Vitamin A is a collective term for a variety of powerful antioxidants, known as retinoids, which are found in animal-based foods. Some plant carotenoid pigments can also be converted into vitamin A when needed. The best known benefits of vitamin A are in the eye, but vitamin A provides important health benefits throughout the body as it regulates the switching on and off of genes involved in growth and repair.
The health benefits of vitamin A
Vitamin A binds to receptors within the nucleus of cells to regulate the way in which genes are activated. It is vital for the production of numerous proteins, including enzymes, hormones and growth factors.
Vitamin A is needed to maintain:
- normal growth and development
- sexual health and fertility
- healthy skin, teeth, bones and mucous membranes
- healing of sores, wounds, and burns
- colour vision.
Within the eye, vitamin A is converted into a pigment, rhodopsin, which is also known as visual purple. When exposed to light, this pigment absorbs energy and stimulates nerve endings in the back of the eye (retina) that relay sensory information to visual cortex of the brain where it is interpreted to form images. This was one of the first recognised functions of vitamin A and is why it originally gained the name of retinol.
Within the EU, the European Food Safety Authority has authorised health claims that vitamin A contributes to:
- Normal iron metabolism
- The maintenance of normal mucous membranes
- The maintenance of normal skin
- The maintenance of normal vision
- The normal function of the immune system
- The process of cell specialisation.
Animal sources of vitamin A
In a typical Western-style omnivore diet, around 70% of vitamin A comes from animal sources, and 30% comes from plant sources. The fat-soluble forms of vitamin A include retinoic acid, retinal and retinol and are already in a form that your cells can use.
Food sources of retinol vitamin A include:
- animal and fish liver
- oily fish and cod liver oil
- dairy products
- milk and dairy products
- butter and margarine (which fortified to contain as much vitamin A as in butter).
The reason why milk, cream and butter from the Channel Islands looks so rich is because Guernsey cows cannot convert dietary carotenoids to vitamin A as efficiently as other breeds. The pigments therefore remain in higher quantities to impart a rich, golden-yellow hue to the milk fat.
The vitamin A obtained from plants is in a water-soluble form of carotenoid pigments, some of which you can convert into retinoids if your levels are low. This conversion occurs mainly in the intestines but can also occur elsewhere in the body.
Around 600 carotenoids are known, but only a few such as beta-carotene, alpha-carotene and beta-cryptoxanthin have pro-vitamin A activity. The conversion of carotenoids to vitamin A is inefficient, especially where intakes of other micro-nutrients such as zinc are low.
Originally, it was thought that around 6 mcg betacarotene is needed to produce 1 mcg of retinol. Now it is recognised that this conversion varies from between 10 to 1 and 28 to 1 depending on how beta-carotene is stored within a particular plant. A new average estimate of 12 to 1 is now used until more data becomes available.
Other carotenoids such as lycopene, lutein and zeaxanthin cannot be converted into vitamin A, but have other beneficial effects on health (such as lutein protecting against macular degeneration).
Absorption of vitamin A
Normally, around 80% of vitamin A in your diet is absorbed in the small intestines, along with dietary fats.
Vitamin A is easily destroyed by exposure to light and heat, however, so once you cook food by boiling or frying, its vitamin A content falls by 40% within one hour (within a stew, for example) and by 70% after two hours in the roasting pan.
Vitamin A deficiency
The fat-soluble forms of vitamin A are stored in the liver so deficiency is uncommon in people who eat a healthy diet – whether animal or plant-based. Your blood levels are also tightly controlled and do not alter significantly until liver stores are severely depleted. Deficiency is sometimes diagnosed in people with absorption problems, chronic liver disease or alcohol dependency associated with a poor diet.
One of the first signs of vitamin A deficiency is a loss of sensitivity to green light, followed by difficulty in adapting to dim light (night blindness), hence the old saying that ‘carrots help you see in the dark’. More severe deficiency leads to dry, burning, itchy eyes, hardening of the cornea (the transparent part of the front of the eye) followed by corneal ulceration – a condition known as xerophthalmia. Lack of vitamin A also increases the risk of cataracts.
It is estimated that as many as half a million people worldwide go blind from vitamin A deficiency each year, which is a tragedy as this nutritional disease is totally preventable.
Worldwide, lack of vitamin A is a public health concern in more than half of all countries, especially those in Africa and Asia. An estimated 250 million preschool children have a vitamin A deficiency and, as a result, up to half a million children become blind every year, half of whom succumb to severe infections within a year of losing their sight. This makes it a leading cause of preventable blindness and mortality in children. High-dose vitamin A drops can reduce mortality from overall infections by a quarter, and halves the mortality rate in vulnerable children with acute measles.
