Primary prevention of CVD: diet

Abstract

Introduction

Diet is important in the cause of many chronic diseases. Individual change in dietary behaviour has the potential to decrease the burden of chronic disease, particularly cardiovascular disease (CVD).

Methods and outcomes

We conducted a systematic review and aimed to answer the following clinical question: What are the effects of dietary advice in generally healthy adults without existing CVD or increased CVD risk factors to improve cardiovascular outcomes (mortality, cardiovascular events, and cardiovascular risk factors)? We searched: Medline, Embase, The Cochrane Library, and other important databases up to April 2010 (Clinical Evidence reviews are updated periodically, please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA).

Results

We found 14 systematic reviews or RCTs that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions.

Conclusions

In this systematic review we present information relating to the effectiveness and safety of the following interventions: advice to increase fibre intake alone, advice to increase fruit and vegetable intake alone, advice to reduce and/or modify fat intake alone, and advice to reduce sodium intake alone.

Key Points

Diet is an important cause of many chronic diseases.

• Individual change in behaviour has the potential to decrease the burden of chronic disease, particularly cardiovascular disease (CVD).

This review separately examines different elements of advice to modify diet, and whether these improve cardiovascular outcomes in healthy people without existing CVD or elevated risk factors.

• We have excluded non-clinical outcomes such as behavioural change (e.g., change in the proportion of saturated fats in the diet, or change in the number of servings of vegetables per week).

Intensive advice to reduce sodium intake alone may reduce blood pressure compared with no advice in healthy people without hypertension.

• Intensive advice seems to reduce sodium intake as measured by sodium excretion.

• However, it is unclear whether advice to reduce sodium intake reduces mortality or cardiovascular events as we found insufficient evidence.

• The intensive advice interventions used in some studies may not be practicable in routine clinical practice.

We found insufficient evidence from RCTs on the effects of advice to reduce and/or modify fat intake alone on cardiovascular outcomes.

Some trials have found that advice to increase fruit and vegetable intake alone may improve blood pressure and BMI compared with no advice; however, other studies have found no difference.

We found insufficient evidence from RCTs on the effects of advice to increase fibre intake alone on risk factors for CVD, CVD events, or death.

This review has examined the effects of separate elements of dietary advice alone in improving cardiovascular outcomes. The next update of this review will include the effects of any dietary advice (broad dietary advice including combinations of any elements) on improving cardiovascular outcomes.

About this condition

Definition

Diet is important in the cause of many chronic diseases. Individual change in dietary behaviour has the potential to decrease the burden of chronic disease, particularly cardiovascular disease (CVD). This review focuses on the evidence that specific interventions to modify and improve diet may reduce CVD risk. Clinically overt ischaemic vascular disease includes acute myocardial infarction, angina, stroke, and peripheral vascular disease. Many adults have no symptoms or obvious signs of vascular disease, even though they have atheroma and are at increased risk of ischaemic vascular events because of one or more risk factors. In this review, we have taken primary prevention to apply to people who have not had clinically overt CVD. Population: In this review, we have included studies in free living healthy adults (aged 18 years or older) with no evidence of clinically overt CVD, and with <10% of the population with existing cardiovascular risk factors (hypertension, dyslipidaemia, or diabetes), but have included studies in people with impaired glucose tolerance. We have included dietary advice given by healthcare professionals such as nurses, doctors, dieticians, and paper-based self-help resources, and have excluded web-based or electronic advice and public health measures. Primary prevention of CVD in people with hypertension or dyslipidaemia is covered in separate Clinical Evidence reviews, as is secondary prevention of CVD in people with existing CVD (see reviews on secondary prevention of ischaemic cardiac events, and angina (chronic stable).

Incidence/ Prevalence

CVD was responsible for 39% of UK deaths in 2002. Half of these were from coronary heart disease (CHD), and one quarter were from stroke. CVD is also a major cause of death before 75 years of age, causing 34% of early deaths in men and 25% of deaths before 75 years of age in women. CHD deaths rose dramatically in the UK during the 20th century, peaked in the 1970s, and have fallen since then. Numbers of people living with CVD are not falling, and the British Heart Foundation estimates that there are about 1.5 million men and 1.2 million women who have or have had a myocardial infarction (MI) or angina. Worldwide, it is estimated that 17 million people die of CVDs every year, and >60% of the global burden of CHD is found in resource-poor countries (10% of disability-adjusted life years [DALYs] lost in low- and middle-income countries and 18% in high-income countries). The USA has a similar burden of heart disease to the UK; in 2002, 18% of deaths in the USA were from heart disease, compared with 20% in the UK. The USA lost 8 DALYs per 1000 population to heart disease and a further 4 DALYs per 1000 population to stroke, and the UK lost 7 DALYs per 1000 population to heart disease and 4 DALYs per 1000 population to stroke. Afghanistan has the highest rate of DALYs lost to heart disease (36 DALYs per 1000 population), and France, Andorra, Monaco, Japan, Korea, Dominica, and Kiribati have the lowest (1–3 DALYs per 1000 population). Mongolia has the highest rate for stroke (25 DALYs per 1000 population lost) and Switzerland the lowest (2 DALYs per 1000 population lost).

