Vitamin A

# Overview
Vitamin A is an essential human nutrient. It exists not as a single compound, but in several forms. In foods of animal origin, the major form of vitamin A is an alcohol (retinol), but can also exist as an aldehyde (retinal), or as an acid (retinoic acid). Precursors to the vitamin (a provitamin) are present in foods of plant origin as some of the members of the carotenoid family of compounds.
All forms of Vitamin A have a Beta-ionone  ring to which an isoprenoid chain is attached. This structure is essential for vitamin activity.
- retinol, the animal form of Vitamin A, is a yellow, fat-soluble, vitamin with importance in vision and bone growth.
- other retinoids, a class of chemical compounds that are related chemically to Vitamin A, are used in medicine.

# Discovery of Vitamin A
The discovery of Vitamin A stemmed from research dating back to 1906, indicating that factors other than carbohydrates, proteins, and fats were necessary to keep cattle healthy. By 1917 one of these substances was independently discovered by Elmer McCollum at the University of Wisconsin-Madison, and Lafayette Mendel and Thomas Osborne at Yale University. Since "water-soluble factor B" (Vitamin B) had recently been discovered, the researchers chose the name "fat-soluble factor A".

# Sources
Vitamin A is found naturally in many foods.  Each of the following contains at least 0.15 mg (which is equal to 150 micrograms (mcg). See Recommended Daily Intake below.) of Vitamin A or beta carotene per 1.75-7 oz. (50-200 g): butter, lemon, sweet potatoes, carrots, collard greens, milk, beetroot, pumpkin, spinach, beef, apple, winter squash, apricots, cantaloupe melon, mango, liver, (beef, pork, chicken, turkey, fish) eggs, broccoli, and leafy vegetables.

# Recommended daily intake
Vitamin A US Dietary Reference Intake:
- 900 micrograms for men
- 700 for women.
- Upper limit - 3,000 micrograms.
(Note that the limit refers to retinoid forms of vitamin A. Carotene forms from dietary sources are not toxic.)

# Equivalencies of retinoids and carotenoids
Vitamin A intake is often expressed in international units (IU) or as retinol equivalents (RE), with 1 IU = 0.3 micrograms retinol. Because the production of retinol from provitamins by the human body is regulated by the amount of retinol available to the body, the conversions apply strictly only for Vitamin A deficient humans. The absorption of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.
Conversion of carotenoids into retinol relies on adequate intake of vitamin C, zinc and protein.

# Symptoms of deficiency
Night blindness, corneal drying (xerosis), triangular gray spots on eye (Bitot's spots), corneal degeneration and blindness (xerophthalmia), impaired immunity, hypokeratosis (white lumps at hair follicles), keratosis pilaris, softening of the cornea (keratomalacia).

# Symptoms of overdose
As vitamin A is fat-soluble, disposing of any excesses taken in through diet is a lot harder than with water-soluble vitamins B and C. As such, vitamin A toxicity can result. This can lead to nausea, jaundice, irritability, anorexia (not to be confused with anorexia nervosa, the eating disorder), vomiting, blurry vision, headaches, muscle and abdominal pain and weakness, drowsiness and altered mentality.
In chronic cases, hair loss, drying of the mucous membranes, fever, insomnia, fatigue, weight loss, bone fractures, anemia, and diarrhea can all be evident on top of the symptoms associated with less serious toxicity.