Digestion of carbohydrates, fats and proteins primarily takes place in the small intestine, and their products go into the bloodstream or lymphatic system. Digestion and absorption happen in a very orderly way within the small intestine, and involves the help of many enzymes, or proteins that the cells use to speed the reactions.
The Small Intestine
The small intestine has three sections: the duodenum, the jejunum and the ileum. The duodenum is approximately 12 inches long, notes Kim Barrett, Ph.D., professor of medicine at the University of California School of Medicine. This first section regulates digestion, and both the pancreas and the gallbladder have secretions that enter into it. The next section is the 8-foot jejunum, where most of the absorption takes place. The ileum, or the last section, is 12 feet long.
The Digestive Enzymes of the Duodenum
As explained by Dr. Barrett, carbohydrate digestion begins in the mouth with enzymes in the saliva. It continues within the duodenum with the amylase enzyme that secretes into the duodenum from the pancreas. In addition, the intestinal cells use the sucrase, maltase and lactase enzymes. Fats break down with the help of the lipase enzyme, which secretes from the pancreas. Protein digestion starts in the stomach. In the duodenum, the trypsin enzyme from the pancreas and the intestinal peptidase enzymes continue the process.
The Digestive Enzymes of the Jejunum
The digestion of carbohydrates and fats finishes in the upper part of the jejunum, or the second section of the small intestine, while most of the absorption of carbohydrates and fats takes place in the duodenum and jejunum, advises Andrew Shelton, M.D., assistant professor of surgery at Stanford University School of Medicine. The main purpose of the jejunum is to absorb nutrients, which is why it has special structures called villi and microvilli. The jejunum absorbs most proteins as well.
The main function of the ileum is absorption, especially of bile acids, the fat-soluble vitamins and vitamin B-12. Bile, made in the liver, helps to break down fats. When the ileum absorbs the bile acids, it sends them through the bloodstream to the liver to become part of bile. The fat-soluble vitamins, which dissolve in fat, are vitamins D, A, K and E. Thus, damage or surgical removal of the ileum can result in problems absorbing the fat-soluble vitamins and vitamin B-2, as explained by Atenodoro Ruiz, Jr., M.D., in "The Merck Manual for Healthcare Professionals."