy vegetables like spinach and deep orange fruits like cantaloupe.
While a vitamin A deficiency is rare in countries such as the US where food sources are abundant and governments have elected to further fortify foods, incidences do occur, mostly though in cases of alcohol abuse and improper nutrition. In developing countries however, it is estimated that 250,000 to 500,000 children go blind each year as a result of a Vitamin A deficiency.2
While a vitamin A deficiency is rare in countries such as the US where food sources are abundant and governments have elected to further fortify foods, incidences do occur, mostly though in cases of alcohol abuse and improper nutrition. In developing countries however, it is estimated that 250,000 to 500,000 children go blind each year as a result of a Vitamin A deficiency.2
Signs of Vitamin A deficiency
According to doctors, night blindness is usually the first sign of a vitamin A deficiency. Ancient medical records show that even centuries ago, Egyptians commonly uses liver to cure this ailment. Researchers now know that a lack of vitamin A will cause the cornea of the eye to dry out, inflicting damage to both the retinal and the corneal cells. When ancient Egyptians ate liver, a rich source of vitamin A, they were inadvertently replenishing vitamin A supplies.
Modern science shows that vitamin A is necessary to regulate the immune system and is very important for infection resistance. Studies clearly indicate that when a deficiency is present the body is less likely to resist even common infections. This is further evidenced by cases in developing countries that show literally millions of children die needlessly each year from complications of infections such as measles.
Diminished vitamin A also lowers the ability of particular cells lining the lungs to fight off disease-causing micro-organisms, commonly resulting in pneumonia or other respiratory infections. As well, diarrhea and mild respiratory infections such as a cold can quickly take a turn for the worse in those with lowered immune function.
These children are also at risk for retarded bone growth and death from serious illness. To make matters worse, any stores of vitamin A are rapidly lost in cases of diarrhea, further complicating an already dire situation. In the US, the World Health Organization (WHO) and the United Nations International Children’s Emergency Fund (UNICEF) strongly recommend vitamin A supplementation for children diagnosed with measles and who live in a community where vitamin A intake is less than 1 %3.
Modern science shows that vitamin A is necessary to regulate the immune system and is very important for infection resistance. Studies clearly indicate that when a deficiency is present the body is less likely to resist even common infections. This is further evidenced by cases in developing countries that show literally millions of children die needlessly each year from complications of infections such as measles.
Diminished vitamin A also lowers the ability of particular cells lining the lungs to fight off disease-causing micro-organisms, commonly resulting in pneumonia or other respiratory infections. As well, diarrhea and mild respiratory infections such as a cold can quickly take a turn for the worse in those with lowered immune function.
These children are also at risk for retarded bone growth and death from serious illness. To make matters worse, any stores of vitamin A are rapidly lost in cases of diarrhea, further complicating an already dire situation. In the US, the World Health Organization (WHO) and the United Nations International Children’s Emergency Fund (UNICEF) strongly recommend vitamin A supplementation for children diagnosed with measles and who live in a community where vitamin A intake is less than 1 %3.
Vitamin A and Zinc deficiency are related
Commonly associated with vitamin A deficiency is a further deficiency in zinc, also available through natural food sources. This essential mineral is necessary to transport vitamin A from the liver to other body tissues. When in the liver, zinc synthesizes a substance in retinol, called retinal building protein (RBP), which in turn, carries vitamin A to depleted tissues within the body.
Researchers have been able to pinpoint particular precursors to a vitamin A deficiency, especially a low protein diet coupled with restricted caloric intakes, ultimately leading to poor zinc levels, all of which are necessary in the production of RBP.
Researchers have been able to pinpoint particular precursors to a vitamin A deficiency, especially a low protein diet coupled with restricted caloric intakes, ultimately leading to poor zinc levels, all of which are necessary in the production of RBP.
Fat malabsorption
Research has shown that a number of diseases which over time lead to fat malabsorption, can also result in a vitamin A deficiency. These include Crohn’s disease, an inflammatory bowel disease affecting the small intestine and in which 30-60 % of sufferers also experience chronic diarrhea, severely limiting their ability to absorb essential fats. As well, Celiac disease, a genetic disorder that results in an intolerance of the protein gluten, often leads to chronic cases of diarrhea, again interfering with fat absorption. Pancreatic diseases also lead to poor fat metabolism. Since the pancreas is responsible for the secretion of enzymes important in fat absorption, those who suffer from this disorder are at a greater risk of vitamin A deficiency.
Plant Vitamin A has antioxidant properties
Animal sources of Vitamin A are more easily absorbed by the body than plant sources, but the carotenoids found in plant sources have excellent antioxidant properties not found in animal sources. Numerous studies show the importance of antioxidants as protectors against free radicals, the harmful by-products resulting from oxygen metabolism. These free radicals are to blame for many chronic diseases and can be antecedents to cancerous cell development.
Vitamin A has proven essential for tissue repair and growth. Studies indicate that immediately following severe incidences of burns, blood levels of vitamin A are very low. Supplementation helps to boost the immune system and promote tissue regrowth.
