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Vitamin A is a group of fat soluble nutrients essential for growth, vision and immune response. Unlike most water soluble vitamins, the body does not immediately remove excess vitamin A. Instead, it is stored in the liver, where excess over time can eventually damage a patient’s health.

Certain internal proteins are necessary for vitamin A to move through the body. Chylomicrons, a type of protein, must be present to move vitamin A from the intestines (where absorption takes place) to the liver for storage. Retinol-binding protein is required to move the vitamin from the liver to cells for use.

Vitamin A itself is not present in food. It is produced in the body once certain precursors in foods are ingested. As foods or supplements are digested, the precursors are converted into a usable form of the vitamin. Once it is in a useable form, vitamin A is active in the entire body, including:

• All body cells. Vitamin A is crucial for cell differentiation – the process that determines each new cell’s function. This occurs throughout life, but is significantly more active during fetal development and childhood growth. During these periods, vitamin A deficiency is known to cause severe debilitation.
  
  
• Epithelial cells. Most of the body’s vitamin A function is performed in these cells that line the skin and internal surfaces (mucous membranes), such as the respiratory, urinary and gastrointestinal tracts. Vitamin A helps synthesize proteins necessary for the function and production of mucus. Mucus creates a barrier that helps prevent invasive organisms and harmful substances from damaging tissues.
  
  
• Eyes. Vitamin A is needed by the retina (in the back of the eye) to convert light energy into nerve signals. It is also necessary to maintain the health of the cornea, the membrane that covers the outside of the eye.
  
  
• Immune system. Vitamin A is needed to create white blood cells, which fight infection.
  
  
• Skeletal system. Throughout life, and especially during growth, the body must take apart sections of bone and then rebuild them. As part of this process, vitamin A is necessary for dissolving the old bone.

To help consumers and medical professionals to determine if they are getting enough vitamin A, the U.S. National Academy of Sciences established dietary reference intakes (DRIs) for the nutrient. DRIs provide the average daily intake of vitamin A necessary to support a healthy body – based on age and gender (including whether women are pregnant or breastfeeding). Tolerable upper intake levels (UL) were also determined. Patients who consume more than the UL for vitamin A increase potential health risks. These recommendations are measured in micrograms per day. One microgram is equivalent to ¼ gram.

Vitamin A DRIs