I became interested in the health value of fermented foods after I noticed a curious coincidence. Humans have three mysterious food preferences: for (a) sour food, (b) food with umami flavor, and (c) food with complex flavor. I realized that all three preferences made bacteria-laden food more attractive. Bacteria change sugars to acids, increasing sourness. They break down proteins, creating glutamate, which produces umami flavor. And the many chemicals they introduce into a food make its flavor more complex. After I noticed this, I came across many studies that supported the idea that fermented foods are good for health. I also found studies that suggest the bacteria in our digestive system are crucial to health.
This raised the question: What fermented foods to eat? How many? How often? To begin to answer these questions, it would help to know how bacteria in our food help us be healthy. There were two obvious answers:
1. Stimulate the immune system. The bacteria in fermented food are inherently safe: they are specialized to reproduce on/in food, which is so different than inside the human body. But the immune system doesn’t know this. If this was one benefit of fermented food, you could study which ones to eat by measuring immune system activation. Unfortunately, that is nearly impossible.
2. Improve digestion. Many people have digestive problems and some of them are helped by fermented foods. Obviously they contain bacteria that digest food. I don’t have digestive problems so I can’t study this by figuring out which fermented foods help.
Recently, I have begun to think there is a third reason:
3. Place competition. To make us sick, outside bacteria need to stick inside us. To digest our food, the surfaces of our digestive system, such as the inside of our intestines, is much more porous than other surfaces, such as our skin. It is our digestive system, therefore, that is most vulnerable to dangerous microbes. The totally-safe microbes in fermented foods compete for sticky spots with other, more dangerous microbes. If there are plenty of safe bacteria — say, billions in a serving of yogurt — they may do a lot to protect us against the dozen or so similar dangerous bacteria we might get from touching the same surface as a sick person. I think of a wooden floor where the lumber is not quite well-fitted. If you want to protect what’s below that floor from black sand (dangerous), an excellent method would be to pour an enormous amount of white sand (safe) on the floor.
If Effect #3 (place competition) is the main reason fermented food protects us from disease, it implies that dead bacteria work as well as live bacteria (in contrast, live bacteria do not digest food, Effect #2). This might explain the potency of alcoholic beverges such as wine, where most of the bacteria are dead. It also suggests that what matters is diversity of where bacteria stick and how much they stick. It might someday be possible to feed people (non-radioactive) bacteria and learn where in the body they end up.