Our health, diet, and routine all change throughout the year. Warm summers are typically associated with time spent outdoors and more physical activity, while shorter winter days bring with them reduced sunlight, festive food, and increased risk of infection. One aspect of health that often goes overlooked is the seasonal gut microbiome. The gut microbiome is the community of trillions of microorganisms living in our digestive tract. It plays an important role in digestion, metabolism and immunity, and its composition is shaped by many of the factors that change throughout the year, including diet, lifestyle, and environment.
This article explores what the research shows about the seasonal gut microbiome, how diet and lifestyle may influence gut bacteria, and what this could mean for gut health in winter and throughout the year.
In healthy adults, the gut microbiome is relatively stable over time. In one long-term study, around 60% of bacterial strains were present after five years, and these strains were predicted to persist for many decades 1. This consistent population of microbial strains is also resilient, meaning it tends to return to its normal composition after a disturbance 2. However, there is considerable variability around that constant baseline, and the portion of the gut microbiota that is changeable can have a significant impact on health and wellbeing.
Short-term dietary changes have been shown to alter the microbiome from day to day. For example, switching to an entirely animal-based diet shifted gut composition within one day of the food reaching the gut, while switching back returned it to normal within two days 3. Larger disturbances can have even longer-lasting effects: gastrointestinal infections can cause lasting changes, with some bacterial groups completely replaced 4.
The extent to which the microbiome changes, and the impacts this can have on health, vary considerably between individuals 5. Where significant changes do occur, they can be beneficial, but can also cause digestive symptoms, impact immune and metabolic activity, and alter responses to medicines 6–9.
Some of the clearest evidence for the seasonal gut microbiome comes from the Hazda, a hunter-gatherer community in Tanzania whose diet changes considerably with the seasons 10. During the wet season, the Hazda diet is composed mostly of honey and berries, while in the dry season they eat more fibre-rich tubers and meat. Researchers analysed 350 stool samples from 188 Hazda community members over a year and found that some bacterial groups fell to undetectable levels in the wet season and returned in the dry season. This pattern repeated consistently over the following year.
Other communities where diet is heavily seasonal also show consistent patterns in gut microbiota variation over the course of a year. In the Hutterites, a communal farming population in North America, around 30% of gut bacteria is made up of species of Bacteroidetes in the summer, when fresh produce is more widely available, compared with only 11% in the winter 11. There is also evidence from more urbanised areas. One study conducted in Ukraine in 2020 found that Actinobacteria were higher in the summer and lower in the winter, when fibre intake was higher across the population. The Firmicutes-to-Bacteroidetes ratio was highest in summer 12. Interestingly, these two studies found different directions in the shift of Bacteroidetes, showing how many factors affect microbial diversity, including genetics, daylight, temperature, and daily activities. All of these factors are liable to change in populations across the world.
As we’ve seen, many studies into seasonal gut microbiota focus on the role of a changing diet. Even in industrialised countries, where imported foods are widely available in supermarkets and diet is less seasonal, intake still varies throughout the year. A systematic review of 26 studies found that the intake of fruit, vegetables, meat, and alcohol varies across the seasons, and energy intake tends to be higher in the winter 13. Winter diets typically include more energy-rich comfort foods and festive meals, while summer is associated with lighter meals like salads and fresh fruit.
These changes can significantly impact health. Dietary fibre is fermented by gut bacteria to produce short-chain fatty acids (SCFAs), which support gut barrier function and help regulate inflammation 14. Red meat, on the other hand, is associated with gut activity that is linked to cardiovascular disease 15. The American Gut Project studied the impact of plant diversity on the microbiome and found that people who ate more than 30 different plant types per week had more diverse microbiome composition than those who ate 10 or less 16.
While the link between diet, gut microbiome, and health is well established, seasonal differences may be less clear in countries like the UK and USA, where imported fresh food is available year-round. In a study of healthy adults living in metropolitan areas in the US, intake of energy, nutrients, and food groups did not differ between seasons 17.
Diet isn’t the only aspect of our daily routine that changes throughout the year. Physical activity, sleep, amount of daylight, and time spent outdoors vary considerably across the year, and may all impact the gut microbiome. Physical activity often falls in colder, darker months, and people who exercise regularly show shifts in their microbiome towards SCFA-producing bacteria 18. Stopping physical activity, for example in worse winter weather, can reverse the impacts of exercise.
