- Researchers are reporting that the human gut microbiome may fluctuate at different times of the day and between seasons.
- They said that light, temperature, humidity, and pollen may play a role in causing changes to the gut microbiome during the year.
- They added that diet as well as fasting periods, such as during sleep, may cause microbiome changes on a daily basis.
The balance of microbes found in the gut may vary from morning to evening and even between seasons.
Research being presented this week at the Digestive Disease Week 2023 conference reports that the microbiome can vary throughout the year in populations around the world.
“The seasonal variations we see in conditions like allergies or the flu occur in context of completely different microbiomes,” Carolina Dantas Machado, PhD, the study’s lead author and a researcher at the University of California San Diego, said in a press release.
“We may need to put our understanding of how seasons affect health and disease in context of a microbiome that is much more variable and dynamic than we have previously thought,” she added.
The findings have not yet been published in a peer-reviewed journal.
Details from the gut microbiome study
For their study, the researchers examined data from about 20,000 stool samples from countries around the world. The samples were collected as part of the American Gut Project between 2013 and 2019.
The researchers analyzed the time, date, and location that the samples were collected.
They reported that nearly 60% of certain related groups of bacteria had a 24-hour cycle that was distinct.
They also found a similar pattern with seasonal fluctuations in some kinds of bacteria, with certain bacteria having one or two distinct patterns throughout the year.
A type of bacteria found in the gut called actinobacteria was observed to fluctuate throughout the day. In the morning, there were lower levels of the bacteria with much higher levels at the end of the day.
“You can imagine that the gut environment is radically different in terms of nutrient and water availability and pH when the person is sleeping compared to right after they eat breakfast,” Amir Zarrinpar, PhD, a co-author of the study and principal investigator at the University of California San Diego lab, said in a press release.
Seasonally, a similar effect was seen with other bacteria.
A bacteria known as proteobacteria dropped to low levels in the winter but rose throughout the year before peaking in summer.
Reaction to the gut microbiome research
Dr. Emeran Mayer, a leading expert in the study of the microbiome and the founding director of the Goodman Luskin Microbiome Center at the University of California Los Angeles, said the results of the study aren’t surprising.
“These findings are not surprising at all since diurnal variations in the geometry of the gut microbiome in mice have previously been reported and since profound diurnal variations in the gut microbiome of the Hazda people in Africa have been described. In both cases, the main factor has been diurnal and seasonal variations of food intake,” Mayer told Medical News Today.
“Seasonal fluctuations could play a role in adaptions to seasonal variations in food intake, like a lower percentage of fresh fruits and vegetables in winter, and higher intake in meat and simple carbs in winter,” he added. “One could speculate this may have an influence of seasonal fluctuations in mood.”
How seasons, diet affect bacteria
The researchers suggested that light and temperature may have an impact on seasonal changes to the microbiome.
Pollen and humidity, they added, may also have an impact.
Dr. Ami Bhatt, an associate professor of medicine in the Hematology and Genetics Department at Stanford University in California, says fasting could explain some of the daily variation.
“Some blood chemicals, for example bilirubin and stress hormones, can vary with the diurnal cycle – so it isn’t too far off base to think that the microbiome might vary with diurnal (light/dark) cycles or with prolonged periods of fasting,” she told Medical News Today. “Since most people don’t wake up to eat in the middle of the night, the morning microbiome likely represents a more ‘fasted’ state, on average.”
Future research on the human gut
The microbiome accounts for roughly half the cells in the human body.
The researchers say a fluctuation in microbiome could have wide ranging impacts in health and medicine.
Experts say more research will be needed before firm conclusions can be made, particularly about seasonal microbiome variations.
“I do wonder if confounders, such as the ambient temperature being different in the summer vs. the winter might have affected the samples during shipping and thus affected the results that were observed. Certain bacteria, for example Proteobacteria, were previously shown to ‘bloom’ in some shipping conditions that were used for the AGP samples,” Bhatt said.
“It’s probably too soon to conclude that seasonal variations of the microbiome are the norm. We’ll have to wait to see when this full research study is published to figure out exactly what these researchers studied – and then can draw some more thoughtful conclusions,” she added.
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