Vitamin absorption

Vitamins do not all enter the body through the same door. They fall into two groups, and what separates them is whether they get along with fat or with water.

The fat-soluble ones, the vitamins that dissolve in fat, cannot get in on their own. They have to ride inside a micelle, a tiny clump that forms in the small intestine with its greasy side facing in and the side that tolerates water facing out. And that micelle does not come out of nowhere: it takes bile and enzymes from the pancreas. Once inside the intestinal cell, the enterocyte, the vitamin is packaged into carriers called chylomicrons that do not go straight into the blood: first they travel through the lymph. Because they end up stored in tissues, these vitamins stay in the body for quite a while.

The water-soluble ones, on the other hand, dissolve in water, and the body has nowhere to keep the extra. Whatever it does not use leaves.

All of them, from both groups, are absorbed in the small intestine. And that is what makes the fat route worth understanding: when someone does not absorb fat well, the vitamins that were traveling with it are left behind, and vitamin A deficiency can show up.