Vitamin A deficiency is very common in those with Human Immunodeficiency Virus (HIV). Preliminary studies suggest that supplemental vitamin A may defer the progression from HIV is Acquired Immunodeficiency Syndrome (AIDS) as well as help with such symptoms as diarrhea. It was also shown that increasing levels of vitamin A could possibly prevent pregnant mothers from passing the virus on to their baby.4
Research continues on the effectiveness of Vitamin A in relation to a variety of diseases and conditions. Some studies suggest that vitamin A could possibly help women who have a history of miscarriage. As well, the vitamin may be helpful for Alzheimer ’s disease, in cases of tuberculosis, gingivitis, intestinal parasites, food poisoning, and ulcers of the stomach, small intestine, cornea and legs.
Vitamin A has proven essential for tissue repair and growth. Studies indicate that immediately following severe incidences of burns, blood levels of vitamin A are very low. Supplementation helps to boost the immune system and promote tissue regrowth.
Vitamin A deficiency is very common in those with Human Immunodeficiency Virus (HIV). Preliminary studies suggest that supplemental vitamin A may defer the progression from HIV is Acquired Immunodeficiency Syndrome (AIDS) as well as help with such symptoms as diarrhea. It was also shown that increasing levels of vitamin A could possibly prevent pregnant mothers from passing the virus on to their baby.4
Research continues on the effectiveness of Vitamin A in relation to a variety of diseases and conditions. Some studies suggest that vitamin A could possibly help women who have a history of miscarriage. As well, the vitamin may be helpful for Alzheimer ’s disease, in cases of tuberculosis, gingivitis, intestinal parasites, food poisoning, and ulcers of the stomach, small intestine, cornea and legs.
Vitamin A supplements and the risk of cancer
On the other hand, diets rich in vitamin A, especially beta-carotene are proving to lower the risk of many cancers including breast, colon, cervical and esophageal cancer. One cancer study of 29,000 chronic male smokers however, shows the incidence of lung cancer actually rises by 18 % in those receiving beta-carotene supplementation and the death rate also rises by 8 %.5,6
Another similar study was quickly halted after researchers discovered subjects had a 46 % higher risk of dying after taking 25,000 IU of retinyl palmitate. In cases of bone marrow disorders such as chronic myleogenous leukemia (3-5 % of all leukemia cases in children), a seven year study concluded however, that vitamin A, when used in conjunction with chemotherapy, actually improved survival rates.7
Studies into cases of osteoporosis also concluded that women whose dietary intake is higher than 1,500 mcg per day (twice the RDA) are at a greater risk of decreased bone density and hip fractures.8 Researchers however, did not find any association between beta-carotene (plant sources of vitamin A) and hip fractures or bone density loss, suggesting possibly that high intake of animal sources is not advisable.
On a smaller scale, vitamin A in the synthetic form retinol, is found in a growing number of oral and topical cosmetics and medicinal preparations. Sold by prescription only in some cases, these remedies are useful for acne and psoriasis and are showing great promise in cases of rosacea, warts and damage caused by sun exposure. Drugs such as Accutane ® and Roaccutane® prescribed for acne however, can be toxic in very high doses and should never be used by pregnant women.
Another similar study was quickly halted after researchers discovered subjects had a 46 % higher risk of dying after taking 25,000 IU of retinyl palmitate. In cases of bone marrow disorders such as chronic myleogenous leukemia (3-5 % of all leukemia cases in children), a seven year study concluded however, that vitamin A, when used in conjunction with chemotherapy, actually improved survival rates.7
Studies into cases of osteoporosis also concluded that women whose dietary intake is higher than 1,500 mcg per day (twice the RDA) are at a greater risk of decreased bone density and hip fractures.8 Researchers however, did not find any association between beta-carotene (plant sources of vitamin A) and hip fractures or bone density loss, suggesting possibly that high intake of animal sources is not advisable.
On a smaller scale, vitamin A in the synthetic form retinol, is found in a growing number of oral and topical cosmetics and medicinal preparations. Sold by prescription only in some cases, these remedies are useful for acne and psoriasis and are showing great promise in cases of rosacea, warts and damage caused by sun exposure. Drugs such as Accutane ® and Roaccutane® prescribed for acne however, can be toxic in very high doses and should never be used by pregnant women.
Recommended dose of Vitamin A
The recommended daily dose of vitamin A varies by age group, but on average for healthy females over 19, 2,333 IU of retinol is adequate and for males 19 and over, 3,000 IU is standard.Pregnant women should be very careful using synthetic forms of vitamin A because it is proven to cause birth defects. As well, too much vitamin A is very toxic and can cause liver damage and failure and in some cases, death. Symptoms of vitamin A poisoning include hair loss, irritated eyes, fatigue, nausea, diarrhea, dry skin and lips, muscle and joint pain and lingering headaches. Toxic doses in adults are 25,000 IU per day and 10,000 IU per day in children.
Vitamin A is found in a variety of supplements so those taking more than one should check the levels of the vitamin in each supplement to avoid overdose. There are several possible drug interactions associated with vitamin A, so it is recommended that those taking any medications, speak with a trained health professional prior to taking vitamin A supplements.
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