Shorter winter days also mean less exposure to sunlight, lowering vitamin D production in the skin. A small study of 21 women showed that exposing the skin to UVB light increased gut microbial diversity, although this was only apparent in women who had vitamin D deficiency 19. Sleep can have a significant impact too: microbial diversity is higher when sleep time is longer 20.
Interestingly, simply being outside can impact the microbiome through increased contact with microbes from soil and plants. In a study conducted in Finland, adding forest vegetation to playgrounds in urban nurseries increased the diversity of skin and gut microbiota in children within one month 21.
As well as bringing colder, shorter days, winter is also accompanied by a rise in colds, flu, and other respiratory infections. Research now shows that the gut may play a significant role in immunity and is also impacted by illness. As we’ve seen, SCFAs produced by bacteria support gut barrier function, an important line of defence against infection. The role of SCFAs in controlling inflammation is also important in ensuring that inflammatory responses are effective and appropriate 14. The rise in illness in winter also means an increase in antibiotic use. The impact of antibiotics on gut bacteria can be substantial, with some species taking over 6 months to recover following treatment 22.
The COVID-19 pandemic provided clear evidence of the impact of the gut microbiome on immunity. In a study of 100 patients, the composition of the microbiome was associated with disease severity and inflammatory markers in the blood, while anti-inflammatory bacteria such as Faecalibacterium prausnitzii were depleted during infection 23.
Research on the seasonal gut microbiome is still developing, but knowing how and when the gut microbiome is liable to change can help guide healthy habits and target interventions. Maintaining an active lifestyle in the winter and taking vitamin D where necessary can help prevent unwanted seasonal gut microbiome shifts. Restrictive or unhealthy seasonal diets are likely to reduce gut diversity, so ensuring a consistent, balanced diet and taking proactive steps such as probiotics can help maintain a healthy seasonal gut microbiome.
Specific probiotics may also help to promote strong immune responses during winter months. A fibre-rich, diverse diet with fermented foods can support SCFA production in the gut to regulate immunity. A review of 24 trials found that probiotics reduce the number of people who develop respiratory infections and decrease the need to take antibiotics 24.
In healthy adults, the gut microbiome is largely stable but responsive, and seasonal shifts have been observed in several populations. These shifts are most pronounced where diets depend on local, seasonal food availability. Human studies are limited and do not currently demonstrate a direct link between the seasonal gut microbiome and health outcomes.
However, it is clear that diet and lifestyle shape the gut microbiome throughout the year. For winter gut health in particular, a fibre-rich diet with plenty of plant variety, fermented foods, regular physical activity, good sleep and vitamin D in line with UK guidance are all well-supported strategies. Probiotics are well-studied and may be a useful tool in protecting against respiratory infection and reducing antibiotic use.
The Biomel Professional Page is a dedicated resource created by our nutrition team to provide healthcare
professionals with reliable, up-to-date information in the gut health space. The content is research-backed,
educational, and strictly non-commercial, focusing on scientific insights beyond our product range.
For customer-friendly resources, the Biomel
Gut Health Articles offers accessible information designed
to support individuals in making informed decisions about their digestive wellness.
© 2026 Biomel. All rights reserved.
* Biomel contains billions of live active cultures and Vitamins B6 to support normal energy-yielding metabolism, normal functioning of the nervous system, normal psychological function, reduction of tiredness and fatigue and the regulation of hormonal activity, Vitamin D to support the normal function of the immune system and Calcium to support the normal function of digestive enzymes. Chicory root fibre contributes to normal bowel function by increasing stool frequency. Biomel Complete Gut contains 3g of chicory root fibre per serving. Full beneficial effect is obtained with a daily intake of 12g chicory root fibre.
** Consumer experiences are one person’s experience and may not reflect the experience of all users. Individual results may vary. Testimonials are only from verified customers.
*** Biomel is formulated using ingredients supported by over 300 published scientific studies.
Our products are not intended to diagnose, treat, cure, or prevent any disease.
The information provided on this site is for informational purposes only and is not intended as a substitute for advice from your physician or other health care professional.
