The balance of microbes in the human gut varies from day to night and by season, which may help explain the challenges in defining a “normal” gut microbiome.
The human body houses around 40 trillion bacteria, viruses, and fungi, creating a microbiome that corresponds roughly to the number of human cells at a 1:1 ratio. At DDW 2023, held May 6-9 in Chicago, Ana Carolina Dantas Machado, PhD, presented the latest findings about how the gut microbiome constantly changes during the day and with seasons.
Using data from the publicly available global American Gut Project, she and her team analyzed 18,219 adult stool samples. They reported diurnal signals at various taxonomic levels, with distinct 24-hour cycles in about 60% of the phyla. These findings suggest that human diurnal rhythms might not be apparent in studies with small sample sizes and few controlled confounding variables, though they may still influence study results.
One notable example was the Actinobacteriota phylum, which was identified at lower levels in stool samples collected in the morning compared with those taken during the day. The researchers hypothesized that this could be linked to light exposure.
Seasonal changes were more pronounced with the Proteobacteria phylum, which showed lower levels in winter and a rise during the spring, peaking in summer. Moreover, seasonal fluctuations could be influenced by location, climate, pollen, humidity, and other environmental factors.
These findings offer a potential explanation for the increased susceptibility of humans to common colds and flu during specific seasons, as the microbiome is known to influence the immune response. “The fluctuating microbiome also plays a role in the way drugs are metabolised, which could alter the results of clinical studies if not considered,” emphasized Dr. Dantas Machado.
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