Aetiology/ Risk factors

Deaths from CHD are not evenly distributed across the population. They are more common in men than in women; 67% more common in men from Scotland and the north of England than in the south of England; 58% more common in male manual workers; twice as common in female manual workers than in female non-manual workers; and about 50% higher in South Asian people living in the UK than in the average UK population. In the UK there are 14% more CHD deaths in the winter months than in the rest of the year. CVD in the UK generally results from the slow build-up of atherosclerosis over many decades, with or without thrombosis. The long development time of atherosclerosis means that small changes in lifestyle may have profound effects on risk of CVD over decades. However, while there is strong evidence from epidemiological studies of the importance of lifestyle factors (such as smoking, physical activity, and diet) in the process of development of CVD, adjusting for confounding can be difficult, and the long timescales involved make proving the effectiveness of preventive interventions in trials difficult. In practice, risk factors — rather than disease outcomes — are often the only practical outcomes for intervention studies in low-risk people. Such risk factors include blood pressure, body mass index (BMI), serum lipids, and development of diabetes.

Prognosis

Improvements in diet may lower the risk of cardiovascular disease by exerting favourable changes on CVD risk factors (obesity, high blood pressure, elevated serum lipids, diabetes).

Aims of intervention

To modify diet in order to reduce risk of CVD and death, with minimum adverse effects.

Outcomes

Mortality: total mortality or mortality attributable to CVD. Cardiovascular events: such as MI, angina, stroke, and heart failure. Cardiovascular risk factors: changes in risk factors such as serum lipids, weight, blood pressure, and glucose tolerance. Adverse effects. We have excluded behavioural change as an outcome. For example, the change in the number of servings or weight of vegetables consumed in a week after advice to increase vegetable consumption. Although many studies report these intermediate outcomes, we have only evaluated clinical outcomes in this review.

Methods

Clinical Evidence search and appraisal April 2010. The following databases were used to identify studies for this systematic review: Medline 1990 to April 2010, Embase 1990 to April 2010, and The Cochrane Database of Systematic Reviews March 2010 (online) (1990 to date of issue). When editing this review, we used The Cochrane Database of Systematic reviews 2010, Issue 3. An additional search within The Cochrane Library was carried out for the Database of Abstracts of Reviews of Effects (DARE) and Health Technology Assessment (HTA). We also searched for retractions of studies included in the review. Abstracts of the studies retrieved from the initial search were assessed by an information specialist. Selected studies were then sent to the contributor for additional assessment, using predetermined criteria to identify relevant studies. Quality issues relating to included studies: Study design criteria for inclusion in this review were: published systematic reviews of RCTs and RCTs in any language, and containing >100 individuals of whom >80% were followed up, with at least a 6-month follow-up. We included open RCTs. We included RCTs undertaken in healthy free living adults without existing CVD and with <10% of the trial population with CVD risk factors (hypertension, dyslipidaemia, diabetes), in whom the intervention of interest was applied. We have included RCTs in which dietary advice was given by healthcare professionals such as nurses, doctors, dieticians, and paper-based self-help resources, and have excluded web-based or electronic advice and public health measures. We have excluded RCTs in which food was also supplied or supplements given in addition to advice, where incentives (cash or other) were employed, where increased exercise was part of the intervention, or where weight loss was an aim of the trial. Ideally, to avoid confounding and allow judgements on the application of different elements of advice in clinical practice, we would have only included RCTs in which the advice given related to that single intervention only (e.g., advice on reducing or modifying fat intake only). However, much of the advice given in RCTs involved more than one element. On occasion, we have included RCTs in which the advice of interest may have been given in combination with other advice, but was a major part of the overall package of advice given. In such cases, we have clearly identified the range of advice given. We have excluded RCTs in which the intervention included a range of different advice in which the contribution of different individual elements was unclear. We have only reported RCTs that reported clinical outcomes in the benefits section of this review (changes in mortality, cardiovascular events, or cardiovascular risk factors [BP, lipid levels, weight, glucose tolerance]). Many RCTs we found reported behavioural change as an outcome (e.g., change in the number of servings of vegetables consumed in 1 week, or alteration in the proportion of saturated fats consumed in the diet); we have excluded these data and only report on the prespecified clinical outcomes. For the option on advice to reduce and/or modify fat intake, we have excluded RCTs that gave cholesterol-lowering advice only. We have occasionally reported data in the benefits section, often as pooled analysis, which may have included some participants outside our population group of interest, or in which the intervention was outside our intervention of interest, but we felt was still of relevance. In such cases, we have clearly described the populations and interventions included in the analysis, and how these differed from our populations and interventions of interest. We have also reported data of less direct relevance as background information in the comments section, because of the paucity of RCTs reporting cardiovascular outcomes in our population group or interventions of interest. We included systematic reviews of RCTs and RCTs where harms of an included intervention were studied applying the same study design criteria for inclusion as we did for benefits. In addition we use a regular surveillance protocol to capture harms alerts from organisations such as the FDA and the MHRA, which are added to the reviews as required. General reporting: To aid readability of the numerical data in our reviews, we round many percentages to the nearest whole number. Readers should be aware of this when relating percentages to summary statistics such as relative risks (RRs) and odds ratios (ORs). We have performed a GRADE evaluation of the quality of evidence for interventions included in this review (see table ). The categorisation of the quality of the evidence (into high, moderate, low, or very low) reflects the quality of evidence available for our chosen outcomes in our defined populations of interest. These categorisations are not necessarily a reflection of the overall methodological quality of any individual study, because the Clinical Evidence population and outcome of choice may represent only a small subset of the total outcomes reported, and population included, in any individual trial. For further details of how we perform the GRADE evaluation and the scoring system we use, please see our website (www.clinicalevidence.com). Changes at this update: Since the last published version of this review, the review has been restructured, a backsearch has been undertaken and previously reported evidence re-evaluated, new evidence up to the current search date of April 2010 has been added, the minimum size criteria for inclusion of individual studies in this review has been raised (from 20 to 100 people), behavioural outcomes have been excluded, and the minimum duration of follow-up reduced from 1 year to 6 months.

