Do you have a mouth? Sure, we all do! Do you want to know what happens after digestion when you put things in your mouth? I bet you do.
For simplicity we will pretend food is made up of four things: fat, protein, carbs, and vitamins/minerals. Each of these things are digested and absorbed differently in different places in your Gastrointestinal (GI) tract.
As food enters your mouth carbs are the first to get digested. Human saliva contains amylase, and enzyme which digests carbs. If you put bread in your mouth it will quickly tun sweet as the amylase breaks it down and it turns into simple sugars and binds to the sweet receptors on your tongue. This causes hormones to get your pancreas ready for carbs. Insulin is released in preparation to transport the carbs to the organs of your body. The sugars can be absorbed through the lining of your mouth (mucus membrane) and all throughout your GI system. When the carbs hit your bloodstream the insulin opens up the organs through GLUT receptors. Adipose tissue (body fat) and muscle use the same receptor: GLUT4.
Not much happens here except some passive diffusion across the mucus membrane. There is a false sphincter where the stomach meets the esophagus. If this is loose or if intra abdominal pressure is high then stomach contents can squirt up from the stomach to the esophagus while the stomach churns food. This is one way heartburn occurs.
This is where the magic happens. The stomach is super complex and majestic in its ability to reduce almost any organic material to nutrient rich mush. I cant get into all the autocrine, intracrine, and paracrine functions of the hormones and neurotransmitters of the stomach here. To give you a hint of its complexity, the stomach and the intestines have their own nervous system: the enteric system. Ever wonder why nerves gives you butterflies in your stomach? Thats why. Many of the neurotransmitters In the brain have specific functions in the GI system and thats why anxiety may manifest as stomach pain or Irritable Bowel Syndrome.
The super duper simple version is this: food enters the stomach and based on its contents several things may happen. If it is 92% liquid, then it begins to be absorbed through the mucus membrane and it passes through the pylorus into the duodenum. If it is over 8% solute and it has a significant amount of fat then the pyloric sphincter is closed and the contents can’t leave the stomach. This is through a very complicated paracrine process. If protein is present then HCl is produced by the stomach, again a very complicated process. The stomach churns the food + acid mixture for as much as 6 hours before the pylorus opens to let the mush bolus through to the duodenum. This is why your post workout meal 1 should be a shake thats 92% water, then it is absorbed through the mucus membrane throughout the whole GI track and can deliver the pre digested nutrients like amino acids and simple carbs to your blood so insulin can carry it to your muscles within the first hour. And the next meal should not contain any fat so the pylorus does not remain closed, this will allow your food to be digested and transported to the blood fast. Often man made foods like chocolate, alcohol, fried foods, and things that are spicy can cause an overproduction of stomach acid. this results in heartburn. The common approaches are chewing tums or rolaids, which buffer the acid, or taking a Proton Pump Inhibitor. This causes the proton pump which dumps HCl into the stomach to break down and the pancreas just produces more gastrin over stimulating the stomach. The decreased acid results in the protein in food not being fully denatured. this means it may not be broken down and absorbed right once it passes to…