This study investigates the digestion and absorption processes in the small intestine of normal adult humans. Using a transintestinal intubation technique, researchers collected intestinal content at various levels after a balanced liquid test meal. A reference substance, polyethylene glycol (PEG), was included in the meal to track the absorption of different substances. The study found that the absorption of fat, glucose, and protein begins in the distal duodenum and is largely completed in the first 50-100 cm of the jejunum. Fat absorption is more proximal than glucose and protein. The test meal was diluted in the duodenum and concentrated further down the small intestine. The absorption of fat, glucose, and protein was calculated based on PEG concentrations in the meal and collected samples. The study also examined the presence of enzymes in the intestinal content, including lipase, trypsin, chymotrypsin, and amylase, which showed varying concentrations along the small intestine. The concentration of bile acids and phospholipids was highest in the duodenum and proximal jejunum, decreasing as they were absorbed. The pH of the intestinal content increased gradually from around six in the duodenum to values close to eight in the lower ileum. The study concluded that the small intestine is the primary site for the digestion and absorption of nutrients, with the duodenum and proximal jejunum being the main areas for fat absorption. The results indicate that the small intestine efficiently absorbs a wide range of nutrients, with the absorption of fat reaching up to 90-95%, glucose up to 100%, and protein up to 80-90%. The study also highlights the importance of bile in the digestion and absorption of fats, with bile acids and phospholipids playing a key role in the process. The findings contribute to a better understanding of the digestive and absorptive functions of the human small intestine.This study investigates the digestion and absorption processes in the small intestine of normal adult humans. Using a transintestinal intubation technique, researchers collected intestinal content at various levels after a balanced liquid test meal. A reference substance, polyethylene glycol (PEG), was included in the meal to track the absorption of different substances. The study found that the absorption of fat, glucose, and protein begins in the distal duodenum and is largely completed in the first 50-100 cm of the jejunum. Fat absorption is more proximal than glucose and protein. The test meal was diluted in the duodenum and concentrated further down the small intestine. The absorption of fat, glucose, and protein was calculated based on PEG concentrations in the meal and collected samples. The study also examined the presence of enzymes in the intestinal content, including lipase, trypsin, chymotrypsin, and amylase, which showed varying concentrations along the small intestine. The concentration of bile acids and phospholipids was highest in the duodenum and proximal jejunum, decreasing as they were absorbed. The pH of the intestinal content increased gradually from around six in the duodenum to values close to eight in the lower ileum. The study concluded that the small intestine is the primary site for the digestion and absorption of nutrients, with the duodenum and proximal jejunum being the main areas for fat absorption. The results indicate that the small intestine efficiently absorbs a wide range of nutrients, with the absorption of fat reaching up to 90-95%, glucose up to 100%, and protein up to 80-90%. The study also highlights the importance of bile in the digestion and absorption of fats, with bile acids and phospholipids playing a key role in the process. The findings contribute to a better understanding of the digestive and absorptive functions of the human small intestine.