The process behind eating

The process of food digestion begins in our mouth and proceeds all the way through the GI tract ending at the anus.

Anatomy of the gastrointestinal tract

The gastrointestinal tract extends from our mouth to our anus. It is a long flexible muscular tube that keeps the food slowly and steadily moving from one end to the other, allowing sufficient time for digestive reactions to occur.

During the transition through the GI tract, our food becomes lubricated with fluid, producing a consistency that aids movement through the tract. At various stages along the tract, different digestive enzymes are secreted. These work on the food breaking it down into the individual nutrients, and smaller components ready for absorption. Left-over food residue makes its way to the large bowel, where excess water is absorbed, leaving behind a paste that is solid enough to pass.

The key sections of the GI tract for digestion that we will explore include: 

• mouth
• oesophagus
• stomach
• small intestine (made up of 3 sections – duodenum, jejunum and ileum)
• large intestine (or colon
• rectum
• anus

We’ll now use the example of a bowl of spaghetti bolognaise to break down the digestion process, step by step.

What’s in a bowl of spaghetti bolognaise?

Ingredients:

• pasta, (carbohydrate – mostly starch, and some dietary fibre, protein, small amount of fat, vitamins and minerals)
• beef (protein, fat, vitamins and minerals)
• tomato pasta sauce (vitamins and minerals, small amount of fat, tiny amount of protein and carbohydrate)
• mixed vegetables (corn, peas and carrots – water-soluble vitamins and some minerals)
• grated cheese (protein, some fat, vitamins and minerals)

Mouth

First, the mouth chews and swallows the food with little digestion.

• Carbohydrate: salivary enzymes start to break down some carbohydrate (starch) in the pasta.
• Fat: minimal digestion takes place here, even though salivary juices do contain lipase. Some hard fats can begin to melt as they reach body temperature.
• Protein: foods containing protein start to be moistened.
• Fibre: teeth will start to break-down fibre found in the pasta and any vegetables.

Stomach

Next, the stomach collects and churns the food with some digestion.

• Carbohydrate: digestion of carbohydrates will continue until the food has been mixed with the gastric juices. The gastric juices contain hydrochloric acid which neutralises the salivary enzymes, causing carbohydrate digestion to cease whilst in the stomach. Once inactivated the salivary enzymes are digested along with other proteins.
• Fat: gastric lipase begins to digest fat in the meat, sauce, and cheese. This fat can form a separate layer on top of the more watery gastric juice.
• Protein: the acid in the gastric juice will start to uncoil the protein found in the meat, cheese, and pasta. As the proteins uncoil this exposes the proteins to the protease enzymes which then are able to start digesting the proteins.

Notes:

• The stomach enzymes work well at the pH of the stomach (very acidic, about pH ‘2’).
• When the chyme leaves the stomach, digestion of all three macronutrients (carbohydrate, fat and protein) has begun.

Small Intestine

After this, the small intestine digests and absorbs the food.

• Carbohydrate: digestion of carbohydrates will speed up here. The pancreas secretes pancreatic enzymes via the pancreatic duct into the duodenum which digest and break-down starch into smaller fragments. Then enzymes on the surface of the intestinal cells will complete the digestion of the starch component of the carbohydrates. Disaccharides from the sauce and any vegetables are digested by enzymes on the surface of the intestinal cells. Monosaccharides require no digestion and cross the intestinal wall once they encounter the intestinal cells into the hepatic portal vein. 
• Fat: when fat arrives in the duodenum, this triggers the secretion of bile from the bile duct into the duodenum. Bile helps to emulsify the fat, putting it into suspension in the watery intestinal juices, giving access to the pancreatic enzyme, lipase which breaks down fat into individual components which can be absorbed through the cells in the small intestinal wall into the lymph.
• Protein: uncoiled proteins are digested by the pancreatic and intestinal proteases, breaking these down into smaller peptides and individual amino acids which are then absorbed  through the cells in the small intestinal wall and into the hepatic portal vein.
• Vitamins and minerals: these are absorbed along the length of the small intestine, mostly in unaltered form.
• Water: water is absorbed along the small intestine.
• Fibre: dietary fibres move through the small intestine undigested. Some fibres attract water and so a semisolid mass can start to form. Some minerals, cholesterol and fat can become bound up in this dietary fibre.

Large Intestine

Lastly, the large intestine absorbs water and some nutrients, and eliminates waste.

• Fluids and some minerals: water and some minerals are absorbed.
• Some dietary fibre: intestinal bacteria ferment some types of fibre, producing medium/short chain fatty acids which can provide energy for the cells in the colon or be absorbed. Also produced is more water and gas.
• Most dietary fibre: attracts water forming a bulky, soft stool assisting with movement through the bowel and making the stool easier to pass. Excreted via the anus; fat, cholesterol and some minerals can be bound up to this fibre and so also excreted. The residue which is finally excreted has little nutritional value.