Symptoms that may suggest vitamin A deficiency
- Loss of sensitivity to green light
- Difficulty adapting to dim light (night blindness)
- Increased susceptibility to infection
- Scaly skin with raised, pimply hair follicles (keratosis pilaris)
- Flaking scalp
- Brittle, dull hair
- Loss of appetite
- Reduced male fertility
Symptoms that can occur with major vitamin A deficiency
- Growth retardation
- Impaired hearing, taste and smell
- Dry, burning, itchy eyes (xerophthalmia)
- Hardening of the cornea
- Corneal ulceration
- Visual loss, including blindness
- Low resistance to infection
Vitamin A and immunity
Vitamin A plays an important role in maintaining immunity and strengthening resistance against viral and bacterial infections, and you need more vitamin A when you are fighting an infection. Vitamin A is involved in the production of immune cells needed to line the mucus membranes of the respiratory and intestinal tracts – two of your front line defences against infectious diseases.
Because of its role in immunity, optimum intakes of vitamin A help to protect against viral sore throats, the common cold, influenza, wart virus, conjunctivitis, cold sores, acute bronchitis and possibly shingles. It is also thought to have a protective action against inflammatory bowel disease, peptic ulcers, Candida yeast infections and in reducing allergies.
Vitamin A and heart disease
Several studies suggest that natural dietary intakes of vitamin A and some carotenoids have a protective effect against coronary heart disease.
A large analysis of the results of 15 trials suggests that people with the highest intake of betacarotene from plant sources have a 22% lower risk of coronary heart disease than those with the lowest intakes.
Skin health: Vitamin A derivatives are used medicinally, on prescription, to treat severe acne, psoriasis and sun damage (photoageing) including wrinkles.
Vitamin A and cancer
Some studies suggest that vitamin A may have a protective effect against certain cancers, including cancer of the ovaries, endometrium, stomach, prostate, breast and possibly colon cancers. However, a more recent study, involving over 13,000 people, did not find any significant associations between vitamin A intakes and mortality (or any other antioxidants, whether dietary or supplemental) after adjusting for other confounding factors.
Vitamin A and safety
High dose betacarotene supplements have been linked with an increased risk of lung cancer in smokers, and in those with previous occupational exposure to asbestos. Although this is controversial, it’s prudent for smokers to avoid taking betacarotene supplements, and to obtain their vitamin A and pro-vitamin A carotenoids naturally by eating plenty of red, yellow, orange and green fruit and vegetables.
How much vitamin A do you need?
For conversion between micrograms and International Units, 1mcg vitamin A = 3.33 IU
Typical intakes of vitamin A
The average intake of vitamin A from food sources in people following a western-style diet is 1100mcg as retinol, and 2.3mg in the form of carotenoids.
Toxicity of vitamin A
Do not exceed the recommended dose of any supplement containing vitamin A. Intakes of just double the recommended daily amount of vitamin A can cause health problems. Excess can cause headache, irritability, blurred vision, nausea, weakness, fatigue, abdominal pain and loss of appetite. Long term excess intakes of vitamin A may cause hair loss, reduced bone mineral density and increase the risk of liver cirrhosis.
Polar bear liver is said to contain so much retinol that consuming just 100g can prove lethal!
Vitamin A and pregnancy
Although vitamin A is important for a healthy pregnancy, daily intakes of more than 15,000 IU (4500 mcg) of pre-formed vitamin A (retinol) have been associated with a more than three-fold greater risk of delivering a baby with congenital defects compared with intakes of 5000 IU (1500 mcg) daily. In women whose vitamin A intake came mainly from supplements, those obtaining 10,000 IU (3000 mcg) daily were almost five times more likely to deliver a baby with congenital abnormalities.
The researchers concluded that, among women taking more than 10,000 IU vitamin A per day, one infant in 57 had abnormalities due to the supplement. The most dangerous time to take excess vitamin A during pregnancy seems to be within the first seven weeks of fetal development.
Pregnant women are therefore advised not to take supplements containing pre-formed retinol, or to eat foods that have a high vitamin A content, such as liver and liver products. You should also avoid cod liver oil, which contains high quantities of vitamin A.
The safest way to obtain vitamin A during pregnancy is in the form of natural carotenoids found in fruit and vegetables, or mixed carotenoids in supplements, which are converted into retinol when needed.
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