Vitamin A is an essential nutrient that you need for proper vision, gene transcription, immune function, bone growth and the production of red blood cells. Vitamin A is actually a generic term for a number of related compounds such as retinol and retinal. Each one has a specific function in the body. The liver is necessary for the metabolism and storage of these various forms of vitamin A.
Storage
Vitamin A is a fat-soluble vitamin---a group that also includes vitamins D, E and K. This means that fats help absorb and dissolve vitamin A in the digestive system. Unlike water-soluble vitamins, which the body flushes out regularly, the liver can store fat-soluble vitamins in its fatty tissue for later use. Little is known about the molecular processes that are responsible for the storage and metabolism of vitamin A, according to Dr. William Blaner of the Columbia University Department of Medicine. However, two liver cell types known as hepatocytes and hepatic stellate cells appear to be the main sites of vitamin A storage. These cells are also a major repository for a number of other nutrients such as proteins and cholesterol.
Mobilization and Metabolism
The liver interacts with vitamin A through a class of molecules called retinol binding protein, or RBP, which delivers the vitamin to and from storage in the liver tissue and mobilizes it into circulation. Fat cells appear to create RBP, although it's also present in the liver, blood and small intestines. Once mobilized, vitamin A often requires conversion into one of its various forms. The liver plays a role in the metabolism of retinol, for instance.
Toxicity
Deficiencies of fat-soluble vitamins tend to be rarer than deficiencies of water-soluble vitamins. The liver generally stores up to two years of vitamin A. However, this also means that toxicity of fat-soluble vitamins is more common. Signs of vitamin A toxicity usually begin appearing with the long-term consumption of at least 8,000 to 10,000 micrograms---or 8 to 10 mg---per day. The liver may be even more vulnerable than other organs to large doses of vitamin A. One of the symptoms of severe over-consumption is the enlargement of the liver.
Dietary Liver
Because the liver contains such large stores, it's also an excellent source of vitamin A in the diet. For example, 1 tsp of cod liver oil---a fat derived from the liver of cod---contains 1,350 micrograms, which represents more than the entire recommended dietary allowance, according to the Linus Pauling Institute. In addition, if you eat liver and take vitamin A supplements, you can be at risk for bone fractures, according to the UK Scientific Advisory Committee on Nutrition. Post-menopausal women and older individuals are at the highest risk of bone fractures from the overconsumption of vitamin A and may want to avoid eating liver altogether.
Member Comments