1. Why the Tennis Court Size for the Gut's Inner Surface Was a Revised Number
For a long time, textbooks stated that the inner surface area of the digestive tract was 180 to 300 square meters, roughly the size of a tennis court. In 2014, researchers Helander and Fendrich from the University of Gothenburg revised this estimate to about 30 to 40 square meters, with an average of about 32 square meters [3]. This is the size of a studio apartment. The reason the old numbers were too large is that during dissection or surgery, the gut tissue relaxes and stretches [3]. According to the same report, the large intestine accounts for about 2 square meters, while the mouth, esophagus, and stomach combined do not reach 1 square meter. Most of the area belongs to the small intestine [3].
Even though it is smaller than a tennis court, 32 square meters is still a large surface area for a tube inside the body. How can a tube that is only a few meters long have such a large area? This estimate assumes the small intestine is about 2.9 meters long [3].
2. How a Thin Small Intestine Tube Can Have the Surface Area of a Room
The small intestine is about 6 meters long and makes up about 80% of the digestive tract [1]. However, the small intestine is a stretchy tube. Its length changes depending on whether it is stretched out or not. In studies, measurements range from the 3-meter range to nearly 7 meters [5]. Therefore, we say it is "about 6 meters" to allow for this variation. The inner wall has small projections called villi, and on their surface, there are even smaller projections called microvilli. These two layers of bumps increase the inner surface area, allowing more nutrients to be absorbed [2].
Think of a towel. A towel with a textured surface has more surface area than a smooth piece of paper of the same size. The projections in the intestinal wall work on the same principle to increase the area. However, villi are part of the intestinal wall itself, not hair that grows on top of it. Most nutrients and water are absorbed in the small intestine [1]. Where does the remaining content go?
3. What the Large Intestine Does With What the Small Intestine Does Not Absorb
The contents that pass through the small intestine enter the large intestine with about 1 liter of water [1]. The large intestine is about 1.5 meters long. Here, it mainly absorbs water and minerals such as sodium. Indigestible dietary fiber is fermented in the large intestine and becomes stool, which is sent to the rectum [1]. The rectum is a place where stool is temporarily stored. Bacteria living in the large intestine are deeply involved in this fermentation process. It is reported that most bacteria in the body live in the large intestine [6]. How many bacteria are there in total?
4. Why the Idea That Bacteria Outnumber Human Cells 10 to 1 Was Revised
The idea that "bacteria in the body are 10 times the number of human cells" was widely shared for a long time. In 2016, Sender and colleagues estimated that the body contains about 3.8 × 10^13 bacteria (about 38 trillion) and about 3.0 × 10^13 human cells (about 30 trillion). They reported the ratio is about 1.3 to 1 [4]. Dividing 38 by 30 gives about 1.27, which is certainly not 10 times. This estimate is for a standard 70-kilogram man. Because numbers vary from person to person, this does not mean "everyone has 1.3 times more bacteria."
The "10 times" figure is said to originate from an estimate by microbiologist Luckey in 1972, which overestimated the bacteria in the gut [6]. Both the "tennis court" area and the "10 times" bacteria count are examples of long-used numbers that were later recalculated. Even so, about 38 trillion bacteria is not a small number. How does the gut coexist with them?
5. How the Gut Accepts So Many Bacteria Living in It
Bacteria that live in symbiosis in the gut are called gut microbiota. Many of them are difficult to culture in a lab [7]. Instead of expelling bacteria, the host body controls the quality and quantity of symbiotic bacteria through mucosal immune mechanisms, according to a joint announcement by JST and the University of Tokyo in 2014 [7]. Peyer's patches in the intestinal wall contain M cells, with immune cells waiting underneath. Also, an antibody called IgA is very important for maintaining the balance of gut bacteria. It binds to bacteria and weakens their movement [7]. This shows the gut is not just an organ that absorbs nutrients, but also an organ that interacts with bacteria. Can the gut move without waiting for instructions from the brain?
6. Can the Enteric Nervous System Move the Gut Without Waiting for the Brain?
The intestinal wall contains the enteric nervous system. It regulates reactions such as peristalsis (movement) and water secretion from the mucosa, mainly independently of the central nervous system [8]. It is sometimes called the "second brain." Scientific journals report more than 100 million neurons and describe this as more than the number in the spinal cord or peripheral nervous system [8]. Estimates range from 100 million to 600 million depending on the source, so we say "hundreds of millions." Although it regulates independently, the gut does not "think." It adjusts these responses to local conditions, such as movement when food passes or water secretion.
7. How to Feel the Size of the Gut at Home With a String and a Room
You can try to understand the size of the gut at home. First, get a string 6 meters long and lay it straight on the floor. If you have a tape measure, you can measure it. This gives you an idea of the length of the small intestine. Second, convert 32 square meters into tatami mats. One tatami mat is about 1.6 square meters, so 32 divided by 1.6 is about 20 mats. If you consider 30 to 40 square meters, that is about 19 to 25 tatami mats. Compare this to the number of tatami mats in your house. If you want to read the original sources, the area revision is in a 2014 paper in *Scandinavian Journal of Gastroenterology*, and the bacteria count recalculation is in a 2016 paper in *PLOS Biology*.