Dr Megan Rossi on why we feel sluggish in winter and how to support gut health

Gut Health specialist, Dr Megan Rossi shares why so many of us feel more sluggish in the colder months, how the gut plays a starring role and her top research-backed tips for boosting energy and strengthen immunity ahead of talk at New Scientist Live this October.

Why do we start to feel more sluggish in colder months?

There are a few factors at play here. As the days get colder and darker, you might naturally spend less time outdoors and exercise less, which can impact both energy levels and mood, as physical activity is a powerful mood booster.

Being indoors more often increases exposure to airborne viruses, so our immune system is under more pressure. It’s not the bacteria themselves causing colds – it’s the viruses – but it’s the entire gut microbiome, bacteria included, that affects immune resilience.

Eating habits also tend to shift towards heavier comfort foods, often higher in ultra-processed ingredients. These can leave you feeling sluggish and even contribute to digestive issues such as reflux and constipation if you’re eating less fibre-rich plants. On top of that, autumn and winter bring an increase in cold and flu viruses, and when your immune system is working harder to fight off infections, it can drain your energy reserves.

Shorter days also mean less light exposure, which disrupts your circadian rhythm – the body’s natural sleep-wake cycle. It takes time to adjust to darker days, and this affects not only your melatonin levels (the hormone that regulates sleep), but also your gut microbes, which follow a circadian rhythm too.

Is winter weight gain inevitable?

Not at all. Psychological factors play a role, but nourishing your gut bacteria with fibre-rich foods helps regulate appetite and weight through hormones like GLP-1. It is a little bit harder to maintain a healthier diet and exercise as much with shorter days. But I don’t recommend that people accept putting on weight as it can then become a slippery slope with people going on extreme diets come January to shed the gains, which ultimately damages the gut metabolism connection making you more vulnerable to putting on weight. Maintaining a balanced diet is key to avoiding weight gain.

What happens to our guts during this time? And why?

Shorter days affect our circadian rhythm, which also impacts gut bacteria. Diet changes, more comfort or ultra processed foods, fewer fresh fruits can reduce microbial diversity, weakening the immune system.

Your gut microbes thrive when you feed them a variety of high-fibre, plant-based foods. But in the colder months, you find yourself reaching for fewer fresh, fibre-rich foods, which can reduce the diversity of your gut microbiome. With fewer ‘good’ anti-inflammatory microbes being nourished as a result, their ability to support your health is limited, a bit like a team with fewer skilled players.

A drop in physical activity can also negatively affect your microbial diversity, adding to the challenge, as exercise has been shown to support a more diverse gut microbiome, independent of diet. Plus, with around 70% of your immune system living in the gut, as it ramps up to defend against seasonal viruses, you may notice the impact on both your digestive system and energy levels.

What about supplements and probiotics?

Vitamin C has little evidence for cold prevention for most people; it’s better sourced from food. Vitamin D is crucial in winter because lower sunlight reduces natural production and it helps regulate the gut immune system.

On the probiotic front, it’s worth being really specific about the strains (the scientific name for each different bacteria) you choose. Not all probiotics offer the same benefits, and just as you wouldn’t take a vitamin D supplement for an iron deficiency, being precise about probiotic strains is key if you want the immune benefits. For example, the combination of Lactobacillus rhamnosus GG (LGG) and Bifidobacterium BB-12 – the exact formulation you’ll find in SMART STRAINS for your immune system – has been shown in clinical trials to reduce the severity of colds and flu by over 30% and cut duration by around 2 days. In infants, this formulation of the two strains has been found to halve the risk of infections compared with placebo (a fake intervention).

What can we do about it?

Even if fresh produce isn’t always available, you can stock up on frozen options like mixed berries or mango and even mixed green veggies or cruciferous veggies like cauliflower. They’re just as nutritious and the mixed variety will quickly help boost the variety of plant foods in your diet. This might mean topping your morning porridge with mixed berries or blending hidden veggies into your pasta sauces.

Comfort foods can still work in your favour, too such as beans, chickpeas and lentils which are packed with fibre and are an easy addition to pasta dishes, soups and stews. Try to get as much plant diversity as possible, as different fibres feed different microbes to support all aspects of your health. Plus, hitting your 30g of fibre a day helps nourish your gut bacteria so that they can produce anti-inflammatory chemicals that support your immune system, strengthen your gut lining and even help keep your bowel movements regular.

Any practical tips for supporting gut health in winter?

Maintain consistent sleep patterns and seek morning light or use a 2,500–10,000 lux light device to regulate your circadian rhythm
Eat a diet rich in fibre, plants, legumes, frozen berries, and fermented foods
Include omega-3-rich fish, garlic, and mixed seeds
Limit alcohol to one or two standard drinks; excessive alcohol can make the gut lining leaky, causing low-grade inflammation

Catch Dr Megan Rossi at New Scientist Live which takes place on October 18-19 at ExCeL, London. To book tickets visit: New Scientist Live 2025 – Ticket types .

Megan’s Smarts Strains probiotic range is available via Smartstrain.com