Absortion of Vitamins

Vitamins are organic compounds that your body needs in small amounts to maintain physiological processes and sustain overall health. Vitamins are available in most of the foods you eat, as well as several supplemental forms. There are a total of 13 vitamins that are categorized into two classes -- fat-soluble and water-soluble -- based on how your body absorbs them.

Fat-Soluble Vitamins

Fat-soluble vitamins, which include A, D, E and K, need dietary fat to be properly absorbed. When you eat fat-soluble vitamins, they combine with fatty acids and bile, which transport them to the walls of your small intestine. The fat-soluble vitamins are then able to travel through the intestinal mucosa in your small intestine, where they combine with fat and other lipids contained in chylomicrons, a type of lipoprotein in your blood. The fat-soluble vitamins then enter your lymphatic system and circulate throughout the body before finally entering your bloodstream. Your body uses the fat-soluble vitamins it needs and then stores any excess in the liver, muscle and fat tissues.

Water-Soluble Vitamins

The water-soluble vitamins, which include vitamin C and the B vitamins, are absorbed with water. These vitamins dissolve in water and pass through your small intestine directly into your bloodstream. Most of the water-soluble vitamins are absorbed in the upper portion of your small intestine. The only exception is vitamin B12, which is absorbed in the lower part of the small intestine. Once water-soluble vitamins are absorbed, your body uses what it needs and then excretes the excess through your urine.

Other Factors

Several factors, such as dosage, mode of administration and form of the vitamin, may affect absorption of vitamins. For example, taking larger doses of thiamine and vitamin B12 actually decreases the amount of the vitamin that your body absorbs, according to an article in the March 1963 issue of "The American Journal of Clinical Nutrition."

The form of the vitamin, such as liquid or solid, may also affect the rate at which the vitamin is absorbed. Manufacturers of liquid vitamins claim that these vitamins are more quickly absorbed because they do not have to be broken down as much in your digestive tract. There is unsubstantial evidence to support this claim, however.

Considerations

Because vitamin absorption takes place in the small intestine, digestive diseases, such as Crohn's disease and ulcerative colitis, may interfere with proper vitamin absorption. Instead of passing through the small intestine into your blood or lymph system, vitamins are excreted from your body via watery diarrhea. If you have a digestive condition, it increases the likelihood of developing vitamin deficiencies.