Carbohydrate digestion begins in the mouth with the breakdown of starch by salivary amylase. However, the complete digestion of carbohydrates occurs in the small intestine. The pancreas secretes pancreatic amylase, which breaks down carbohydrates into simple sugars such as glucose, fructose, and galactose. These simple sugars are then absorbed into the bloodstream through the walls of the small intestine.


Protein digestion starts in the stomach with the secretion of hydrochloric acid and pepsinogen. The acid converts pepsinogen into pepsin, which breaks down proteins into smaller peptides. The pancreas also secretes enzymes such as trypsin, chymotrypsin, and carboxypeptidase which further break down peptides into amino acids. The amino acids are then absorbed into the bloodstream through the walls of the small intestine.


Fat digestion begins in the small intestine with the secretion of bile from the liver and gallbladder. Bile emulsifies fat, breaking it down into smaller droplets. Pancreatic lipase then breaks down the fat droplets into fatty acids and glycerol. These fatty acids and glycerol are then absorbed into the bloodstream through the walls of the small intestine.


In conclusion, the complete digestion of carbohydrates, proteins, and fats occurs in the small intestine. The breakdown of these macronutrients into their basic components allows for their absorption into the bloodstream, where they can be used for energy or stored for later use.

The site of complete digestion of carbohydrates fats and proteins in humans is small intestine