Question

where in the gastrointestinal tract are nutrients first absorbed into the body? give some reasons this cannot happen prior to this point.

Answer 1

The digestion starts in the mouth, where salivary amylase act on starch to produce maltose. Saliva also has a lipase that starts digesting fats. Food is then passed to the stomach via the esophagus. Proteins are partially digested by pepsin in the stomach. However, major protein, carbohydrate and lipid breakdown occurs in small intestine via pancreatic enzymes that are secreted in small intestine.

The enterocytes also have brush border enzymes such as sucrose and maltase that further breakdown sugars to monosaccharides. Absorption of nutrient occurs majorly in the small intestine. Large intestine is involved in absorbing water, excretion of waste products and production of vitamins such as vitamin K. Some digestion does occur in large intestine as well.

The reason why absorption of nutrient only occurs in small intestine:

a. Stomach acid will inactivate most digestive enzymes except pepsin. Pepsin also does not completely digest protein to amino acids. It only forms peptides.

b. The digestion is majorly occurring in the small intestine due to secretion of pancreatic enzymes. The pH of small intestine is optimum for activity of pancreatic enzymes.

c. The presence of villi on inner wall of small intestinal epithelium provides a larger surface area that aids in absorption of breakdown product of digestion. Further, these absorbed nutrients can directly enter the blood via the basement membrane. Fatty acids and monoglycerides can absorbed by enterocytes via diffusion and enter the lymphatic system via lacteals. Thus, the small intestine is specialized organ for digestion and absorption. There are specific transporters present in enterocytes for absorption of glucose and amino acids. No such transporters are present in stomach.

d. The stomach lacks a higher surface area for absorption. It only allows absorption of alcohol and aspirin, which are lipid soluble compounds.

e. Esophagus lacks any digestive enzymes and hence, does not digest or absorb nutrients.