In the 1880s, Dr. Nikolai Lunin ascertained that a substance in milk was essential for growth. The problem was that he couldn’t distinguish the substance.

That took until 1931, when Dr. Paul Karrer identified a structure — which he called vitamin A — as the unknown growth factor Dr. Lunin had described decades earlier.

Vitamin A is actually made up of a group of related molecules found only in animal food products. The best sources include:

• Butter

• Cod liver oil

• Eggs

• Whole milk

Many yellow and orange colored plants contain substances called carotenoids (such as beta-carotene), which are precursors to vitamin A.

This means that they can be converted to vitamin A within the body. However, the process is not efficient.

Vitamin A has many functions in the body, including aiding vision. Specifically, the eye’s retina depends on adequate vitamin A to function optimally. Deficiency of this nutrient is the number one cause of blindness worldwide.

Vitamin A also helps regulate and stimulate the immune system.

The first line of defense in the immune system — the cells lining the skin and the mucosal tracts — depend on vitamin A to function.

For more than 20 years, I have given patients large doses of vitamin A (25,000 to 100,000 IU) to treat infections. This has worked especially well when combined with vitamin D (100,000 IU) and vitamin C (10,000 to 20,000 mg) daily for a few days.

Because it is vital for the development of most of the major organs of the body, vitamin A deficiency during pregnancy is known to cause birth defects.

It is also needed for red blood cell production, and for years has been used to treat skin disorders.

The recommended daily allowance (RDA) for vitamin A is 3,000 IU for men and 2,333 IU for women. But beware: Vitamin A is fat soluble, and therefore toxicity is a risk.

The doses I recommended for treating acute infections are not to be used long term. Generally, I recommend using large doses of vitamin A for less than seven days.

The best results occur when it is used in conjunction with vitamin D, because these nutrients share many of the same transport molecules and receptors.