You might imagine that the quantity and diversity of the microorganisms making up any one individual’s microbiome are reasonably fixed and unchanging. However, our microbiomes may be more volatile than we believe.
Scientists in Israel have proposed a mechanism through which the composition and functionality of our microbiota are regulated throughout the day to achieve a state of metabolic homeostasis synchronized with the body’s own circadian cycle (Thaiss et al, 2014). Homeostasis is the process of the body maintaining a condition of balance or equilibrium within itself, even when experiencing external changes. A simple example of homeostasis is our ability to maintain an internal temperature of around 98.6 degrees Fahrenheit, whatever outside temperature. Metabolic homeostasis within the microbiome represents its ability to maintain balance in the chemical processes necessary for the maintenance of life occurring within its cells and organisms.
A mechanism of this nature may explain previous reports of the microbiome’s tendency to quickly adapt to changing circumstances, such as a new diet (David et al, 2013).
A circadian rhythm is any biological process that displays a self-driven oscillation of about 24 hours. Circadian rhythms can generally be affected by external circumstances, and have been widely observed in plants, animals, fungi, and cyanobacteria (a large group of bacteria that obtain their energy through photosynthesis).
Of course, plants also photosynthesize, and in the same way that they do so during daylight hours and take in oxygen at night, the human body is programmed to perform certain functions best at particular times of the day.
Before the advent of electric light in the home, people woke as the sun rose and went to bed when it set. There were no external factors affecting their circadian clocks. However, once homes were lit with electricity people could continue to go about their lives after dusk, ignoring their circadian rhythm in favor of more work or leisure time. This led to questions being asked about the effects such a lifestyle might have on individuals’ health. For example, studies have indicated that sleeping for less than eight hours a day could lead to weight gain, or be related to mental health problems such as depression. There are even suggestions that too little sleep may raise the risk of developing cancer (Shanmugam et al, 2013). As we begin to better understand the human microbiome, and how the microorganisms living within us affect our health, it is logical to ask what happens to them if we change our schedules.
The Israeli study mentioned above shows that the gut bacteria living in both humans and mice exhibit diurnal oscillations, not only in functionality but also in composition, and that these 24-hour cycles are governed by the feeding rhythm of the host. The study also concludes that misalignments in the circadian cycle can result in metabolic imbalance and dysbiosis (a lack of bacterial balance), and that these may, in turn, have relevance in the diagnosis of modern human diseases.
In order to study how it affects the microbiome, the scientists induced jet lag in mice, noting that it changed the composition and functionality of the microbiome, suggesting that chronic jet lag can cause significant disruption to the microbiome’s balance and composition. This is principally the result of significant changes in the host’s feeding rhythms.
The learning is clear. Next time you consider cramming for an exam, working late, or partying until dawn, think about the effect it could have on your microbiome and how this will, in turn, affect you.
1. Thaiss et al (2014). Transkingdom Control of Microbiota Diurnal Oscillations Promotes Metabolic Homeostasis
2. David et al (2013). Diet rapidly and reproducibly alters the human gut microbiome
3. Shanmugam et al (2013). Disruption of circadian rhythm increases the risk of cancer, metabolic syndrome and cardiovascular disease