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What is Vitamin B12 or Cobalamin + Food Sources of Vitamin B12 + Roles in the Body, Requirements and Deficiency and Toxicity

What is vitamin A? 
Vitamin A in foods includes members of two chemical families, the retinoids such as retinol, retinal, and retinoic acid, and the carotenoids such as alpha-carotene, beta-carotene, and other carotenes. However, in order for a carotenoid to have vitamin A activity it must first be converted to a retinoid in the body. Therefore, carotenoids are often referred to as provitamin A. Although there are hundreds of carotenoids found in nature, only about 50 may be converted to vitamin A. Furthermore, only about a half dozen of those carotenoids are found in the human diet in appreciable amounts. Because of its availability in the diet and relatively efficient conversion to a vitamin A, beta-carotene may be the most significant carotenoid with regard to conversion to vitamin A. 


What foods provide vitamin A? 
Vitamin A in the retinoid form is found in animal products with better sources being liver, fish oils, eggs, and vitamin A-fortified milk and milk products. Meanwhile, carotenoids are found in plant sources—mainly in orange and dark green vegetables and some fruits (squash, carrots, spinach, broccoli, papaya, sweet potatoes, pumpkin, cantaloupe, and apricots). In fact, the term carotenoid is derived from the species name for carrots. Nutrition supplements tend to provide vitamin A in the form of retinyl acetate or retinyl palmitate or as beta-carotene. 

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What does vitamin A do in the body?

Vitamin A is crucial to growth, health and maintenance for many reasons including:


- Eye Health – Within the eye lies a complex neural/sensory processes that allow us to see. Vitamin A is fundamentally involved in this process and is also involved in maintaining the health of the cornea, which is the clear outer window of the eye. Because of this relationship, poor vitamin A status in the human body is often recognized by changes in vision, as will be discussed.


- Maintenance of Mucus Producing Tissue - Vitamin A is also indispensable for the maintenance and regulation of growth of many types of cells in the body. Cells that produce mucus, a lubricating and protecting substance, are particularly sensitive to vitamin A status. These types of cells are found lining the digestive tract and lungs and also in the eye’s cornea.


- Growth of Body - Vitamin A is also essential for normal growth and development of the human body as a whole. It is now clear that vitamin A acts in certain cells throughout the body at the genetic level. This means that some of the function of vitamin A is related to its ability to interact with DNA and affect the manufacture of certain proteins. This seems to be very important in the proper development and maintenance of various tissues throughout the body.


Do carotenoids have a role in health without be converted to vitamin A?

Not all of the beta-carotene eaten will be converted to vitamin A. Much of it, along with other carotenoids, will go unchanged and have different functions in the body. For instance, beta-carotene and other carotenoids such as lutein and lycopene can function as antioxidants. In this capacity, the carotenoids function more as nutraceuticals helping to protect the body’s cells against free radicals. Thus, eating a diet rich in fruits and vegetables will not only support vitamin A intake but also provide carotenoids that help protect us from disease.


How much vitamin A do we need?

The RDA for for vitamin A is 700 and 900 µg for women and men, respectively. During pregnancy and lactation the RDA increases to 770 and 1300 µg respectively. DRI/RDA Table provides recommended levels for people across the lifespan. Often vitamin A content in foods is listed as µg and retinol equivalents (RE). Retinol Equivalents are used because we derive vitamin A from retinoids and carotenoids and REs and the level of activity is not the same for the various forms. For instance, carotenoids are absorbed from the digestive tract with about half the efficiency of the retinoids. Also, once inside the body, they must be converted to a retinoid, a process that varies in efficiency from one carotenoid to another. In order to account for the inherent differences in obtaining vitamin A activity from retinoids versus the carotenoids, vitamin A is listed in REs. One microgram of retinol equals 1 RE, whereas it takes 12 µg of ß-carotene to equal 1 RE and 24 µg of other carotenes to equal 1 RE. Also, International Units (IU) are an older methods of expressing vitamin activity and is still used on some packaging. One IU is equal to 0.3 µg of retinol.


What happens if too little vitamin A is consumed?

When vitamin A is deficient from the diet for many months the body’s internal stores are decreased and deficiency is revealed in the form of:


- Night Blindness - Night blindness is an inability to adapt to dim lighting and is usually accompanied by a prolonged transition from dim to bright light.


- Xeropthamia - Occurs when the mucus-producing cells of the cornea deteriorate and no longer produce mucus; a hard protein called keratin is produced instead. Keratin in combination with a decreased presence of mucus will dry out and harden the cornea of the eye. Xerophthalmia means dry, hard eyes.


- Drying of Body Linings - Inadequate mucus secretion of cells lining the respiratory, digestive, urinary, and reproductive tracts will greatly affect the function and health of these tissues as well. They are subject to drying and infection. Dry, hard skin is an observable sign of a vitamin A deficiency.


How common is vitamin A deficiency?

Vitamin A deficiency is one of the more recognized nutrient deficiencies worldwide, as roughly 2 million children in developing countries go blind each year as a result of vitamin A deficiency. International relief efforts to improve health conditions in these countries are attempting to correct this deficiency by giving children large amounts of vitamin A a couple of times per year. Hopefully, the doses are large enough to provide adequate vitamin A storage to last until the next treatment.


Can vitamin A become toxic?

Toxicity of vitamin A is seemingly just as severe as a deficiency. If a person consumes as little as ten times the RDA for vitamin A for several months, signs and symptoms such as bone pain, hair loss, dryness of the skin, and liver complications may develop. If toxicity persists it can eventually result in death. The risk of vitamin A toxicity from eating a balanced diet is low. Even those of us eating very large amounts of carotenoid-containing fruits and vegetables are not at significant risk of toxicity. This is due to the much lower rate of digestive absorption and conversion of carotenoids to vitamin A. Most people who develop vitamin A toxicity seem to do so through use of supplements. Recently, research has revealed that retinol intakes exceeding 5000 IUs daily might increase the risk of osteoporosis. Furthermore, vitamin A toxicity during pregnancy can result in birth defects. We will look more closely at this situation in Family/Your Health.



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