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Vitamin A in a nutshell:
Vitamin A is extremely important in the activation and deactivation of genes responsible for skin and membrane growth and maturation, including membranes of the retina in the eye.  It’s also very important in the embryonic development of the fetus, helping formation of the heart, eyes and ears.  Vitamin A is also important in the immune response of the skin, stomach and urinary tract.  However, vitamin A is toxic in high doses, so please read this page for more information.

Best source of Vitamin A for Juicers
Vitamin A can be obtained from carrots, kale, parsley, spinach, watercress, broccoli & romaine lettuce.

What Is Vitamin A?

Vitamin A is a group of six naturally occurring compounds that the human body can transform into a chemical called retinol, which is important for regulating how fast cells grow, especially in the skin, eyes, and central nervous system. It's important to get enough vitamin A without getting too much.

Alternate names: Vitamin A; retinol. The human body can use alpha-carotene, beta-carotene, gamma-carotene, beta-cryptoxanthin, and retinal to make retinol. Nutritional supplements may contain vitamin A in the form of retinyl acetate or retinyl palmitate.

In plants, the compounds that can be converted into vitamin A serve as antioxidants that protect the plant against the effects of sun and heat. In animals, alpha-carotene and beta-carotene, which are absorbed from plant foods, perform some of the antioxidant functions of vitamin A. Animals and humans have to have the retinol form of vitamin A not only as an antioxidant but also as a regulator of gene function.

What Does Vitamin A Do in the Human Body?

One of the most important functions of vitamin A in the human body is to act as a hormone that regulates the expression of genes. Vitamin A is carried into a cell on specialized transporter proteins that present it to certain genes in the cell's DNA. It locks onto these genes to turn them on and disengages from these genes to turn them off.

Vitamin A activates genes that are responsible for the maturation and multiplication of cells in the skin and membranes all over the body, including the retina of the eyes, the linings of the nose and throat, and the linings of the reproductive tract in both sexes. Vitamin A is sometimes also a cofactor to vitamin D and thyroid hormone, which cannot perform all of their functions without vitamin A.

Vitamin A is essential for activating genes that direct the formation of the heart, eyes, and ears of human beings in the embryo stage. It is needed for the production of growth hormone. Throughout life, it activates genes in stem cells in the bone marrow to transform them into red blood cells, and it also activates genes that cause the white blood cells known as T-cells to mature into their active form. Vitamin A is especially important to immune protection of the skin, urinary tract, and stomach.

Vitamin A is also important to the health of the retina. The retina is a membrane at the back of the eye that transmits pulses of energy to the optic nerve when light falls on it. Vitamin A combines with a protein called opsin to make substance known as rhodopsin in the rods in the retina.

Rhodopsin is critical for night vision. When small amounts of light fall on the cones in the retina, rhodopsin is transformed into the retinal form of vitamin A with the release of electrical energy to the optic nerve. The retinal is then transformed back into retinol so the process can be repeated over and over again.

What Happens When We Don't Get Enough Vitamin A?

Vitamin A deficiencies leave people prone to chronic infections, and chronic infections further deplete vitamin A. The problem is especially acute for children who are deprived of vitamin A, usually because they are fed a diet consisting only of rice, corn, or millet. Many children who do not get vitamin A still die of diarrhea, pneumonia, and measles. Survivors suffer loss of vision and chronic infections due to dry eyes and dry skin.

What Happens When We Get Too Much Vitamin A?

One of the most famous figures of the early twentieth century was the intrepid arctic explorer Robert Peary. One of the first people ever to reach the North Pole, Peary and his comrades at one point were so low on food that they were forced to kill and eat a polar bear.

The polar bear got its revenge, however, when the explorers and their pack dogs ate polar bear liver. Several of the dogs died, and Peary himself lost some of his fingers even without exposure to frostbite. The reason for the death of the dogs and the near-death of Peary and his fellow explorers was the enormous amount of vitamin A in the livers of polar bears, walruses, and seals. As little as 1 oz (28 grams) of polar bear liver can deliver a fatal dose of vitamin A.

Most of us don't have any trouble avoiding polar bear liver in our diets, but about 75% of the population of developed countries are exposed to chronically high levels of vitamin A. The human body can convert alpha-carotene, beta-carotene, gamma-carotene, and beta-cryptoxanthin into vitamin A as they are needed. It's essentially impossible to get too much of these plant chemicals, although it is possible to cause temporary discoloration of the skin. Excesses of these precursors of vitamin A are usually eliminated in a few weeks, and they do not cause any kind of toxicity.

