Not all fibre helps: some supplements raised LDL cholesterol in pooled trials

Fibre supplements are usually discussed as if fibre were a single substance, yet dietary fibres vary widely in their chemical and physical complexity and in how readily gut microbes can use them. Whether those differences change what fibre actually does for health has been unclear, particularly for adults with overweight or obesity, a group in which fibre is often recommended. This review set out to test whether the specificity of a fibre, meaning its structure and how the gut microbiota utilises it, shapes its effects on weight, blood sugar, blood fats and blood pressure.

The authors searched PubMed, Cochrane Library, Web of Science and EMBASE for randomised controlled trials published between 1 January 2012 and 10 November 2023. Eligible trials compared dietary fibre supplementation against placebo in adults with overweight or obesity, without any deliberate calorie restriction. Thirty-four trials covering 1804 participants made the cut, testing doses from 1.5 to 40 g per day over 3 to 16 weeks. Trials were grouped by fibre specificity and analysed with random-effects models, with further analysis of whether population, duration or dosage changed the results.

Overall, fibre supplementation significantly reduced HbA1c by 0.13%, fasting insulin by 0.82 μIU/mL and the HOMA-IR insulin resistance score by 0.33. The subgroups told different stories. Low-specificity fibres cut fasting insulin by 1.09 μIU/mL, while low-to-intermediate-specificity fibres produced the biggest metabolic gains: HbA1c down 0.8%, fasting insulin down 2.08 μIU/mL and HOMA-IR down 0.61. Intermediate-specificity fibres were linked with a 2.85 kg drop in body weight but a 9.03 mg/dL rise in LDL cholesterol. Mixed fibres were associated with a 3.85 mmHg rise in systolic blood pressure.

This analysis extends the fibre literature by treating fibre as a family of different substances rather than one intervention, and its subgroup results complicate the comfortable idea that any fibre is automatically good. It fits the broader evidence that fibre supports blood sugar control, while raising new questions about why certain fibre categories tracked with worse cholesterol or blood pressure. Those subgroup findings rest on smaller pools of trials and considerable disagreement between studies, so they should be read as signals to investigate rather than settled effects. Head-to-head trials of different fibre types would be needed to resolve them.

This is a reason to think fibre supplements are not interchangeable: the average benefits for blood sugar control were real but modest, and the type of fibre appeared to matter for what else changed. The authors suggest that an individual's gut microbiota and specific health goals are relevant when thinking about fibre supplementation. For a general reader the takeaway is informational: fibre as a category looks helpful for metabolic markers, but the details of which fibre does what are still being worked out.