Fibre lowers blood pressure, but only psyllium moved the needle on its own

Dietary fibre, especially the viscous, gel-forming soluble kind, has an established place in reducing cardiometabolic risk factors. Whether it also lowers blood pressure has remained controversial, with trials pointing in different directions. Blood pressure is one of the most consequential numbers in cardiovascular risk, so even small, cheap, food-based reductions could matter when spread across a population. This review set out to quantify what viscous fibre supplementation actually does to blood pressure, and whether the individual fibres differ from one another.

The researchers searched MEDLINE, Embase and Cochrane databases for randomised controlled trials of at least four weeks testing supplementation with five viscous fibre types: beta-glucan from oats and barley, guar gum, konjac, pectin and psyllium. Study data were pooled using random effects models. Twenty-two trials with 1,430 participants informed the systolic blood pressure analysis and twenty-one trials with 1,343 participants the diastolic analysis, at a median dose of 8.7 g per day (range 1.45 to 30 g) over a median follow-up of seven weeks.

Viscous soluble fibre lowered systolic blood pressure by a mean 1.59 mmHg and diastolic pressure by 0.39 mmHg, both statistically significant but modest. When the five fibre types were examined separately, only psyllium produced a significant systolic reduction on its own, at 2.39 mmHg. There was substantial disagreement between studies in both the systolic and diastolic analyses, which makes the precise size of the effect uncertain.

Viscous fibre already has good evidence for other cardiometabolic risk factors, and this analysis adds blood pressure to the list, resolving some of the controversy in the direction of a real but small effect. The psyllium-specific finding suggests the different fibres should not be treated as interchangeable. Open questions include why the studies varied so much, whether people with higher starting blood pressure benefit more, and how supplemental fibre compares with fibre eaten in whole foods.

This is a reason to see viscous fibre, and psyllium in particular, as a small ally for blood pressure rather than a fix: the average systolic reduction of 1.59 mmHg is modest for any individual. The authors suggest that adding viscous fibre to a habitual diet may still have value for cardiovascular risk via blood pressure, so if that is a number you watch, fibre appears to help at the margins, alongside rather than instead of the bigger levers.