To understand how digestion works, we are going to follow the journey of an apple through the human body. Why an apple? Well, apples are extremely healthy foods. Their consumption has been linked with lower incidence chronic diseases and they are rich in many different antioxidants (to learn more about antioxidants read ‘5 Antioxidant Superfoods‘). Nutritionally, apples contain plenty of sugars and some fiber (see ‘The Importance Of Fiber‘), along with a small proportion of proteins and fat.
In our moth, the apple is converted into smaller pieces by the teeth. Saliva is mostly made of water, but it also contains some special proteins that help to cleave the apple constituents into smaller nutrients. So digestion really starts in the mouth! Saliva also lubricates the smaller pieces of apple, easing their way through the esophagus and into the stomach.
In the stomach is where the heavy part of digestion takes place. Specialized cells from the stomach walls wall release gastric acid, which is mainly composed of hydrochloric acid. This acid has a dual role: on one hand, it directly breaks the nutrients of the apple into smaller components, which are easier to absorb. On the other, it lowers the pH, activating specific proteins that also contribute to digestion.
From the stomach, our digested apple goes into the intestine. Here is where most of the absorption takes place. Nutrients, mostly sugars in our case, cross the intestine cells and are released into the bloodstream. When the concentration of sugar in the blood increases, the beta cells of the pancreas start to release insulin. Insulin promotes the uptake of sugar by the body cells and stimulates muscle building. It also activates mechanisms to accumulate the excess of sugar, either by formation of glycogen in the liver and muscles, or by transforming it into fat, which is then stored in the adipose tissue. Sugar levels in the blood are also sensed by our hypothalamus, which produce signals to curb our hunger accordingly (“Added Vs Naturally Occurring Sugar“).
As the digested apple progresses through the intestine, nutrients and water are absorbed. Our gut flora, formed by a large group of different bacteria, help out in the digestion process. In our specific case, the role of these bacteria is critical, as they are required to fully cleave the fiber and other types of carbohydrates contained in the apple.
At the end of the intestine, our apple has been converted into faeces. Faeces contain undigested elements of the apple, along with waste products that are formed during the digestion process. This may sound gross, but it is advisable to keep an eye on the color and shape of your stool, as it is a marker of how well your digestive tract is working.
It’s amazing how many things happen within us while we eat an apple. Our bodies are highly complex and regulated machines, and we should treat them with great respect. Think about it next time you eat.
And remember: An apple a day keeps the doctor away!