At the top of your intestines and bottom of your stomach, your food hits a red light that tells your brain you’re full.The vagus nerve delivers this signal. This nerve comes from the brain and causes the stomach to contract. The nerve’s also the main cable controlling the relaxation section of the nervous system (the parasympathetic system).When the GI tract senses fat, it produces a peptide called CCK, which switches the vagus nerve on. CCK stands for cholecystokinin, but you can think of it as Crucial Craving Killer. Its main function is to tell your brain (via the vagus nerve) that your stomach feels full.CCK provides a very short-term, intense message. This contrasts leptin, which provides a more long-term satiety message.After the food spends time in your stomach, it slowly leaves that reservoir and goes into the small intestine via the duodenum. This is the first part of your intestines, which comes right after the stomach.Then CCK puts up a digestive detour sign—a very physical sign that you feel full. The pylorus (opening at the end of the stomach) slams shut, which keeps food from moving into the small bowel. This is how you get full both mentally and physically. High-saturated fat diets lead to less CCK sensitivity, so you wouldn’t feel as full after eating a steak!QUIZ: What’s Your Eating Style?Your food then enters the small intestine, and has a head-on collision with bile. Bile’s the thick, green digestive fluid that the liver secretes. This is also stored in the gallbladder and released into the small intestine. CCK has yet another effect. It causes the gallbladder to contract.The pancreas releases lipases, which break the fat into smaller particles. The particles then interact with bile to form a compound that your body’s cells easily absorb. Bile surrounds fat like soap surrounds grease on our hands. This way, it can be scrubbed from the intestinal wall and better digested and absorbed.Food continues influencing how hungry you feel when it reaches the bloodstream. Elevated blood sugar tells your brain it’s time to take your plate to the sink. When your blood sugar’s low, it stimulates hunger.Many of us submit to the lure of simple sugars (jelly, soft drinks, cake). The trouble is they create a rebounding effect. You feel blah, so you grab a candy bar. This sugar surge acts like an electrical jolt—instantly you feel more energy. Less than two hours later, that energy surge (elevated blood sugar levels) drops.What’s that mean? You’re back to feeling sluggish. This rebound effect can put your body in biological turmoil. You eat to feel better, but what you’re eating is making you listless. So then you always feel like you have to eat. Couple that with a desire for the taste (that your pleasure center stimulates in your brain), and you have a recipe for disaster.At the bottom end of your small intestine (before it joins your large intestine), food hits the ileal break. This is yet another signal that you’re full. At this juncture, a traffic signal slows the slurry of contents into the large intestine. It’s called the ileocecal valve.Some foods naturally reduce the squeezing required to overcome the valve signal. Your body feels that you’re still digesting and not ready to evacuate those foods yet.Very little nutrient absorption occurs in the colon (save for your bacteria, which gorge on it). Once the food passes the ileocecal valve, not much more happens except that you reabsorb water while consolidating the waste you’ve formed.The outcome: A traffic backup in your gut. If you try to send more cars down the road, the problem will turn into a fuller feeling. This is one reason why fiber kills cravings. Fiber slows down the transit of food from your small to large intestine, maintaining that full feeling.MORE: Exercise & Your Appetite: The Truth