Background
Cardiovascular disease (CVD) is the leading global cause of morbidity and mortality [
1]. Insufficient intake of foods high in dietary fibre has been identified as one of the leading dietary risk factors that contribute to the burden of non-communicable diseases [
2]. Our systematic review and meta-analyses [
3] provide convincing evidence from prospective cohort studies and clinical trials that a high dietary fibre intake can reduce cardiometabolic events and premature mortality in generally healthy populations. We have identified comparable benefits in the management of adults with type 1 and type 2 diabetes [
4]. Active pharmacological management of cardiometabolic risk in patients with established cardiovascular disease has reduced the risk of further cardiovascular events and improved survival [
5]. The extent to which dietary fibre can further reduce risk for those with CVD and treated with cardioprotective drugs following an acute event has not been clearly established.
We have addressed this gap in the literature by conducting a systematic review and meta-analysis of the available data. We have identified prospective observational studies reporting on fibre intakes in those with pre-existing CVD and trials in which the effects of increasing fibre on cardiovascular risk factors have been examined in people with established CVD. We have also considered trials in which the effects of dietary fibre have been examined in hypertensive individuals because they are a readily identifiable group at high risk of developing CVD [
6]. As many of those with diagnosed CVD or hypertension are likely to be treated on cardioprotective medications, this research is intended to determine the extent to which dietary fibre is a useful adjunct to the pharmacological management of this high risk group of patients.
Discussion
We have considered the role of dietary fibre as a potential adjunct therapy alongside cardioprotective drugs in the management of established cardiovascular disease and hypertension. The findings indicate a reduced risk of premature mortality with higher fibre intakes when compared with lower intakes, and an improvement in key cardiometabolic risk factors when increasing fibre intakes. Risk reduction for premature mortality from the prospective observational studies was evident from data that controlled for medication use, while meta-regression from trials of adults with hypertension did not indicate anti-hypertensive medication use was a determining factor in the reported outcomes. As such, the current analyses indicate benefits with higher fibre intakes independent to what is achieved in pharmacological management.
The consideration of data from both trials of increasing fibre intakes and studies of higher intakes over time add confidence in the beneficial effects of dietary fibre intake, as the improvements in blood pressure, blood lipids, and body weight would be expected to reduce premature mortality, as was observed. There were more data available from trials of participants with hypertension than CVD, these analyses support and add to what was observed with CVD participants with improvements in blood pressure, blood lipids, bodyweight, and glycaemic control observed.
Higher dietary fibre intakes have demonstrated previous benefit in evidence synthesis on the prevention of premature mortality and non-communicable disease occurrence [
3] and in diabetes management [
4]. This review however is the first meta-analysis to consider the role of dietary fibre in the management of pre-existing hypertension and CVD. Furthermore, our methodology included use of meta-regression analyses to explore initial heterogeneity observed in trial data and increase confidence in the observed results. Although common, it is potentially misleading to report initial heterogeneity values without some further consideration of where it is derived. As an example, all nine data points from trials of increased dietary fibre and systolic blood pressure in patients with hypertension indicated a beneficial effect; however, the initial
I2 was high (99%). Meta-regression techniques did not identify a single underlying reason for this heterogeneity, and standardisation of dose to 5g of fibre per day still produced appreciable benefits. From this we conclude that the initial heterogeneity is statistical heterogeneity due to the low variability around each point estimate, rather than underlying differences between trials beyond the interventions delivered. Our use of GRADE protocols to assess the certainty of evidence for dietary fibre intakes in these populations is a further addition to the existing literature, and a key addition for guideline development and clinical recommendations.
Current guidelines for CVD and hypertension management focus on pharmacological aides [
35,
36] or if dietary, total dietary fat intake and fat quality [
37]. Fewer guidelines recommend dietary fibre as part of a cardioprotective dietary pattern [
38] or in lipid management [
39]. The current work provides confirmation on the role of dietary fibre in human health, and the direct translatability of the findings into dietary and clinical guidelines make it a substantial contribution to the field.
Increasing dietary fibre intake led to high certainty of substantial improvements in blood pressure in adults with hypertension. These improvements were observed regardless of the use of antihypertensives. High blood pressure not only results in deleterious mechanical stress on blood vessels but also on the myocardium, leading to the development of hypertensive heart disease and congestive heart failure [
1,
40]. Several pathways of action may explain this finding, such as dietary fibre’s role in reducing LDL cholesterol and triglyceride uptake [
41] improving the elasticity of blood vessel walls to decrease vascular resistance and maintain adequate tissue perfusion without requiring a subsequent rise in heart rate to maintain stroke volume [
42]. As a less direct mechanism, higher fibre intakes improved insulin sensitivity in this and previous works [
4], with insulin sensitivity believed to play a role in endothelial dysfunction and hypertension [
43]. Another major contributor to endothelial function is nitric oxide, which may be increased by increased fibre intake. Consuming foods high in dietary fibre may provide additional antioxidants [
44], reducing the role of oxidative stress in the pathogenesis of atherosclerosis [
45].
Other potential mechanisms for the beneficial effects observed with higher fibre intakes may relate to concomitant intakes of inorganic nitrate, or reduced body weight. High fibre foods such as vegetables also contain other beneficial nutrients that are metabolised into compounds such as nitric oxide, which may improve blood pressure through greater bioavailability for use in vasodilation [
46,
47]. The current work found some support for reductions in body weight with higher fibre intakes, as shown in evidence synthesis of the general population [
3] and those with diabetes [
4], with weight loss beneficial in the treatment and prevention of hypertension [
48]. Recent work has shown that the intake of whole grains, a considerable source of dietary fibre, when compared with refined grains leads to great measures of satiety [
49], providing some rationale for why higher fibre diets may reduce energy intake through increased satiety.
The present study has many strengths, primarily the parallel consideration of the effects of increasing fibre intake from controlled trials and higher fibre intakes in prospective cohort studies enabled us to consider mechanisms supporting hard outcomes [
50]. We followed recognised procedures for conducting systemic review and meta-analysis [
7,
8] as well as an assessment of the certainty of evidence to support clinical and dietary guidelines [
19]. To our knowledge this is the first meta-analysis to consider fibre for CVD and hypertension management, adding novelty to our work. The primary limitation of this work was the lack of relevant data available. Although only four cohort studies were identified, and it is never possible to fully exclude confounding from observational studies, follow-up duration was reasonable (weighted mean 8.6 years) and the cohorts were conducted in three distinct populations. Trials were generally of a limited number of participants, with the majority of studies of 12 weeks duration. Such limitations in the data increase the chance of observing spurious effects, although we considered that uncertainty when assessing the evidence. Further trials and cohorts of those with CVD or hypertension are needed, with some currently underway [
51]. We varied from the protocol of this review by considering only trials of at least six weeks intervention duration rather than the stated two weeks. This decision was made before searches were conducted to better consider meaningful change in a broader range of cardiometabolic risk factors beyond blood lipids and blood pressure. A wider variety of interventions considering multiple food sources of fibre would increase confidence in the presented findings and may provide further evidence on the place of high dietary fibre intakes as an adjunct therapy in CVD and hypertension management.
The findings from this meta-analysis support the incorporation of high fibre foods in CVD and hypertension management, with improvement in cardiometabolic risk factors supporting the observed reduction in premature mortality. However, further trials and cohort analyses in this area would increase confidence in these results.
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