1. Do bacteria really outnumber human cells 10 to 1?
You might have read that bacteria outnumber human cells by a ratio of 10 to 1. This claim appeared in many science materials. But in 2016, researchers updated the counting method. They used a standard adult male weighing 70 kg (about 154 pounds) as a reference. The study estimated there are about 38 trillion bacteria and about 30 trillion human cells. The ratio is roughly 1 to 1. The total weight of all these bacteria is estimated at about 0.2 kg (0.44 pounds) [1].
This weight is about 0.3% of the total body weight. To visualize this, it is roughly the weight of three eggs. These numbers are estimates, not exact counts, so we cannot say they are perfectly equal. Still, if you compare the 38 trillion bacteria to the world population of about 8 billion people, the bacteria are more than 4,000 times the number of people. If that many bacteria live in your body, what do they eat to survive?
2. Where does dietary fiber go when humans cannot digest it?
Dietary fiber is found in vegetables and beans. It is a group of components that human digestive enzymes have trouble breaking down. It is hard to digest in the small intestine, so some of it reaches the large intestine [2]. In the large intestine, gut bacteria use this fiber.
One type of bacteria in the Bacteroides genus has 308 genes related to breaking down polysaccharides and sugars, and mechanisms to take them in. In contrast, the human body is thought to have relatively few enzymes for breaking down carbohydrates [3]. Sugars that humans cannot use become food for bacteria. It is easy to think that humans "share" their leftovers with bacteria. However, it is not about sharing. The bacteria have the ability to grow using whatever reaches them. So, what do they leave behind after eating?
3. What do gut bacteria produce when they eat dietary fiber?
When gut bacteria ferment water-soluble dietary fiber, they produce short-chain fatty acids. One of these, butyric acid, serves as nutrition for the cells of the large intestine. It is also said to help create an environment where helpful bacteria can grow [4].
Research also covers acetic acid, another short-chain fatty acid. A 2021 announcement from RIKEN reported that in a study on mice, acetic acid changed how immune antibodies called IgA reacted to bacteria. This suggests acetic acid helps control gut bacteria [5]. It is thought that there is a cycle where the body uses substances made by bacteria for nutrition and regulation. How much is known about the connection to the immune system?
4. Did mice eating more fiber have more anti-inflammatory immune cells?
In 2013, RIKEN and a joint research group published a study in the science journal Nature. They gave mice a diet high in dietary fiber. Compared to mice with less fiber, these mice had more active gut bacteria and produced more butyric acid. In their large intestines, regulatory T cells, which suppress inflammation, increased [6].
Another experiment involved germ-free mice, which had no bacteria at all. When bacteria from the order Clostridiales were introduced to these mice, regulatory T cells in the large intestine also increased. This shows that the state of the immune system changes depending on the type of bacteria. These results are from mice, so we cannot say for sure that humans react the same way. Still, the study showed one example of the path from food to bacteria to immunity. Do bacteria only make these types of substances?
5. Do gut bacteria make vitamin K2, and do we still need to eat it?
Gut bacteria also make vitamin K2 (menaquinone). Bacteria such as E. coli and Bacteroides produce it. However, it is not clear how much is absorbed or how much of the needed amount this covers. Some explanations say it does not meet the required amount, while others suggest it meets at least part of it [7].
For B vitamins, researchers studied the genomes of 256 types of human gut bacteria. They predicted that 40% to 65% of these bacteria have pathways to make each of the 8 types of B vitamins [8]. This is a prediction based on genes. How much is actually made and how much humans absorb is a different issue. Just because bacteria can make vitamins does not mean you do not need to get them from food. Based on this information alone, we cannot say that eating is unnecessary.
6. How can you find dietary fiber by reading food labels?
You can try comparing dietary fiber amounts on food labels today. Look at the nutrition facts on bread, cereal, beans, and processed vegetables you have at home. It is easier to compare if you look at the same serving size. Note that some products do not have this information listed.
You can also read the RIKEN announcement. The RIKEN website has a press release for the 2013 study in Japanese. Try comparing the diagrams and explanations of the experiments with the text to see how the research was done.