Vitamin A, or retinol, is essential for acute night vision, strong immunity, healthy skin and normal fetal development. Vitamin A is fat-soluble and stored within the body for long periods of time, which makes deficiency symptoms relatively uncommon. Additionally, vitamin A is found in a variety of foods commonly eaten by Americans, such as meat and dairy products. Vitamin A is toxic in large doses, more so than any other vitamin.
Functions of Vitamin A
Vitamin A is required for the maintenance of moist mucous membranes that line the eyes, oral and nasal cavities, lungs and digestive system. As such, it contributes to immunity by forming a defensive barrier, which traps and eliminates debris, toxins and potentially harmful micro-organisms. Vitamin A is also a powerful antioxidant that scavenges free-radicals, which cause inflammatory reactions and tissue damage, especially in blood vessels. Furthermore, vitamin A is necessary for the production of rhodopsin, a light-sensitive pigment needed within the retina to detect movement and images in the dark. "Vitamins: Fundamental Aspects in Nutrition and Health" states that rhodopsin allows most people to distinguish among about 500 shades of gray in low-light conditions.
Recommended Amounts
The National Institutes of Health states that the recommended daily amounts of retinol is 300 mcg for infants, 900 mcg for men and 1,300 mcg for lactating women. Good sources of retinol include pig, beef and fish livers, eggs, some cheeses and fortified dairy products. The body can also synthesize retinol from carotenoids, such as beta-carotene, but about three times as much food is required, "Nutritional Sciences" reports. Beta-carotene is converted to retinol in the liver and stored there, which reduces the odds of deficiency, but increases the risks of toxicity. Good sources of beta-carotene include carrots, sweet potatoes and other orange, fleshy vegetables.
Night Blindness
An early symptom of vitamin A deficiency is impaired vision in low-light conditions, commonly called night blindness. Without adequate levels of rhodopsin, the retina portion of the eye isn't able to register as many shades of gray. Night blindness is of special concern to those working night shifts, driving in the dark or serving in the military.
Xeropthalmia
The membranes of the eye are dependent on vitamin A for moisture and lubrication, which is for comfortable blinking. The "Textbook of Medical Physiology" states that the average person blinks about 10 million times a year. Long-term vitamin A deficiency leads to xeropthalmia, or dry eye, which can cause scaring and ulceration of the cornea and significantly reduce day-time vision as well.
Other Symptoms of Deficiency
Chronic vitamin A deficiency also causes reduced immune response, higher rates of infection and skin conditions, such as acne, eczema, dandruff and boils.
References
- "Vitamins: Fundamental Aspects in Nutrition and Health"; G. Combs; 2008
- National Institutes of Health: Office of Dietary Supplements: Vitamin A Fact Sheet
- "Nutritional Sciences"; Michelle McGuire; 2007
- "Textbook of Medical Physiology -- Tenth Edition"; Arthur C. Guyton, et al.; 2000
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