Abstract: We included 23 RCTs (1513 participants randomised) examining the effect of dietary fibre. The risk of bias was unclear for most studies and studies had small sample sizes. Few studies had an intervention duration of longer than 12 weeks. There was a wide variety of fibre sources used, with little similarity between groups in the choice of intervention. None of the studies reported on mortality (total or cardiovascular) or cardiovascular events. Results on lipids suggest there is a significant beneficial effect of increased fibre on total cholesterol levels (17 trials (20 comparisons), 1067 participants randomised, mean difference -0.20 mmol/L, 95% CI -0.34 to -0.06), and LDL cholesterol levels (mean difference -0.14 mmol/L, 95% CI -0.22 to -0.06) but not on triglyceride levels (mean difference 0.00 mmol/L, 95% CI -0.04 to 0.05), and there was a very small but statistically significant decrease rather than increase in HDL levels with increased fibre intake (mean difference -0.03 mmol/L, 95% CI -0.06 to -0.01). Fewer studies (10 trials, 661 participants randomised) reported blood pressure outcomes where there is a significant effect of increased fibre consumption on diastolic blood pressure (mean difference -1.77 mmHg, 95% CI -2.61 to -0.92) whilst there is a reduction in systolic blood pressure with fibre but this does not reach statistical significance (mean difference -1.92 mmHg, 95% CI -4.02 to 0.19). There did not appear to be any subgroup effects by the nature of the type of intervention (fibre supplements or provision of foods/advice to increase fibre consumption) or the type of fibre (soluble/insoluble) although the number of studies contributing to each subgroup were small. All analyses need to be viewed with caution given the risks of bias observed for total cholesterol and the statistical heterogeneity observed for systolic blood pressure. Adverse events, where reported, appeared to mostly reflect mild to moderate gastrointestinal side-effects and these were generally reported more in the fibre intervention groups than the control groups. Studies were short term and therefore did not report on our primary outcomes, CVD clinical events. The pooled analyses for CVD risk factors suggest reductions in total cholesterol and LDL cholesterol with increased fibre intake, and reductions in diastolic blood pressure. There were no obvious effects of subgroup analyses by type of intervention or fibre type but the number of studies included in each of these analyses were small. Risk of bias was unclear in the majority of studies and high for some quality domains so results need to be interpreted cautiously. There is a need for longer term, well-conducted RCTs to determine the effects of fibre type (soluble versus insoluble) and administration (supplements versus foods) on CVD events and risk factors for the primary prevention of CVD.

Plain-language summary: We searched scientific databases for randomised controlled trials (clinical trials where people are allocated at random to one of two or more treatments) looking at the effects of dietary fibre intake in healthy adults or those at high risk of developing CVD. We did not include people who already had CVD (e.g. heart attacks and strokes). The evidence is current to January 2015. Twenty three trials fulfilled our inclusion criteria. All of the trials were short term and so could not examine the effect of fibre intake on CVD events. All of the trials examined the effects of fibre intake on lipid levels (lipids are fat-like substances, including cholesterol found in the blood), blood pressure or both. Pooling the results showed a beneficial reduction in total cholesterol and LDL cholesterol (sometimes called 'bad' cholesterol), and diastolic blood pressure with increasing fibre intake. There were no clear patterns for the type of fibre used (soluble or insoluble fibre) or the way in which fibre was provided (via supplements or food stuffs) but their were few studies in each group so results are uncertain. Overall the risk of bias was unclear with few studies judged to be at low risk of bias (so less chance of arriving at the wrong conclusions because of favouritism by the participants or researchers), and for some there was a high risk of bias for some of the quality criteria. The results of this review need to be interpreted cautiously bearing this in mind. There is a need for longer-term well-conducted RCTs to determine the effects of fibre intake on CVD events and to further explore effects by the type of fibre and the way in which increased fibre is provided.