Category: Natural chemicals
Introduction and description
Vitamin A is “a group of unsaturated nutritional organic compounds, that includes
- retinoic acid, and several provitamin A carotenoids, among which
- beta-carotene is the most important”.
It has multiple functions in the body. Vitamin A plays a role in a variety of functions throughout the body, such as:
Gene transcription - Vitamin A, and more specifically, retinoic acid, appears to maintain normal skin health by switching on genes and converting immature skin cells into mature epidermal cells
Immune function - A study in Burkina Faso, for example, showed major reduction of malaria morbidity with combined vitamin A and zinc supplementation in young children.
Embryonic development and reproduction - retinoic acid, reverses the growth-stunting effects of vitamin A deficiency, as well as early stages of xerophthalmia [inability to produce tears]. However, vitamin deprived rats show infertility (in both male and females) and continued degeneration of the retina, showing that these functions require retinal or retinol.
Bone and tooth metabolism - its formation, replacement and recycling
Hematopoiesis - the formation of blood cellular components
Skin and cellular health - Vitamin A also functions as an irreversibly oxidized form of retinol known as retinoic acid, an important hormone-like growth factor for epithelial and other cells.
Vitamin A can be found in two principal forms in foods:
Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance.
The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin, function as provitamin A in herbivores and omnivore animals, which possess the enzyme (15-15'-dioxygenase) which cleaves beta-carotene in the intestinal mucosa and converts it to retinol.
Carnivores are poor converters of carotenoids, so there is no point in giving your cat carrots thinking they will do it good. They won't.
Because Vitamin A is not one substance but several, in measuring Vitamin A content a system of equivalences has been used, an international unit (IU) equal to 0.3 μg of retinol, 0.6 μg of β-carotene, or 1.2 μg of other provitamin-A carotenoids and a new unit, the retinol activity equivalent (RAE) introduced in 2001. Each μg RAE corresponds to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of "dietary" beta-carotene, or 24 μg of the three other dietary provitamin-A carotenoids.
The absorption of provitamins depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin. More about this in a moment.
Vitamin A imbalance
Vitamin A deficiency is estimated to affect approximately one third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually. Approximately 250,000–500,000 children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Southeast Asia and Africa.
Due to the unique function of retinal, one of the earliest and specific manifestations of vitamin A deficiency is impaired vision, particularly in reduced light – night blindness. Persistent deficiency gives rise to a series of changes, the most devastating of which occur in the eyes. First there is dryness of the conjunctiva (xerosis) as the normal lacrimal and mucus-secreting epithelium is replaced by a keratinized epithelium. This is followed by the build-up of keratin debris in small opaque plaques (Bitot's spots) and, eventually, erosion of the roughened corneal surface with softening and destruction of the cornea leading to total blindness.
Other effects of deficiency include impaired immunity (increased risk of ear infections, urinary tract infections, Meningococcal disease and so on), skin diseases, tooth decay, maldevelopment of the fetus, plus many other problems related to its function.
Night blindness was recognized by the ancient Egyptians and Greeks, and many authorities from Galen onward advocated liver as a curative. Outbreaks of night blindness were linked to nutritional causes in the 18th and 19th centuries by von Bergen, Schwarz, and others. Corneal ulceration was reported in 1817 by Magendie .... Subsequently, corneal epithelial defects, often in association with night blindness, were recognized in malnourished individuals subsisting on diets now recognizable as deficient in vitamin A by Budd, Livingstone, von Hubbenet, Bitot, Mori, Ishihari, and others.
During World War I, Bloch conducted a controlled clinical trial of different diets among malnourished Danish children with night blindness and keratomalacia. In 1913, Ishihara proposed that a "fatty substance" in blood is necessary for synthesis of both rhodopsin and the surface layer of the cornea, and that night blindness and keratomalacia develop when this substance is deficient. ....By 1922 McCollum and colleagues distinguished two vitamins within the fat-soluble fraction, later named vitamins A and D......
Vitamin A deficiency remains a serious problem in developing countries as indicated by global surveys beginning in the 1960s. Millions of children were shown to be vitamin A deficient, with resultant blindness, increased susceptibility to infection, and increased childhood mortality. Beginning in the 1960s, intervention trials showed that vitamin A deficiency disorders could be prevented in developing countries with periodic vitamin A dosing, and in the 1980s and 1990s, large randomized, double-blind, placebo-controlled clinical trials demonstrated the marked efficacy of vitamin A supplementation in reducing childhood mortality.
Excess vitamin A, which is most common with high dose vitamin supplements, can cause birth defects. The fetus is particularly sensitive to vitamin A toxicity during the period of organogenesis.
Excessive dietary intake of beta-carotene can lead to carotenodermia, which causes orange-yellow discoloration of the skin. Otherwise the symptoms of Vitamin A overdose are those of poisoning.
Since vitamin A is fat-soluble, disposing of any excesses taken in through diet takes much longer than with water-soluble B vitamins and vitamin C. This allows for toxic levels of vitamin A to accumulate. The level at which poisoning can occur is lower for children and those with kidney disease.
