This essay is written by Jongsik Jon Chun, a professor at the Department of Biological Sciences, Seoul National University and translated by ChunLab staffs. The author is also actively engaged in R&D as a major board member at ‘ChunLab, Inc.’
There’s a proverb in Korea, “If a close acquaintance buys an estate, you get a stomachache.” meaning you turn green with envy when your happiness does not prevail over the other’s fortune. It is said that a proverb similar to the one in Korea exists in almost every country. This essay explains the relation between happiness and microbiome.
If you have suffered from a stomachache because of an affluent friend, here are some possible explanations of why. In fact, our brain and intestine are closely related. According to recent research, there is an unknown invisible hand in force that controls our brain and intestine. Today, we will explore that mysterious entity.
In our body, there are various types of hormones that control our feelings. Among them, the happiness hormone called serotonin receives the most attention. Unlike the hormones, dopamine, that is related to pleasure, there is no addiction associated with serotonin. Serotonin is a representative hormone that generates positive emotions in multiple aspects of our lives, including appetite and sleep.
The interesting to note how this hormone serotonin is mainly found in the large intestine while it seems more ideal to be found in the brain. More than 80% of the serotonin in our body (the same hormone found in the brain) is made and stored by enterocytes not by the brain. This means our large intestine is largely contributing in our happiness. According to the recent studies, the production of serotonin by enterocytes has a strong correlation with intestinal microorganisms.
First, a bacteria-free sterile mouse without any enterocytes produces 60% less serotonin than the regular mouse with enterocytes. Of course, this does not signify that the sterile mice are 60% less happier than the regular mice. Although it is difficult to verify this hypothesis as we lack human counterparts, it seems positive that more than half of our happiness-hormones are produced by the intestinal microorganisms ordering enterocytes to produce more serotonin. The happiness-hormones made in the intestine spread throughout the body via the blood vessels. Then, what are these wonderful microorganisms? There are scientists worldwide who are striving to find this out. This is possibly not exclusive to a few bacteria but rather a consortium-like collaboration of tens of different types of bacteria. Furthermore, the composition of the consortium is likely to differ from each ethnic group to person as well.
As of today, discovered intestinal microorganisms controlling our brain are in four ways. First, the microorganisms directly create a substance that affects the brain. The substance is absorbed in the intestine and moves through the blood vessels to the brain, thereby controlling our feelings. Second, the microorganisms order the enterocytes to produce hormones like serotonin. Third, they send signals to the nerve cells in the large intestine linked to the brain via high-speed neural network. Through this method, the order of intestinal microorganisms’ is immediately delivered to the brain. The latter is largely related to brain diseases such as autism and dementia, where the microorganisms pose a negative effect on our immune system. Although there may be other methods, there is minimal understanding on these four ways with modern science. Humankind has just started to figure out the relationship between the intestinal microorganisms, the human intestine and the brain, results of evolution between humans and millions of bacteria during tens of millions of years. Hence, wouldn’t the system involve a substantial amount of complexity?
The proposition “You need to eat well to stay happy” becomes a solid truth when you take account of the intestinal microorganisms, in other words, microbiomes. It is important to consider not only the nutrition that we take in through digestion but also foods for the intestinal microorganisms. Foods such as cookies, cake and soft drinks that immediately make us feel good do not apply to this context. In the upcoming holidays, let us try to avoid processed food and eat less meat but eat some vegetables and greens which are foods for the microorganisms! Especially to those who have a rich friend with lots of estates.