At concentrations present in the diet, which vitamin is absorbed primarily by diffusion?
**Core Concept**
The absorption of vitamins in the gastrointestinal tract can occur through different mechanisms, including active transport, facilitated diffusion, and passive diffusion. Passive diffusion, the primary mechanism for vitamin B12 absorption, relies on the concentration gradient across the intestinal epithelium.
**Why the Correct Answer is Right**
Vitamin B12 is absorbed in the terminal ileum mainly through a passive diffusion process facilitated by the presence of intrinsic factor. Intrinsic factor, a glycoprotein secreted by the parietal cells in the stomach, binds vitamin B12, allowing it to be absorbed in the ileum. This process relies on the concentration gradient of vitamin B12 in the diet, making it essential for the efficient absorption of this vitamin.
**Why Each Wrong Option is Incorrect**
* **Option A:** Fat-soluble vitamins (A, D, E, and K) are absorbed primarily through active transport and passive diffusion, facilitated by the presence of bile salts and pancreatic enzymes. This is not the primary mechanism for vitamin absorption at concentrations present in the diet.
* **Option B:** Carbohydrates are absorbed primarily through facilitated diffusion and active transport in the small intestine, facilitated by enzymes such as sucrase and maltase. This is not relevant to vitamin absorption.
* **Option C:** Amino acids are absorbed primarily through active transport in the small intestine, facilitated by enzymes such as aminopeptidases and amino acid transporters. This is not relevant to vitamin absorption.
**Clinical Pearl / High-Yield Fact**
The absorption of vitamin B12 is critically dependent on the presence of intrinsic factor and the concentration gradient of vitamin B12 in the diet. Deficiencies in intrinsic factor or impaired vitamin B12 absorption can lead to megaloblastic anemia and neurological dysfunction.
**Correct Answer: C. Vitamin B12**