Around 70% of the body's immune cells reside in the gut-associated lymphoid tissue (GALT). This is one of the largest and most active immune sites in the human body and it's in constant communication with the trillions of microbes that live in the intestines. What you eat, how you sleep and how much you move directly influence this system, especially during seasons when your immune load is higher.
The relationship between the gut and immune system is bidirectional and highly active. Research published in Frontiers in Immunology (2024) describes the gut microbiota as "a pivotal regulator of immune ontogeny, response calibration and mucosal barrier integrity."
In practical terms, the microbiome supports immunity in several ways:
When the microbiome is diverse and well-fed, this system runs efficiently. When it's disrupted through poor diet, stress, alcohol or antibiotics, immune regulation suffers.
Several factors converge in autumn to make immunity more vulnerable:
1. Keep plant diversity high. Different fibres feed different microbes. Aim for around 30 plant varieties a week, including autumn produce like squash, pumpkin, leeks, apples, pears, cabbage, lentils, and root vegetables. Diversity is one of the strongest markers of a resilient microbiome.
2. Get your vitamin D checked and supplement if needed. In Switzerland, the UK, and most of northern Europe, vitamin D supplementation from October through March is broadly recommended. Talk to your healthcare provider about testing your levels and dosing appropriately.
3. Include fermented foods regularly. Kefir, live-culture yoghurt, sauerkraut, kimchi, and miso introduce beneficial microbes and bioactive compounds. A 2021 Stanford study found that a diet high in fermented foods increased microbiome diversity and reduced inflammatory markers within 10 weeks.
4. Prioritise sleep and stress management. These aren't nice-to-haves. Poor sleep and chronic stress both alter gut microbiome composition and immune function within days. Aim for 7-9 hours of sleep and build in daily recovery, whether that's a walk, breathwork, or time offline.
5. Be strategic with alcohol and ultra-processed foods. Both disrupt the gut barrier and reduce microbial diversity. You don't need to eliminate either, but if you're heading into cold and flu season, being deliberate about intake gives your gut more room to do its job.
The evidence for over-the-counter probiotics in general immune support is inconsistent, and outcomes depend heavily on the specific strain, dose, and individual. Some strains, like Lactobacillus rhamnosus GG, have research supporting reduced incidence and duration of upper respiratory infections in certain populations. Others have no meaningful data behind them. Whole-food sources of beneficial microbes remain the most reliable starting point, with strain-specific supplementation reserved for particular clinical situations under professional guidance.
Immunity is not something you build in a week with supplements. It's the result of consistent inputs across sleep, food, movement, stress, and hydration, all filtered through the health of your gut. Autumn is the season to tighten up on the basics before winter arrives, and your microbiome is the smartest place to start.
