Vitamin A Functions
Vitamin A is a fat-soluble vitamin that performs several key functions in the body:
• Helps create and maintain healthy teeth, bones, and soft tissues in the body
• Preserves your skin and mucous membranes so they can protect your body from being invaded by bacteria or viruses that can cause an illness or infection
• Helps create pigments found in the retina of the eye and helps you see, especially at night
• Supports reproduction, the development of an embryo, and breastfeeding
• Regulates and strengthens the immune system
• Acts as an antioxidant to protect cells against damage from free radicals that can contribute to some chronic diseases and unhealthy aging
The two main dietary sources of vitamin A are retinol (preformed vitamin A) and Provitamin A carotenoids.
Retinol, the active or usable form of vitamin A, is found most abundantly in animal foods such as meats and dairy products. The vitamin A from animal foods, also called preformed vitamin A, is better absorbed and used by the body than the vitamin A in plant foods.
Provitamin A carotenoids are orange pigments found in fruits, vegetables, and other plant foods that can be converted in the body (liver) into usable retinol. Beta-carotene, alpha-carotene, and beta-cryptoxanthin are provitamin A carotenoids commonly found in plant foods. Although all these become retinol in the body, beta-carotene is much more efficiently converted than the other carotenoids. Key food sources of beta-carotene include deeply colored fruits and vegetables such as cantaloupe, pink grapefruit, apricots, pumpkin, carrots, sweet potatoes, winter squash, broccoli, spinach, and most dark green leafy vegetables.
Vitamin A recommendations are sometimes expressed as retinol activity equivalents (RAE); this unit of measuring the vitamin A in foods is useful because it reflects the difference in conversion to vitamin A between animal and plant sources of vitamin A. one RAE equals:
• 1 microgram (mcg) retinol
• 12 micrograms (mcg) beta-carotene
Vitamin A is expressed as international units (IUs) on food labels and on supplement packages. One IU of preformed vitamin A equals 3.33 IU.
Deficiencies and Excesses
Vitamin A is considered a nutrient of concern for both children and adults according to the 2005 Dietary Guideline Advisory Committee Report. Although it’s rare to be deficient in vitamin A, symptoms can include night blindness or dry, rough skin. Lower resistance to infections, impaired tooth development, and slower bone growth can also occur.
Too much vitamin A from foods and/or supplements can cause nausea, irritability, and blurred vision. Excessive amounts can cause growth retardation, liver problems, hair loss, and bone pain. Large amounts of carotenoids can cause your skin to turn yellow or orange, but this is not harmful and is reversed once you cut back on whatever you are eating that is rich in carotenoids (often, carrots). It can also increase the risk for bone fractures and osteoporosis. Those who consume high amounts of alcohol, have liver disease or high blood cholesterol levels, or consume little dietary protein can be at increased risk for these effects.
Daily Value (DV) is a dietary reference value for adults of all ages and both sexes that appears on food labels to help Americans compare the amount of nutrients in products in the context of their total diet.
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