Vitamin A is a fat-soluble nutrient that is consumed either as retinol, the purest form, or via carotene, which is a precursor to retinol and stored in the liver. Vitamin A is essential for healthy vision and skin, normal musculoskeletal growth and a strong immune system. In general, vitamins from natural sources are better assimilated within the body than supplements, which are often synthetic varieties. Understanding which foods contain vitamin A is important for establishing a healthy diet.
Functions of Vitamin A
Vitamin A is needed to form and maintain the mucous membranes of the eyes and eye lids, which provide moist barriers against debris and pathogenic microorganisms. The membranes also provide lubrication for blinking, which occurs about 200,000 times per week, according to "Nutritional Sciences." The membranes of the respiratory system and digestive tract also require vitamin A and form the immune system's defensive barrier. Vitamin A aids the immune system's offensive element by contributing to the production of enzymes that scavenge pathogens that penetrate the membranes. Vitamin A is also essential for the production of rhodopsin, which is a light-sensitive retinal pigment needed for night vision. Due to rhodopsin, most people can detect about 500 shades of gray in the dark, according to the "Doctor's Complete Guide to Vitamins and Minerals."
Required Amounts
According to the Office of dietary Supplements, the recommended dietary allowances for vitamin A, as retinol, range from 300 mcg per day for infants, to 1,300 mcg for lactating females. Adult males are said to require 900 mcg daily. Carotenoids, such as beta-carotene, are sources of vitamin A indirectly and are assigned a conversion rate of 3.3. For example, adult females require 700 mcg of retinol daily, which is the equivalent of 2,310 IU of beta-carotene.
Foods Rich in Vitamin A
According to the "American Dietetic Association Complete Food and Nutrition Guide," rich sources of vitamin A, as retinol, comprise animal products and include the livers of beef, pig, fish and chicken, fish oils, eggs, hard cheeses and fortified dairy products. For example, 100 g of stewed pig liver contains 23,000 mcg of vitamin A, whereas 100 g of beef liver pate contains 7,000 mcg and the equivalent amount of eggs have 200 mcg. A less rich source of vitamin A is beta-carotene, which gets converted to retinol in the liver. Beta-carotene is found in fleshy, orange vegetables such as carrots, sweet potatoes and yams, but also in spinach, watercress, mangoes and apricots. For example, 100 g of carrots contain 8,000 mcg of beta-carotene, whereas 100 g of sweet potato contains 4,000 mcg and the equivalent amount of mango has 1,400 mcg.
Deficiency Symptoms
An early symptom of vitamin A deficiency is impaired vision in the dark, or night blindness. Vitamin A deficiency also leads to skin complaints, such as dandruff on the scalp, facial acne, eczema, boils and other skin infections, according to "Professional Guide to Diseases." Chronic vitamin A deficiency leads to xeropthalmia, or dry eye, which can cause cornea damage, ulcers, scarring and eventually blindness.
References
- "Nutritional Sciences"; Michelle McGuire; 2007
- "Doctor's Complete Guide to Vitamins and Minerals"; Mary Dan Eades and Philip Lief; 2002
- Office of Dietary Supplements: National Institutes of Health: Vitamin D
- "American Dietetic Association Complete Food and Nutrition Guide"; American Dietetic Association; 2006
- "Professional Guide to Diseases: Ninth Edition"; Springhouse Publishing; 2009
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