On the other hand, it is possible to get too much vitamin A that is “preformed” and immediately available to the body. Just how much vitamin A is too much depends on:

• Whether or not there is enough zinc in the diet. Zinc is needed for the protein that carries vitamin A inside cells. People who are zinc-deficient need more vitamin A and can tolerate more vitamin A without toxic effects.
• Whether or not there is enough fat in the diet. Fat binds to vitamin A from food and supplements and binds it to the receptors in the lining of the small intestine that carry it into the bloodstream. People who receive very small amounts of fat in their diets can tolerate higher doses of vitamin A.
• Whether or not the liver can make the proteins that transport vitamin A. People who have chronic liver disease need more vitamin A and can tolerate more vitamin A.

When people get enough zinc and enough fat in their diets, a single dose of 25,000 IU (in children under the age of six) to 100,000 IU (in older children, teens, and adults) can cause symptoms of toxicity. When people are deficient in both vitamin A and zinc and their diets provide very little fat, a dose of up to 200,000 IU may actually be beneficial. Signs of acute toxicity from a single dose include fatigue, nausea, loss of appetite, dizziness, dry skin, peeling skin, and swelling in the brain. In infants, there may be swelling at the fontanel, where the skull is not yet closed.

Because the body stores vitamin A in fat and LDL cholesterol, it takes a much lower daily dosage of vitamin A to cause chronic vitamin A toxicity. As little as 25,000 IU of vitamin A per day taken for a month or longer can cause the same symptoms as an acute toxic dose, listed above. Chronic consumption of too much vitamin A can also cause bone spurs and other abnormalities of bone structure as well as chronic liver failure. In extreme cases, as was the case with Peary's sled dogs and a later Swiss explorer who also ate polar bear liver, too much vitamin A can cause death.

Vitamin A supplements are more rapidly toxic than natural sources of vitamin A such as cod liver oil. Anything over about 10 times the recommended daily allowance (RDA), however, can cause long-term problems.

Are You at Risk of Vitamin A Deficiency?

About 40% of children and a large number of adults in societies dependent on a single foodstuff, usually a grain like rice or millet, develop vitamin A deficiency. About 75% of people in modern countries develop a low level of vitamin A toxicity.

You are at risk of vitamin A deficiency if:

• You usually eat a single food at every meal.
• You have any kind of chronic infection. Most people who have HIV or AIDS become deficient in vitamin A.
• You have had gastric bypass surgery or you have a disease that interferes with the absorption of fat-soluble nutrients. People who have Crohn's disease, ischemic colitis, celiac disease, short bowel syndrome, pancreatitis, or similar health conditions sometimes develop vitamin A deficiencies.
• You have alcoholic liver disease.

If you have any of these conditions, you may need to take supplemental vitamin A.

You are at risk of vitamin A toxicity if:

• You take more than 1 teaspoon (5 ml) of cod liver oil daily.
• You eat liver or foods made with liver (Braunschweiger, pate, and some sausages) every day.
• You take more than 10,000 IU (3 mg) of supplemental vitamin A per day on an ongoing basis.
• You consume large amounts of vitamin D and calcium, which amplify the effects of vitamin A in the bones.

Women in the first trimester of pregnancy must not take more than 3,000 IU (1 mg) of vitamin A per day. There is about a 20% chance of spontaneous abortion and about a 10% chance of the birth of a child with serious birth defects (spina bifida, cleft palate, and defects in the eyes, heart, and skull) when women are exposed to more than 300,000 IU (100 mg) during the first three months of pregnancy.

How to Make Sure You Get the Right Amount of Vitamin A

There are very complicated formulas for computing “retinol equivalents” between alpha-carotene, beta-carotene, gamma-carotene, beta-cryptoxanthin, and vitamin A.

• 24 units of alpha-carotene or beta-cryptoxanthin can become 1 unit of the retinol form of vitamin A.
• 12 units of beta-carotene from the diet can become 1 unit of the retinol form of vitamin A.
• 2 units of beta-carotene from supplements can become 1 unit of the retinol form of vitamin A.
• 1 unit of retinol is treated as 1 unit of vitamin A.

There are complicated charts that tell you how many retinol equivalents are available from various serving sizes of various foods, dividing micrograms of alpha-carotene, beta-carotene, and beta-cryptoxantin by the appropriate numbers to estimate vitamin A.

But your body doesn't have to follow the rules. The conversion of plant chemicals into active forms of vitamin A only occurs when the body is deficient in vitamin A. Otherwise, alpha-carotene, beta-carotene, and beta-cryptoxanthin are available to perform other, more familiar tasks as antioxidants.

