Questions: Intestinal Mucosal Absorption and Nutrient Transport

Question 1

After a large carbohydrate-rich meal, glucose concentration in the intestinal lumen may actually be lower than inside the enterocyte. How does the intestine continue absorbing glucose under these conditions?

• A. Glucose diffuses freely through the lipid bilayer without any transporter
• B. SGLT1 uses the inward sodium gradient to co-transport glucose against its concentration gradient via secondary active transport
• C. GLUT2 on the apical membrane drives glucose uptake using ATP directly
• D. Glucose absorption pauses until luminal concentrations rise above cytoplasmic levels

Answer: (B)

SGLT1 (Sodium-Glucose Linked Transporter 1) is a secondary active transporter: it couples glucose uptake to the downhill movement of Na⁺ into the cell, exploiting the sodium gradient maintained by the basolateral Na⁺/K⁺-ATPase. This coupling allows glucose to be transported against its own concentration gradient — energy from Na⁺ flow drives glucose uptake. This is essential for efficient post-meal absorption. GLUT2 is on the basolateral membrane and uses facilitated diffusion (not active transport); GLUT5 handles fructose on the apical membrane.

Question 2

After a high-fat meal, dietary triglycerides enter the bloodstream via which route?

• A. Through the portal vein directly, like glucose and amino acids
• B. Through the thoracic duct and lymphatic system as chylomicrons, bypassing the liver
• C. Through the portal vein after being broken down to free fatty acids
• D. Through passive diffusion directly across the intestinal epithelium into capillaries

Answer: (B)

Fats follow an entirely different route than carbohydrates and proteins. After entering enterocytes as monoglycerides and free fatty acids, they are re-esterified into triglycerides, packaged with cholesterol, phospholipids, and apolipoprotein B-48 into chylomicrons — particles too large to enter capillaries. Chylomicrons are secreted by exocytosis into the lacteals (lymphatic capillaries in each villus) and travel through the thoracic duct to enter systemic circulation at the subclavian vein. This bypasses first-pass hepatic metabolism, which is why high-fat meals raise systemic triglycerides before the liver can process them.

Question 3

Glucose and fructose are both monosaccharides that are absorbed from the intestinal lumen by the same apical membrane transporter.

• T. True
• F. False

Answer: False

Glucose and galactose use SGLT1 (a secondary active, Na⁺-coupled transporter) on the apical membrane. Fructose uses GLUT5, a facilitated diffusion transporter that requires no energy and moves fructose passively down its concentration gradient. This distinction has real physiological consequences: GLUT5 has limited capacity and can be overwhelmed by large fructose loads, while SGLT1 can actively concentrate glucose. Both sugars exit through GLUT2 on the basolateral membrane, but their apical entry mechanisms are entirely different.

Question 4

Fat-soluble vitamins (A, D, E, K) are absorbed via the same lymphatic pathway as dietary fats, and their absorption depends on adequate dietary fat and bile salts.

• T. True
• F. False

Answer: True

Fat-soluble vitamins are hydrophobic and require the same infrastructure as dietary fats: bile salts to form micelles that ferry them to the brush border, passive diffusion across the apical membrane, incorporation into chylomicrons within the enterocyte, and exit via lacteals into the lymphatic system. This is why very low-fat diets or bile salt deficiency can cause fat-soluble vitamin deficiencies, and why fat-soluble vitamin toxicity (e.g., vitamin A overdose) is possible — they accumulate in fatty tissues rather than being excreted in urine like water-soluble vitamins.

Question 5

Why do dietary fats travel through the lymphatic system rather than directly entering portal blood, as glucose and amino acids do?

Model answer: After re-esterification inside enterocytes, triglycerides are packaged into chylomicrons — large lipoprotein particles (80–1200 nm) that are too large to pass through the tight junctions and small fenestrations of intestinal capillaries. The lacteals (lymphatic capillaries) have larger, more permeable openings that accommodate chylomicron exocytosis. This structural constraint forces fats through lymphatics. The consequence is that dietary fat enters systemic circulation via the thoracic duct, bypassing the liver's first-pass metabolism — unlike glucose and amino acids, which go directly to the liver via the portal vein.

The size constraint is the mechanistic reason, but the physiological consequence matters equally. Portal delivery of glucose and amino acids allows the liver to immediately process and regulate their systemic levels. Chylomicron delivery via lymphatics means dietary fat reaches peripheral tissues before the liver can act — explaining why postprandial hyperlipidemia is primarily in systemic rather than portal blood, and why fat-soluble drugs and vitamins accumulate differently than water-soluble ones.