Carbohydrates ingested, are mostly in the form of poly- and disaccharides and need to be broken down into simple sugars, before absorption can take place. Digestion starts in the mouth, through the salivary enzyme, ptyalin. In the intestinal tract, the pancreatic amylase, hydrolyzes the remaining starch to maltose.
No matter what we eat, whether it’s a juicy fillet steak or a crispy apple or a cheese cake, are nothing more and nothing less than proteins, carbohydrates and fats. To convert these three basic food materials into biochemical substances we can use, we need three groups of enzymes, the protein-digesting enzymes (protease), the fat-decomposing enzymes (lipases) and the carbohydrate-decomposing enzymes (amylases). The enzymatic transformation starts as soon as we take the first bite. Our bodies have a preference for carbohydrates and our digestive system acts on them first, then on the proteins and finally on the fats.
Digestive fate of a peanut butter and banana sandwich on whole-wheat, sesame seed bread through the gastro-intestinal (GI) tract
MOUTH: chewing and swallowing with little digestion:
- Carbohydrate digestion begins as the salivary enzyme starts to break down the starch from bread and peanut butter.
- Fiber covering on the sesame seed is crushed by the teeth, exposing the nutrients inside the seeds to the upcoming digestive enzymes.
- Fat digestion is minimal.
- Protein foods are moistened by saliva.
STOMACH: collecting and churning, with some digestion:
- Carbohydrate digestion continues until the mashed sandwich has been mixed with the gastric juices; the stomach acid of the gastric juices inactivates the salivary enzyme and carbohydrate digestion ceases.
- Proteins from the bread, seeds and peanut butter begin to uncoil when they mix with the gastric acid, making them available to the gastric protease enzymes that begin to digest proteins.
- Fat from the peanut butter and seeds form a separate layer on top of the watery mixture.
SMALL INTESTINE: digestion and absorption:
- Carbohydrate digestion picks up when the pancreas sends pancreatic enzymes to the small intestines via the pancreatic duct to break down starch.
- Enzymes on the surfaces of the small intestinal cells complete the process of breaking down starch into small fragments that can be absorbed through the intestinal cell walls and into the hepatic portal vein. Sugars from the banana require little digestion and cross the intestinal cells immediately on contact.
- Protein is digested by the pancreatic and intestinal proteases. Small fragments of protein are liberated through the cells of the small intestinal wall and into the hepatic portal vein. Vitamins and minerals are absorbed.
- Fat from the peanut butter and seeds is emulsified with the watery digestive fluids by bile. Now the pancreatic and intestinal lipases can begin to break down the fat to smaller fragments that can be absorbed through the cells of the small intestinal wall and into the lymph.
LARGE INTESTINE: absorbing and eliminating:
- Fluids and some minerals are absorbed.
- Some fibers from the seeds, whole-wheat bread, peanut butter and banana are partly digested by the bacteria living in the large intestine and some of these products are absorbed.
- Most fibers pass through the large intestine and are excreted as feces; some fat, cholesterol and minerals bind to fiber and are also excreted.
- Carbohydrates are digested partly in the mouth and mostly in the small intestines.
- Fats are digested in the small intestines.
- Proteins are digested in the stomach and small intestines.
- Some fibers are digested in the large intestines (colon) and most are excreted.
- Vitamins and minerals are absorbed in the small intestines; some minerals are absorbed in the large intestines and some are excreted.