Absorption of most amino acids and simple sugars (except glucose) takes place in the small intestine, specifically in the portion known as the jejunum. The jejunum is the middle section of the small intestine and is responsible for the majority of nutrient absorption. After digestion in the stomach and upper part of the small intestine (duodenum), the partially digested food enters the jejunum, where it comes into contact with the mucosal lining of the intestinal wall. The mucosal lining contains numerous finger-like projections called villi, which increase the surface area available for absorption. The villi are covered in microvilli, which are microscopic brush-border membranes that further enhance absorption.

Within the jejunum, amino acids and simple sugars are actively transported across the intestinal epithelium into the bloodstream. Active transport requires energy in the form of adenosine triphosphate (ATP) and involves specific transport proteins located in the cell membranes of the intestinal cells. These transport proteins facilitate the movement of amino acids and simple sugars from the intestinal lumen into the enterocytes, where they enter the bloodstream through the capillaries.

Once in the bloodstream, these absorbed nutrients can be transported to different tissues and organs throughout the body for various metabolic processes, such as energy production, protein synthesis, and cellular repair.