In this field’s early days, researchers took fecal samples from people to investigate the composition of the gut microbiome. Later they noticed that defining the microbiome from a sample taken in the morning was quite different from one taken in the evening: The gut microbiota was not static over the span of the day.
Perhaps this was to be expected. Almost all life on Earth has an endogenous circadian rhythmicity that is genetically determined, but that also responds to changes in light and dark. For human beings, reliable changes occur between day and night in hunger, body temperature, sleep propensity, hormone production, activity level, metabolic rate and more.
These findings on daily rhythmicity in microbiota have really piqued my interest because disruption of our circadian rhythmicity by electric light at night has been my research passion for several decades. As scientists investigate the links between our internal daily patterns, electric light and health, new information about the rhythmicity of our microbiome might hold clues about how this all works together.
The crucial question is whether the microbes simply respond to their host human’s circadian rhythm or whether they can actually alter our rhythm somehow. And does this really matter anyway?
Microbiota calling the shots
A group of researchers from the Weizmann Institute in Israel have now used an array of remarkable DNA technologies to show that the gut microbiota changes location within the gut, and changes its metabolic outputs over the span of the 24-hour day, at least in mice. Amino acids, lipids and vitamins that the microbes release circulate in the host mouse’s blood. As the levels of these molecules in the blood changed throughout the day, they altered the expression of genes in the mouse’s liver that code for many metabolic enzymes.
This is the first clear demonstration of the gut microbiota changing the circadian activity of an essential organ – in this case, the liver, which is the engine of our physiology and crucial to our health.