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Sunday, May 1, 2011

Vitamin A and Carotenoids

[Extracted from the webpage from]

Vitamin A is a group of compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, blood, or other specialized tissue.) Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A also may help lymphocytes (a type of white blood cell) fight infections more effectively.

Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts. When those linings break down, it becomes easier for bacteria to enter the body and cause infection. Vitamin A also helps the skin and mucous membranes function as a barrier to bacteria and viruses.

In general, there are two categories of vitamin A, depending on whether the food source is an animal or a plant.

Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body.

Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids. Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.

Of the 563 identified carotenoids, fewer than 10% can be made into vitamin A in the body. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of all carotenoid-rich fruits and vegetables for their health-promoting benefits.

Some provitamin A carotenoids have been shown to function as antioxidants in laboratory studies; however, this role has not been consistently demonstrated in humans. Antioxidants protect cells from free radicals, which are potentially damaging by-products of oxygen metabolism that may contribute to the development of some chronic diseases.
What foods provide vitamin A
Retinol is found in foods that come from animals such as whole eggs, milk, and liver. Most fat-free milk and dried nonfat milk solids sold in the United States are fortified with vitamin A to replace the amount lost when the fat is removed. Fortified foods such as fortified breakfast cereals also provide vitamin A. Provitamin A carotenoids are abundant in darkly colored fruits and vegetables. The 2000 National Health and Nutrition Examination Survey (NHANES) indicated that major dietary contributors of retinol are milk, margarine, eggs, beef liver and fortified breakfast cereals, whereas major contributors of provitamin A carotenoids are carrots, cantaloupes, sweet potatoes, and spinach.

Vitamin A in foods that come from animals is well absorbed and used efficiently by the body. Vitamin A in foods that come from plants is not as well absorbed as animal sources of vitamin A. Tables 1 and 2 suggest many sources of vitamin A and provitamin A carotenoids.

Table 1: Selected animal sources of vitamin A

FoodVitamin A (IU)*%DV**
Liver, beef, cooked, 3 ounces27,185545
Liver, chicken, cooked, 3 ounces12,325245
Milk, fortified skim, 1 cup50010
Cheese, cheddar, 1 ounce2846
Milk, whole (3.25% fat), 1 cup2495
Egg substitute, ¼ cup2265

Table 2: Selected plant sources of vitamin A (from beta-carotene)

FoodVitamin A (IU)*%DV**
Carrot juice, canned, ½ cup22,567450
Carrots, boiled, ½ cup slices13,418270
Spinach, frozen, boiled, ½ cup11,458230
Kale, frozen, boiled, ½ cup9,558190
Carrots, 1 raw (7½ inches)8,666175
Vegetable soup, canned, chunky, ready-to-serve, 1 cup5,820115
Cantaloupe, 1 cup cubes5,411110
Spinach, raw, 1 cup2,81355
Apricots with skin, juice pack, ½ cup2,06340
Apricot nectar, canned, ½ cup1,65135
Papaya, 1 cup cubes1,53230
Mango, 1 cup sliced1,26225
Oatmeal, instant, fortified, plain, prepared with water, 1 cup1,25225
Peas, frozen, boiled, ½ cup1,05020
Tomato juice, canned, 6 ounces81915
Peaches, canned, juice pack, ½ cup halves or slices47310
Peach, 1 medium3196
Pepper, sweet, red, raw, 1 ring (3 inches diameter by ¼ inch thick)3136

* IU = International Units
** DV = Daily Value. DVs are reference numbers based on the Recommended Dietary Allowances (RDAs). They were developed to help consumers determine if a food contains a lot or a little of a nutrient. The DV for vitamin A is 5,000 IU. Most food labels do not list vitamin A content. The percent DV (%DV) column in the table above indicates the percentage of the DV provided in one serving. A food providing 5% or less of the DV is a low source while a food that provides 10% to 19% of the DV is a good source. A food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet. For foods not listed in this table, refer to the U.S. Department of Agriculture's Nutrient Database
What are the health risks of too much vitamin A
Hypervitaminosis A refers to high storage levels of vitamin A in the body that can lead to toxic symptoms. There are four major adverse effects of hypervitaminosis A: birth defects, liver abnormalities, reduced bone mineral density that may result in osteoporosis (see the previous section), and central nervous system disorders.

Toxic symptoms can also arise after consuming very large amounts of preformed vitamin A over a short period of time. Signs of acute toxicity include nausea and vomiting, headache, dizziness, blurred vision, and muscular uncoordination. Although hypervitaminosis A can occur when large amounts of liver are regularly consumed, most cases result from taking excess amounts of the nutrient in supplements.
What are the health risks of too many Carotenoids

Provitamin A carotenoids such as beta-carotene are generally considered safe because they are not associated with specific adverse health effects. Their conversion to vitamin A decreases when body stores are full. A high intake of provitamin A carotenoids can turn the skin yellow, but this is not considered dangerous to health.

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