Vitamin A, also known as retinol, is a fat-soluble vitamin essential for human development. Vitamin A was in fact one of the first vitamins to be discovered. In 1913, scientists were already touting the benefits of this vitamin to their patients. Some of the most important functions of vitamin A include:

• Vision protection: Vitamin A is best known as "the eye vitamin," since it protects and improves vision. Vitamin A helps with night vision, adaptation to low light, and eye focus. Vitamin A deficiencies are believed to be one of the leading causes of blindness in developing countries.

• Resistance to viral infections: A deficiency of vitamin A is often cited as the leading cause an infection takes longer in disappearing. Vitamin A stimulates the immune system and increases the production of antibodies.

• Reproduction: Vitamin A is essential for normal reproductive cycles and sperm production. Lack of vitamin A can lead to decreased fertility levels.

Vitamin A deficiency is more common in people with malabsorption problems, or those taking certain medications which interfere with absorption, such as Colestid®, a cholesterol-lowering medication, and Neomycin®, an antibacterial drug. Supplements are readily available, and are a good choice for people who are concerned about not getting enough vitamin A from their diets, or for those who may think they are somehow losing part of what they take in.

The best sources of Vitamin A include animal products such as eggs, cheese and milk, and oily fish. Vegetables are not a good source of vitamin A but they do provide carotenoids, which the body can transform into retinol. The best sources of carotenoids include yellow-red and dark-green plants such as carrots, spinach, bell peppers, kale, and turnip greens.

Because vitamin A is stored in the body, it is possible to consume too much. Toxicity symptoms for vitamin A include vomiting, fatigue, and loss of appetite; in acute cases, it can lead to visual changes, depression, and anemia. All these symptoms usually reverse themselves as soon as the intake of vitamin A is decreased.