Which nutrient is not absorbed directly into portal blood?

Long-chain fatty acids are not absorbed directly into portal blood; instead, they enter the lymphatic system after processing. When fats are consumed, they undergo emulsification and are broken down into smaller components in the intestines. The products of fat digestion include long-chain fatty acids and monoglycerides, which combine with bile salts to form micelles. These micelles are then absorbed into the enterocytes of the intestinal lining.

Within the enterocytes, long-chain fatty acids are re-esterified to form triglycerides and packaged into chylomicrons. These are lipoprotein particles that transport lipids away from the intestines. Chylomicrons enter the lymphatic system through the lacteals in the intestinal villi and eventually reach the bloodstream at the thoracic duct, bypassing the portal circulation altogether.

In contrast, monosaccharides and amino acids enter the portal blood directly after being absorbed in the intestines. Medium-chain fatty acids do not require such extensive processing and are able to enter portal circulation more easily, as they bind to albumin in the blood and are transported directly to the liver.

Overall, the absorption pathway of nutrients plays a crucial role in their transport and utilization in the body, making the understanding of these mechanisms fundamental