Vitamin A has the unique distinction of being the first vitamin officially named, thereby earning it the letter A as its identifying mark. It also has the distinction of being much more complicated that researchers originally thought in 1913, the year it was discovered. At that time, the focus was on what we now call preformed vitamin A, or retinol. Vitamin A is absorbed primarily from the small intestine. Absorption of this fat-soluble vitamin is reduced with alcohol use, with vitamin E deficiency, with cortisone medication, and with excessive iron intake or the use of mineral oil, as well as with exercise. Vitamin A is needed at a level of at least 5,000 IU a day, although this may vary due to many factors.
The two forms of vitamin A come from different food sources. Preformed A (retinol) is the main animal-source vitamin A. It is found in highest concentrations in all kinds of liver and fish liver oil, which is a common source for supplements. Egg yolks and milk products, such as whole milk, cream, and butter, are also good sources of vitamin A. Provitamin A, mainly in the form of beta-carotene, is found in a wide variety of yellow-and orange-colored fruits and vegetables, as well as leafy green vegetables.
Vitamin A performs many important functions in the human body. The following are the most common.
- Growth and tissue healing
- Healthy skin
- Lowering cancer risk and supporting immune function
- Regulating genetic processes
Signs of deficiency:
- Night blindness
- Dry, bumpy skin
- Lack of luster in hair and dandruff
- Bone softness
- Abnormal menstruation
- Fatigue and insomnia
- Decrease in appetite and some loss of smell and taste
- Lowered immune function
Source: Haas, E. (2006) Staying Healthy with Nutrition: The Complete Guide to Diet & Nutritional Medicine. New York, Ten Speed.