How your gut's circadian rhythm affects your whole body

Story highlights

  • Research suggests the composition and activity of microbiota has a circadian rhythm
  • This may explain adverse health effects of modern life, such as eating at night

We've known that bacteria live in our intestines as far back as the 1680s, when Leeuwenhoek first looked through his microscope.

Yogurt companies use that information in the sales pitch for their product, claiming it can help keep your gut bacteria happy.
    The bacteria growing on our skin have also been effectively exploited to sell the underarm deodorants without which we can become, ahem, malodorous.
      Until fairly recently our various microbes were thought of as freeloaders without any meaningful benefit to our functioning as healthy human beings.
      However, that view has changed in a big way over the last couple of decades.
      Interest in, and knowledge about, the microbiota has recently exploded. These highly diverse communities of microbes live in and on us in staggering numbers; researchers now estimate that a typical human body is made up of about 30 trillion human cells and 39 trillion bacteria.
      We now recognize they're essential to our health, participating in many important physiological functions such as digestion and metabolism of foods, and immune responses and inflammation; disruption of the gut microbiota might then contribute to a variety of conditions including childhood asthma, obesity, colitis and colon cancer.
      New research is beginning to show that the composition and activity of the microbiota exhibits a daily, or circadian, rhythmicity, just like we do. This offers one pathway to explain a Pandora's box of possible adverse health effects from aspects of modern life, such as eating late at night or too much electric light after sunset.

      The microbial daily routine

      The microbiota is primarily bacterial but also includes viruses and eukaryotes like yeast; the latter are much bigger and more complicated than bacteria, and have a structure similar to our own cells. The total DNA complement of the microbiota is termed the microbiome, and it's what we study to learn about the inner workings of the microbiota.
      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.
      The authors showed this link by administering an antibiotic to mice that kills much of the gut microbiota. Afterward they found significant changes in liver physiology. They could produce the same effect just by changing the feeding times of the mice; mice forced to eat only during the day showed different patterns of microbiota metabolites circulat