Vitamins are important substances that the body uses to help speed up some of the chemical reactions that are necessary to life. Vitamin deficiencies are usually caused by people not consuming enough of a given vitamin, but a vitamin deficiency can also be caused by poor vitamin absorption by the gastrointestinal tract.
One factor which affects how vitamins are absorbed by the digestive tract is their solubility. Some vitamins, including vitamins A, D, E and K are soluble in fats, the Merck Manual explains. Most of the other vitamins are water-soluble. The solubility of a vitamin describes the environment in which the vitamins are easily dissolves; consequently, fat soluble vitamins are only absorbed well in the presence of fats. If you are eating a diet low in fat or have trouble digesting fats, you can also have problems with absorbing these vitamins.
Vitamin absorption can also be impaired by conditions which cause the digestive tract to not absorb nutrients well. These conditions, which are known as malabsorption syndromes, can be caused by surgery on the digestive tract. Malabsorption syndromes can also be the result of infections which cause the lining of the intestines to become inflamed, which decreases their abilities to absorb vitamins and other nutrients. Other condition, such as celiac or Crohn's disease, can also cause inflammation in the intestines, inhibiting vitamin absorption.
Vitamin B-12 is notable for needing another substance to be absorbed by the digestive tract. This substance, which is known as intrinsic factor, is produced by cells in the stomach, MedLinePlus explains. A lack of intrinsic factor can be caused by stomach surgery or by a condition known as pernicious anemia, in which the immune system attacks the cells in the stomach which produce intrinsic factor, leading to poor absorption of vitamin B-12. This can result in anemia and neurological problems.
Vitamin D and Calcium
Vitamin D is a vitamin which is needed for the body to absorb another important mineral, calcium. Vitamin D can be found in some foods, such as fish and beef liver, but most dairy products are also fortified with this vitamin, the Office of Dietary Supplements explains. Vitamin D can also be produced by the body when the skin is exposed to sunlight. Because vitamin D is needed for calcium to be absorbed by the digestive tract, vitamin D deficiencies can cause low calcium levels in the body, which can cause weak bones.