Table 1.

GRADE evaluation of interventions for primary prevention of CVD: diet

Important outcomes	Mortality, cardiovascular events, cardiovascular risk factors, adverse effects
Number of studies (participants)	Outcome	Comparison	Type of evidence	Quality	Consistency	Directness	Effect size	GRADE	Comment
What are the effects of dietary advice in generally healthy adults without existing CVD or increased CVD risk factors to improve cardiovascular outcomes (mortality, cardiovascular events, cardiovascular risk factors)?
3 (2326)	Mortality	Advice to reduce sodium intake alone v no advice	4	0	0	–2	0	Low	Directness points deducted for unclear generalisability of intensive regimen and small number of events (31 in total)
3 (2326)	Cardiovascular events	Advice to reduce sodium intake alone v no advice	4	–1	0	–2	0	Very low	Quality point deducted for incomplete reporting of results. Directness points deducted for unclear generalisability of intensive regimen and use of composite outcome including mortality and morbidity in 1 RCT
4 (2835)	Cardiovascular risk factors	Advice to reduce sodium intake alone v no advice	4	0	0	–2	0	Low	Directness points deducted for unclear generalisability of intensive regimen and use of co-intervention in 1 RCT (advice on fruit and vegetables)
Unclear (unclear)	Mortality	Advice to reduce and/or modify fat intake alone v no advice	4	–1	0	–2	0	Very low	Quality point deducted for incomplete reporting of results. Directness points deducted for inclusion of people with existing CVD and for inclusion of co-interventions (supplements, food)
Unclear (unclear)	Cardiovascular events	Advice to reduce and/or modify fat intake alone v no advice	4	–1	0	–2	0	Very low	Quality point deducted for incomplete reporting of results. Directness points deducted for inclusion of people with existing CVD and for inclusion of co-interventions (supplements, food)
3 (4426)	Cardiovascular risk factors	Advice to increase fruit and vegetable intake alone v no advice	4	0	–1	–1	0	Low	Consistency point deducted for inconsistent results between RCTs (BP, BMI/weight). Directness point deducted for co-intervention in 2 RCTs (advice other than on fruit and vegetables)

Type of evidence: 4 = RCT. Consistency: similarity of results across studies.Directness: generalisability of population or outcomes.Effect size: based on relative risk or odds ratio.

Glossary

Body mass index (BMI)

Calculated by weight (in kilograms) divided by height (in metres) squared.

Low-quality evidence

Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.

Very low-quality evidence

Any estimate of effect is very uncertain.

Primary prevention of CVD: physical activity

Primary prevention of CVD: treating dyslipidaemia

Primary prevention of CVD: treating hypertension

Secondary prevention of ischaemic cardiac events

Angina (chronic stable)

Disclaimer

The information contained in this publication is intended for medical professionals. Categories presented in Clinical Evidence indicate a judgement about the strength of the evidence available to our contributors prior to publication and the relevant importance of benefit and harms. We rely on our contributors to confirm the accuracy of the information presented and to adhere to describe accepted practices. Readers should be aware that professionals in the field may have different opinions. Because of this and regular advances in medical research we strongly recommend that readers' independently verify specified treatments and drugs including manufacturers' guidance. Also, the categories do not indicate whether a particular treatment is generally appropriate or whether it is suitable for a particular individual. Ultimately it is the readers' responsibility to make their own professional judgements, so to appropriately advise and treat their patients. To the fullest extent permitted by law, BMJ Publishing Group Limited and its editors are not responsible for any losses, injury or damage caused to any person or property (including under contract, by negligence, products liability or otherwise) whether they be direct or indirect, special, incidental or consequential, resulting from the application of the information in this publication.