Excessive vitamin A consumption can lead to nausea, irritability, anorexia (reduced appetite), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain, bone fractures, anaemia, diarrhoea, fever, insomnia, drowsiness, and 'altered mental status'. Chronically high doses of vitamin A, and also pharmaceutical retinoids can produce 'pseudotumor cerebri' associated with increased intracerebral pressure. This can cause hallucinations and other spiritual experiences.
Chronic intake of preformed vitamin A can cause osteoporosis and hip fractures because it suppresses bone building while simultaneously stimulating bone breakdown.
Causes of Vitamin A imbalance
- Intestine damage - In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal. If the intestine is not functioning properly vitamin A deficiency can result.
- Nutritional deprivation - Vitamin A is a fat soluble vitamin. It needs lipids. What are lipids? Lipids are a group of naturally occurring molecules that include fats - monoglycerides, diglycerides, triglycerides, phospholipids, and others. In other words we need fats to enable Vitamin A to be absorbed in the small intestine, a low fat diet, for example, results in poor use of vitamin A. If you glance at the list below of sources of Vitamin A you will see that a vegetarian or vegan in theory has nothing to worry about. But they do if they are also thinking they need to cut out fat.
- Zinc deficiency - can also impair absorption, transport, and metabolism of vitamin A because it is essential for the synthesis of the vitamin A transport proteins and as the cofactor in conversion of retinol to retinal.
- Liver disease or gall bladder malfunction – for example if the gall bladder has been removed. Both result in chronic malabsorption of lipids and impaired bile production and release. Bile is produced by the liver and aids the digestion of lipids in the small intestine. Bile is stored in the gallbladder and, when the organism eats, is discharged into the duodenum. Alcoholics with damaged livers are thus liable to get Vitamin A deficiency
- Pharmaceuticals - pharmaceuticals are not as strongly implicated in deficiency of Vitamin A as they are in other forms of Vitamin imbalance; having said this 44 drugs have so far been implicated in Vitamin A deficiency, for the full list see this LIST from eHealthme. The drugs include [this list is not exhaustive]:
- Skin and acne treatments
- Contraceptive pills
- Thyroid treatments
- Mouthwashes [inadvertently swallowed]
- Opioid based analgesics
- Muscle relaxants
- Osteoporosis treatments
- Pain killers and NSAIDs
- Proton pump inhibitors used for stomach problems
- Insomnia treatments
- Cancer treatments
- Obesity treatments - "Orlistat, a weight loss medication, can reduce the consumption of β-carotene by as much as 30%”
- Vitamin supplements – Although cases exists of people overdosing on carrot juice, the principal cause of overdose are supplements. Water-soluble vitamin A supplements are approximately 10 times as toxic as fat-soluble vitamin supplements A 2006 study found children given water-soluble vitamin A and D, which are typically fat-soluble, suffered from asthma twice as much as a control group supplemented with the fat-soluble vitamins.
Chronic, high doses of synthetic β-carotene supplements have been associated with a higher rate of lung cancer in smokers. Additionally, supplemental β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.
- Pharmaceuticals - some of which are based on Vitamin A, some of the pharmaceuticals in the list above appear to cause both deficiency and overdose
Sources of Vitamin A
The full list with all the sources of Vitamin A [excluding manufactured products that have only limited availability outside the USA] is shown in the science section Vitamin A food sources. The list was derived from the USDA Nutrients database. As you will be able to see the list is extremely long and contains rather a lot of duplication, as such if we summarise the contents, the main sources of Vitamin A are:
- Cod liver oil
- Offal - liver principally, giblets and kidney to a lesser extent
- Red peppers
- Grape leaves
- Fish - eel, tuna, sturgeon, cisco, caviar, herring
- Root vegetables - sweet potatoes, carrots, butternut squash, pumpkins, winter squash
- Brassicas and leafy green vegetables - kale, mustard greens, spinach, turnip greens, collards, mustard greens, lettuces, beet greens, swamp cabbage, swiss chard, chicory greens
- Dried fruit - apricots
- Cheese - goats cheese, cheddar cheese, cream cheese, roquefort, gruyere, port salut
- Herbs - parsley, marjoram, dill, cress, coriander, bay leaves, sage, chervil, basil, savory
- Eggs - principally the yolk [there may be some influence here from the feed given to the chickens]
- Red meat - notably lamb
- Malted drink - unspecified so it could be Ovaltine or Horlicks
β-Carotene contributes to the orange color of many different fruits and vegetables. Many yellow and orange fruits, such as cantaloupe, mangoes and papayas, and orange root vegetables such as carrots and yams contain beta-carotene.
How it works
Vitamin A heals, but via poisoning it can cause hallucinations and other forms of experience.
References and further reading
- Handb Clin Neurol. 2010;95:435-44. doi: 10.1016/S0072-9752(08)02129-5. Chapter 29: historical aspects of the major neurological vitamin deficiency disorders: overview and fat-soluble vitamin A. Lanska DJ. Department of Neurology, Veterans Affairs Medical Center, Tomah, WI 54660, USA. email@example.com
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