You get hungry, you eat, the food is digested and then a complex process of transformation begins that turns a burger and fries -- or some veggies and lean protein -- into activity. After your food is digested, it travels through about 27 feet of small and large intestines, where the nutrients are extracted and assigned to specific roles in energy production and body maintenance.
Absorption
After the digestive enzymes in the small intestine break down the macronutrients carbohydrates, proteins and fats into particles tiny enough to be absorbed, the water-soluble nutrients and fatty acids are absorbed into the blood stream. The fat-soluble nutrients are released into the lymphatic system. Cholesterol, vitamins and minerals are not broken down in the small intestine before they are directly absorbed.
Reduction of Nutrients
In order to be absorbed, carbohydrates are reduced to simple sugars, fats are broken down into fatty acids and glycerol, and proteins are reduced to amino acids. Then, the nutrients can pass through the circular folds in the small intestine and be absorbed by the villi, small projections that stick out from the folds to snag nutrients, and the microvilli, the microscopic hair-like structures on the cells of the villi. Once absorbed, nutrients are directed to the bloodstream or lymphatic system.
Metabolism
As soon as the nutrients are absorbed after digestion, they travel via the bloodstream or lymphatic system to the cells. There they begin to create new blood cells, build muscle tissue, fuel various body functions -- like contracting muscles, breathing, controlling body temperature and blood circulation -- and supply energy. Hormones, secreted primarily by the thyroid gland, are the triggers for the process of oxidation that turns nutrients to energy. Metabolism is a term that encompasses all the stages of using nutrients as fuel -- digestion, absorption, oxidation and elimination.
Elimination
Fiber and other non-digestible material moves from the small intestine into the large intestine, where the water is removed and the material is compressed. Salts, fatty acids and some B and K vitamins are absorbed from the large intestine along with the water. Then, the remaining material is prepared to be expelled. In a process that takes up to 48 hours in a healthy digestive system, the non-digestible matter travels through the colon into the rectum and is eventually eliminated.
References
- Oklahoma State University Extension; Digestion, Absorption and Metabolism; Janice Hermann Ph.D., RD, LD
- Nemours: Teens Health: Digestive System
- Illinois Valley Community College; Digestion, Absorption and Metabolism; May 14, 2010
- University of Maryland Medical Center: Metabolism: Overview
- Children's Hospital of Pittsburgh: Organs: Small and Large Intestines
