Vitamin A
About Vitamin A
Vitamin A is a group of fat soluble compounds that exists as preformed retinol in animal foods such as liver, and as provitamin A carotenoids in plants, with beta carotene the most common. Recommended intakes are expressed as retinol activity equivalents and vary by age, sex and life stage. Vitamin A is critical for vision, especially in dim light, because the retinal form combines with the protein opsin to make rhodopsin in the retina. It also regulates cell growth and immunity. Deficiency is a leading cause of preventable childhood blindness worldwide, producing xerophthalmia, in which dry eyes can progress to corneal ulceration and blindness, along with impaired immunity and poor growth. Supplementation programs in low income countries have been associated with lower child mortality from measles and diarrheal disease. Excess preformed vitamin A is toxic, causing nausea and headache acutely and liver damage, bone pain, hair loss and birth defects with chronic high intake. In the ATBC and CARET trials, high dose beta carotene supplements increased lung cancer risk in smokers.
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