If you celebrated Thanksgiving this week, then you likely spent the holiday eating untold amounts of food. From protein-rich foods, like turkey or ham, to foods rich in complex and simple carbs, like stuffing or sweet potatoes (my personal favorite), and everything in between. In order to get the most out of this feast, your digestive system needs a variety of mechanisms to extract the nutrients you need from a wide range of foods. Because these mechanisms evolved at a time when food was scarce and Thanksgiving-like feasts were a distant pipe dream, your body’s digestion system is remarkably efficient at breaking down food and extracting as many nutrients as possible from it. Of course, this efficiency also means that your digestive system spends the holidays much like retail workers during Black Friday—overworked and underappreciated.
As we’ve learned in past blog posts, the human body contains many moving parts and ongoing processes at any given point in time. These processes necessitate a constant supply of raw materials—nucleic acids to replicate DNA, amino acids to build new proteins, fatty acids to form new cellular membranes. Not to mention the amounts of raw energy your cells need to build these molecules and perform all these necessary functions. While oxygen can be obtained mostly passively, since we’re basically swimming in it, most of the other molecules we need must be actively harvested from the food we eat. The process of chemically and mechanically breaking food down into usable base chemicals is known as digestion. Digestion takes place throughout the gastrointestinal (GI) tract, or alimentary canal as it’s also called, where a team of muscles, organs, and enzymes work in concert to pulverize, dissolve, and cleave each bite of food you eat.
As you probably know, the process of digestion starts in the mouth, where the muscles in your jaw begin mechanical digestion by repeatedly mashing the food with the teeth and tongue. At the same time, the saliva in your mouth contains enzymes that begin breaking down carbohydrates in the food. Once the food has been suitably mashed up by your mouth, you swallow it, sending it into the esophagus. Muscles in the esophagus continue moving the food down towards the stomach through a process known as peristalsis, where waves of contracting muscles around the esophagus push the food down towards the stomach. While swallowing is mostly voluntary, peristalsis is involuntary, and the rhythmic contractions allow food to move towards the stomach regardless of gravity (so yes, you can eat upside down, but I wouldn’t recommend it).
In the stomach, muscles continue to churn the food through an acidic chemical broth full of enzymes that begin breaking down some of the proteins and fat-soluble substances. After being thoroughly churned and dissolved in the acid, the food slurry, called chyme, is released into the first section of the small intestine, the duodenum, where most of the chemical digestion takes place. In the duodenum, enzymes from the pancreas cleave proteins into smaller peptides, and other pancreatic enzymes further break the peptides into amino acids that can later be absorbed into the blood. Meanwhile, the pancreatic enzyme lipase breaks fat molecules down into fatty acids and glycerol with the help of bile from the liver. Bile is necessary to break up the naturally hydrophobic fat molecules, so they can be properly exposed to the lipase enzyme. Finally, another enzyme called pancreatic amylase breaks down starch molecules (like those found in sweet potatoes) into smaller sugars that are easier to break up and absorb.
Further digestion of carbs and proteins occurs farther down in the second segment of the small intestine, the jejunum. Just like in the esophagus, peristalsis moves the slurry through the small intestine. These contractions also allow the jejunum to segment out portions of the slurry for more vigorous mixing and more efficient absorption. Small protrusions called villi line the inside of the jejunum, increasing the surface area available for absorbing nutrients directly into the bloodstream. Finally, at the very end of the small intestine, the ileum is responsible for absorbing leftover nutrients, vitamins, and excess water from the slurry before it moves into the large intestine.
The large intestine, which is much shorter than the small intestine, doesn’t perform any direct chemical or mechanical digestion at this point, but bacteria living in the large intestine may break down some of the excess starch, fiber, lactose, and other undigested materials, which can then be absorbed into the bloodstream. Meanwhile, the main function of the large intestine is to absorb excess water and electrolytes from the leftover, indigestible material so that it can be excreted out as waste (it’s a real shitty job). Overall, the digestion system is tightly regulated and optimized to maximize nutrient absorption from any food you may eat, from turkey to sweet potatoes and everything in between.
Science You Can Bring Home To Mom will be back in December! For now, check out last month’s blog post on the science and history of moonshine. Comment or email us at contact@anyonecanscience.com to let us know what you think of this week’s post. And subscribe below for weekly science emails!
