I've been reading through some papers on a gut-brain connection that regulates food intake and blood nutrient balance. I've learned some interesting things.
First of all, when fat hits your small intestine (especially long-chain fatty acids), it sends a message to the brainstem via the vagus nerve. This rapidly inhibits eating behavior.
The hypothalamus can also inhibit glucose production by the liver in response to fat in the bloodstream, by sending it signals via the vagus nerve.
A recent paper that got me interested in all this showed that when you put fatty acids on the upper small intestine, it sends a signal to the brain, which then sends a signal to the liver, increasing insulin sensitivity and decreasing glucose production.
There are two conclusions I can draw from this. First, fat is filling. But we already knew that. The second is that fat may also be able to improve post-meal insulin sensitivity, and not just because it replaces carbohydrate and slows digestion.
One caveat is that the fat receptor cells become desensitized after a few days on a high-fat diet (at least in rats), responding less well to fat over time. Maybe they need to be reset periodically. Intermittent fasting, anyone? Actually, I won't really believe this result until it's replicated in humans. After all, we're better adapted to a high-fat diet than rodents.
The upper small intestine is not just a passive nutrient sponge. It's a very active player in the body's response to food, coordinating changes in food intake and nutrient disposal.