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The 14 recognized vitamins are classified as either fat soluble or water soluble. This is largely based on how they are absorbed into the body. Fat soluble vitamins require specific carrier molecules to be transported through the body for storage in fat cells and the liver. Water soluble vitamins are able to move freely across cell membranes, allowing them to be absorbed directly into the blood. Other general differences include:

The fat soluble vitamins include:

• Vitamin A (retinol, retinal, retinoic acid). A group of nutrients essential for growth, vision and immune response. It is active in the eye, where it helps convert light to nerve signals in the retina (the back of the eye) and helps maintain the health of the cornea (the outer membrane). Vitamin A is active in the skin and cells that line internal surfaces (epithelial cells) and also acts as an antioxidant, which helps protect cells from damage caused by unstable molecules called free radicals. Vitamin A, along with other vitamins and minerals, also has a role in the remodeling of bone.
  
  
• Vitamin D (calciferol). Works with calcium and phosphorous to build and maintain strong bones. When exposed to sunlight, skin cells make vitamin D and in some cases dietary consumption may be unnecessary.
  
  
• Vitamin E (tocopherol, tocotrienol). An antioxidant that is also necessary for the proper function of the reproductive, nervous and muscular systems.
  
  
• Vitamin K (phylloquinone, menaquinone, menadione or naphthoquinone). Required for blood to clot. Without vitamin K, even minor injuries could cause massive blood loss, leading to death. It is also involved in bone formation.

The water-soluble vitamins include:

• B vitamins. In general, each of the B vitamins is involved in growth and the metabolism of carbohydrates, lipids and proteins. Other functions include:
  
  
• Vitamin B1 (thiamine). Found on the membrane of nerve cells, where it aids communication between nerves and cells.
  
  
• Vitamin B2 (riboflavin). Necessary for cell development and function and may be involved in blood cell production. It is also involved in breaking down fats and proteins.
  
  
• Vitamin B3 (niacin, nicotinic acid, nicotinamide, niacinamide). Used in the production of new cells, as well as fatty acids and steroids. Large doses may be prescribed to treat high cholesterol. B3 can also be made from tryptophan (an amino acid).
  

  

• Vitamin B5 (pantothenic acid). An essential component in the processes that make lipids, neurotransmitters and hemoglobin. The body produces some B5 itself, but not enough to meet its needs.
  
  
• Vitamin B6 (pyridoxine, pyridoxal, pyridoxamine). Required in the process to make amino acids and maintain blood glucose levels. Like B5, vitamin B6 is also needed to synthesize neurotransmitters and to make hemoglobin.
  
  
• Vitamin B9 (folate, folic acid). Involved in the synthesis of proteins and DNA, the genetic blueprint for the body. B9 is essential in the earliest stages of pregnancy to prevent neural tube defects in the fetus.
  
  
• Vitamin B12 (cobalamin). Helps maintain the health of the nervous system. It also has roles in the breakdown of fatty acids and amino acids, and the creation of red blood cells and DNA. B12 is the only water-soluble vitamin that requires a specific chemical (intrinsic factor [IF]) to be absorbed into the bloodstream.
  
  
• Biotin (sometimes called B7). Involved in the process that creates the blood sugar glucose. Some biotin is made by bacteria in the alimentary canal, but it must also be consumed with the diet.
  
  
• Choline. Necessary to transport fats through the body and the synthesis of several neurotransmitters.
  
  
• Vitamin C (ascorbic acid). Involved in the production of the connective protein collagen, neurotransmitters, hormones and hemoglobin. Like vitamins A and E, vitamin C also acts as an antioxidant, protecting cells from highly reactive chemicals.

While each vitamin has a specific function, they are also involved in complex interdependent relationships. Without B6, the tryptophan can not be converted to B3, potentially leading to a deficiency. Vitamin A is needed to remodel bone to allow for growth, but too much can counteract the bone formation of vitamins D and K.