It is far more important to make sure you don't get too much vitamin A. The maximum amounts of the vitamin that should be taken on a daily basis are:

• 0–3 years: 0.6 mg or 2000 IU
• 4–8 years: 0.9 mg or 3000 IU
• 9–13 years: 1.7 mg or 5665 IU
• 14–18 years: 2.8 mg or 9335 IU
• 19+ years: 3 mg or 10,000 IU

The RDAs (recommended daily allowances) of vitamin A are:

• 0-6 months, 0.4 mg or 1333 IU
• 7-12 months, 0.5 mg or 1667 IU
• 1-3 years, 0.3 mg or 1000 IU (Toddlers need less vitamin A than infants because their nervous systems are more completely developed.)
• 4-8 years, 0.4 mg or 1333 IU.
• 9-13 years, 0.6 mg of 2000 IU.
• Males over the age of 14, 0.9 mg or 3000 IU.
• Females who over the age of 14 who are not pregnant or breastfeeding, 0.7 mg or 2333 IU.
• Pregnant women aged 18 or under, 0.75 mg or 2500 IU.
• Pregnant women who are 19 or older, 0.77 mg or 2567 IU.
• Breastfeeding women who are 18 or under, 1.2 mg or 4000 IU.
• Breastfeeding women who are 19 or older, 1.3 mg or 4333 IU.

How can you get 1000 IU of vitamin A? That's the amount of vitamin A you can get from:

• 1/4 of a teaspoon (about 1 ml) of cod liver oil (Taking more than 5 teaspoons of cod liver oil daily can cause toxicity symptoms, especially if your diet is also high in calcium).
• 1 egg, or
• 1 cup (240 ml) of whole milk, or
• 3/4 cup (180 ml) of vitamin A-fortified milk of any kind, or
• 1 tablespoon of butter, or
• 1 serving of any vitamin-fortified breakfast cereal.

If you follow a strict vegan diet and you don't eat fortified breakfast cereals, then for your body to produce 1000 IU of vitamin A, you need to consume:

• 1/2 cup of pumpkin,
• 1 cup of sweet potato,
• 1 cup of chopped carrots,
• 1 cup of cooked spinach,
• 1 cup of cooked kale,
• 2 cups of cooked acorn squash,
• 1-1/2 cups of cooked collard greens,
• 1 whole cantaloupe, or
• 10 mangoes.

Some vegans and vegetarians benefit from taking mixed carotenoids or beta-carotene, up to 25,000 IU per day, when they cannot eat orange-colored plant foods.

Using Vitamin A to Treat Disease Conditions

High-dose vitamin A, at levels causing a risk of toxicity, is useful for treating retinitis pigmentosa and acute myolytic leukemia (AML). The anti-acne drugs Accutane, Retin-A, and Tazorac are highly concentrated forms of vitamin A. Any use of high-dose vitamin A or vitamin A equivalents, however, should only be attempted under the supervision of a physician.

Using Vitamin A to Prevent Blindness in Children

Vitamin A deficiency is the leading cause of preventable blindness in children. Children who don't get enough vitamin A may develop cataracts in the lens of the eye, similar to those in older adults, or they may be prone to have severe inflammation and infection of the cornea covering the eye. Children may develop lesions in the retina of the eye or lose their sight to glaucoma. Vitamin A deficiency has to be corrected in the first year of life or the only way to restore sight may be with surgery.

Most children who don't get enough vitamin A do not go totally blind. In most countries where epidemiologists have documented the effects of vitamin A deficiency, only about 1 to 2% of children lose sight completely. About 10% of children develop chronic dry eyes prone to infection, however, and about 20% develop nyctolopia, or night blindness, leaving them effectively blind when the sun goes down. Worldwide, however, the US Institute of Medicine estimates that 13 million children have suffered some degree of loss of sight simply because they did not get enough vitamin A and up to 250 million children are at risk.

Providing children with vegetables containing beta-carotene tends to maintain vitamin A levels rather than increase them. Correcting vitamin A deficiency requires getting a supplement, but a single dose of 200,000 IU just once during the first two years of life may be enough.

Using Vitamin A To Prevent Transmission of HIV

One of the ongoing challenges of the AIDS crisis in Africa is figuring out ways to prevent the transmission of HIV from mother to child. The virus does not usually cross the placenta to infect the child before birth, but bruising and tears to the birth canal can transfer the virus to the baby during labor.

It's not a sure thing that a child born to a mother who has HIV will develop HIV. Even without treatment, the virus is only transmitted about 40% of the time during labor. However, South African doctors have found that giving pregnant women a combination of 30 mg of beta-carotene and 5,000 IU of vitamin A in the form of retinyl ascorbate reduced the rate of transmission by more than half. When babies who were infected with HIV were given just five doses of vitamin A (50,000 IU at the age of one month and at three months, 100,000 IU at six and nine months, and 200,000 IU at twelve and fifteen months), they were 77% less likely to have to be hospitalized for the most common problem of infants and toddlers who have HIV, diarrhea.