Let's talk about the three primary nutrient sources that we consume, carbohydrates (starches), proteins, and fats which will provide the glucose, amino acids, and fatty acids respectively.
Before absorption can occur, we must first prepare and begin to mechanically digest these products which begins in the mouth through chewing, or mastication and the forming of the bolus of food. The chemical digestion also begins here on the carbohydrates by the secretion of salivary amylase enzyme. As the now partially digested food enters the stomach from the esophagus, the mechanical digestion continues by the churning action of the stomach muscles and the chemical digestion of the proteins begins under the action of pepsinogen to pepsin enzyme conversion by hydrochloric acid. The stomach contents now referred to as chyme, incrementally enter the upper third of the small intestine or the duodenum, where the chemical digestion will continue. The gallbladder begins to release bile salts, which mixed along with the pancreatic enzymes, will chemically emulsify the fats for digestion in addition to further enzyme breakdown of all three categories of nutrients.
It is now here in the duodenum where the majority of absorption will occur, as this segment of the small intestine is responsible for claiming the glucose, amino acids, and fatty acids, in addition to various vitamins, and minerals out of food.
Small finger like projections referred to as "villi" line the walls the entire length of the small intestine. The microvilli greatly increase the surface area and are rich in blood and lymphatic (lacteal) vessels for the absorption of the now digested nutrients across the wall. In fact, malabsorption may be the result of a decrease in, or absence of the microvilli in segments of the small intestine. This non-homeostatic loss of microvilli may be the result of a number of digestive health conditions.
The water soluble products move by diffusion through the walls and barriers, like the villi in the small intestine, and into the blood vascular system. Once they are emulsified by the bile salts, fats are broken down into the fatty acids and those along with the fat-soluble vitamins (A, D, E, and K) are absorbed by the villi into lacteals of the lymphatic vessels where they are transported to the liver for storage and released into the body as needed.
