Small intestinal bacterial overgrowth (SIBO) is a condition in which bacteria that should predominantly inhabit the large intestine proliferate in the small intestine, causing a range of digestive symptoms that are often confused with irritable bowel syndrome. Understanding SIBO — its causes, its diagnosis, and the dietary approaches that support treatment — is important for anyone with persistent digestive symptoms that have not responded to standard IBS management.
What SIBO Is and How It Develops
The small intestine normally contains relatively few bacteria compared to the large intestine, where the majority of the gut microbiome resides. SIBO develops when this balance is disrupted — bacteria migrate into or proliferate in the small intestine, fermenting carbohydrates that would normally be absorbed before reaching the large intestine. This fermentation produces hydrogen and methane gases responsible for the characteristic symptoms: bloating (often within 90 minutes of eating), abdominal distension, flatulence, abdominal pain, diarrhoea, constipation, or alternating patterns of both.
Conditions that predispose to SIBO include: slow intestinal motility (common after gastrointestinal surgery or with certain medications); structural abnormalities; low stomach acid; and certain autoimmune conditions. The British Society of Gastroenterology publishes diagnostic and management guidelines for SIBO that inform clinical practice in the UK.
Diagnosis
SIBO is diagnosed primarily through breath testing — hydrogen and methane breath tests measure the gases produced by bacterial fermentation of a lactulose or glucose substrate. A positive breath test provides evidence of bacterial overgrowth, though the sensitivity and specificity of breath testing is imperfect. Clinical diagnosis in the context of typical symptoms and response to treatment is also used.
Self-diagnosis of SIBO based on symptoms alone is unreliable — many conditions produce similar symptoms. Consult your GP or a gastroenterologist if you suspect SIBO. The NHS IBS guidance provides a starting point for understanding the overlap between SIBO and IBS symptoms.
The Low-FODMAP Diet and SIBO
The low-FODMAP diet — which restricts fermentable oligosaccharides, disaccharides, monosaccharides, and polyols — is the most evidence-based dietary intervention for symptom management in SIBO. By reducing the availability of fermentable substrates for bacteria in the small intestine, low-FODMAP eating reduces gas production and the symptoms that result from it. The British Society of Gastroenterology and FODMAP-trained dietitians at King's College London have developed structured protocols for low-FODMAP implementation.
Low-FODMAP is a short-term diagnostic and symptom-management tool, not a permanent diet. The reintroduction phase — systematically adding FODMAP categories back and identifying personal triggers — is as important as the restriction phase.
Elemental and Semi-Elemental Diets
In severe SIBO cases, elemental diets — where nutrients are provided in pre-digested form that requires minimal bacterial action to absorb — can reduce bacterial load in the small intestine by starving the overgrown bacteria of fermentable substrates. These are clinical interventions managed under medical supervision, not self-treatment approaches.
Supporting Recovery
After antibiotic or herbal antimicrobial treatment for SIBO, supporting gut motility (through adequate fibre, movement, and meal spacing) and rebuilding microbiome diversity (through gradual reintroduction of diverse plant foods) helps prevent recurrence. Prokinetic support — stimulating the migrating motor complex that sweeps the small intestine between meals — is often part of post-treatment protocols.
For broader gut health support through diet, see our microbiome diversity guide and our 4-week gut healing plan. Always work with a registered dietitian or gastroenterologist for SIBO management — the British Dietetic Association can help you find a qualified specialist.
Supporting Your Health Through Daily Nutrition
Understanding the principles covered in this article is valuable — but applying them consistently through daily food choices is where the real benefit comes. For London office workers, the quality of the daily work lunch is one of the most controllable nutritional variables in the day. A fresh, balanced, nutritious lunch delivered to your desk removes one decision from a demanding schedule and ensures a consistently good nutritional foundation.
Vanda's Kitchen near St Paul's Cathedral EC4 delivers certified halal, 100% nut-free, freshly prepared corporate catering across the City of London and central London. Our Filipino-inspired menu is built around lean proteins, fresh vegetables, and complex carbohydrates — the nutritional combination that supports energy, performance, and health throughout the working day. Every item we produce carries full allergen labelling in compliance with Natasha's Law, and our entire kitchen is independently certified halal by the Halal Friendly List.
Our Selfridges Food Hall presence confirms the quality standard we maintain. For London teams wanting consistently nutritious, genuinely delicious, allergen-safe daily lunches, Vanda's Kitchen is the straightforward answer. View our team lunch options, WhatsApp us for a same-day response, or send an enquiry. Read our healthy office lunch delivery guide for more on what we offer and how our delivery works.
Frequently asked questions
How is SIBO different from IBS and how do I know which one I have?
SIBO and IBS share many symptoms — bloating, abdominal pain, altered bowel habits — and SIBO is thought to underlie a significant proportion of IBS diagnoses. The difference is mechanistic: SIBO has a specific cause (bacterial overgrowth in the small intestine) confirmed by breath testing, while IBS is a symptom-based diagnosis. A gastroenterologist or your GP can arrange breath testing if SIBO is suspected.
Can SIBO resolve on its own without antibiotics or herbal antimicrobials?
Some mild cases improve with dietary changes and addressing underlying motility issues, but SIBO tends to recur or persist without targeted treatment. The low-FODMAP diet manages symptoms by reducing fermentable substrates but does not eliminate the bacterial overgrowth itself. Most gastroenterologists recommend antimicrobial treatment — either prescribed antibiotics or herbal protocols — alongside dietary support.
What happens if SIBO is left untreated for a long time?
Untreated SIBO can cause progressive nutrient malabsorption, particularly of fat-soluble vitamins (A, D, E, K) and vitamin B12 as bacteria consume these nutrients in the small intestine. Long-term consequences include nutritional deficiencies, worsening gut permeability, and systemic effects including fatigue, cognitive fog, and immune disruption. Persistent digestive symptoms warrant investigation rather than indefinite self-management.
Are probiotic supplements helpful or harmful in SIBO?
The evidence on probiotics in SIBO is mixed and genuinely uncertain. Some strains appear to reduce SIBO recurrence by competing with the overgrown bacteria; others may worsen symptoms in active SIBO by adding more organisms to an already over-populated small intestine. Most SIBO specialists avoid recommending probiotics during active treatment and reconsider them only after bacterial levels have been reduced.
Does SIBO cause weight loss or weight gain?
SIBO can produce either, depending on type and severity. Hydrogen-dominant SIBO is more commonly associated with diarrhoea and weight loss from malabsorption. Methane-dominant SIBO (now sometimes classified as intestinal methanogen overgrowth) is more commonly associated with constipation and, in some research, increased calorie extraction from food and weight gain. Body weight changes alongside digestive symptoms are worth discussing with your GP.