Fibre Is Not Just Fibre
The advice to "eat more fibre" is one of the most common and least helpful recommendations in digestive health. Not because fibre is unimportant (it is critically important) but because different types of fibre have vastly different properties, mechanisms, and clinical applications. Psyllium husk, inulin, methylcellulose, wheat bran, and resistant starch each behave differently in the gut, feed different bacterial populations, and are suited to different conditions. Choosing the wrong type of fibre can actually worsen the symptoms you are trying to fix.
Psyllium Husk
What It Is
Psyllium is a soluble, gel-forming fibre derived from the seed husks of Plantago ovata. When mixed with water, it forms a viscous gel that gives it unique properties in the GI tract. It is only partially fermented by gut bacteria, which is a critical distinction from other soluble fibres.
Key Properties
- Bidirectional stool regulation: psyllium is the only fibre supplement that has been clinically shown to normalise both constipation and diarrhoea. The gel absorbs excess water in diarrhoea and retains water in constipation, producing a consistently well-formed stool
- Low fermentation: because psyllium is only about 30% fermented (compared to 100% for inulin), it produces significantly less gas. This makes it the best tolerated fibre for people with bloating, IBS, and SIBO
- Cholesterol reduction: the gel-forming property traps bile acids, reducing cholesterol reabsorption. The FDA allows a health claim for psyllium's cholesterol-lowering effect
- Blood sugar modulation: slows carbohydrate absorption, reducing postprandial glucose spikes
Best For
IBS (all subtypes), chronic constipation, diarrhoea-predominant conditions, cholesterol management, and blood sugar regulation. Dose: start with 5g daily and increase gradually to 10-15g daily, always with adequate water (at least 250ml per serving).
Inulin and FOS (Fructo-Oligosaccharides)
What They Are
Inulin and FOS are soluble prebiotic fibres found naturally in chicory root, garlic, onions, leeks, and Jerusalem artichokes. They are fully fermentable, meaning gut bacteria break them down completely into short-chain fatty acids (primarily acetate and propionate) and gas.
Key Properties
- Potent prebiotic effect: inulin and FOS are the most studied prebiotics, selectively promoting the growth of Bifidobacterium and Lactobacillus species
- High fermentability: while this produces beneficial SCFAs, it also produces significant gas. This is the primary cause of bloating, flatulence, and cramping that many people experience when taking inulin supplements
- Calcium absorption: FOS enhances calcium absorption in the large intestine, which has positive implications for bone health
Best For
Increasing microbial diversity, specifically boosting Bifidobacterium populations, and supporting SCFA production. Not ideal for people with IBS, SIBO, or bloating, as the high fermentability often worsens these conditions. Start at 2-3g daily and increase very gradually to 5-10g.
Wheat Bran (Insoluble Fibre)
What It Is
Wheat bran is the outer layer of the wheat kernel, composed primarily of insoluble fibre (cellulose and hemicellulose). It adds bulk to stool by absorbing water and increasing faecal mass.
Key Properties
- Mechanical bulking: increases stool volume and stimulates peristalsis through mechanical stretch of the colon wall
- Minimal fermentation: cellulose is poorly fermented, so gas production is low
- Accelerated transit: reduces colonic transit time, which can benefit slow-transit constipation
Limitations
Wheat bran can be irritating to sensitive guts. The coarse particles can aggravate IBS symptoms, and it contains gluten, making it unsuitable for coeliac patients or those with gluten sensitivity. Studies comparing wheat bran with psyllium for IBS consistently show psyllium as superior, with wheat bran sometimes worsening symptoms.
Best For
Simple slow-transit constipation in people without IBS, gluten sensitivity, or significant bloating.
Partially Hydrolysed Guar Gum (PHGG)
What It Is
PHGG is a soluble, prebiotic fibre derived from guar beans that has been enzymatically processed to reduce viscosity and improve tolerability. It is fully fermented but produces less gas than inulin because fermentation is slower and more gradual.
Key Properties
- Well tolerated: studies show PHGG produces significantly less bloating and flatulence than inulin or FOS at equivalent doses
- Prebiotic: selectively feeds Bifidobacterium and increases butyrate production
- SIBO-compatible: uniquely among prebiotic fibres, PHGG has been studied as an adjunct to SIBO treatment and shown to improve eradication rates
- Regulates stool: similar bidirectional effects to psyllium, though the evidence base is smaller
Best For
People who want prebiotic benefits but cannot tolerate inulin or FOS. Particularly useful during and after SIBO treatment. Dose: 5-10g daily.
Resistant Starch
What It Is
Resistant starch is starch that resists digestion in the small intestine and reaches the colon intact, where it is fermented by bacteria. Sources include green bananas, cooked and cooled potatoes, cooked and cooled rice, and raw oats.
Key Properties
- Butyrate production: resistant starch is the most potent dietary promoter of butyrate, the SCFA that fuels colonocytes and maintains barrier integrity
- Insulin sensitivity: improves glucose metabolism through both SCFA signalling and reduced glycaemic load
- Generally well tolerated: produces less gas than inulin when introduced gradually
Best For
Boosting butyrate production, improving metabolic health markers, and supporting colonic health. Introduce gradually starting at 5g daily.
Choosing the Right Fibre
The best fibre depends on your specific condition and tolerance. For most people with gut issues, psyllium husk is the safest starting point because of its low fermentability and bidirectional stool regulation. Once symptoms stabilise, gradually adding prebiotic fibres (PHGG or resistant starch first, then small amounts of inulin) diversifies microbial feeding and increases SCFA production. GutIQ evaluates your digestive patterns and symptom profile to help you identify which fibre strategy is most appropriate for your current gut health status.