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 March 2014 (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 studies 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 examines evidence solely from RCTs and systematic reviews of RCTs.

To reduce confounding, this review has examined the effects of separate elements of dietary advice alone in improving 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 RCTs have found that advice to increase fruit and vegetable intake alone may improve systolic blood pressure at 6 to 12 months compared with no advice; we found insufficient evidence from RCTs on the effects of advice to increase fruit and vegetable intake alone on CVD events or death. However, we also found no harm from advice to increase fruit and vegetables.

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.

RCTs may only provide limited evidence on longer-term outcomes such as mortality or cardiovascular events due to the restricted numbers included in most trials and the length of follow-up needed to identify any differences between groups.

Large observational studies may provide important evidence on these longer term 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 (MI), 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 less than 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, and dietitians, which may have been supplemented by paper-based self-help resources. We 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 32% of UK deaths in men, and 21% of UK deaths in women in 2010. 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 28% of early deaths in men and 19% 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 an MI or angina. Worldwide, it is estimated that 17 million people die of CVDs every year. More than 60% of the global burden of CHD is found in resource-poor countries. The US has a similar burden of heart disease to the UK; in 2002, 18% of deaths in the US were from heart disease, compared with 20% in the UK. The US lost 8 disability-adjusted life years (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 18% more CHD deaths in men, and 21% in women over the winter months compared with 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 for 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; and 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 March 2014. The following databases were used to identify studies for this systematic review: Medline 1966 to March 2014, Embase 1980 to March 2014, and The Cochrane Database of Systematic Reviews 2014, issue 3 (1966 to date of issue). Additional searches were carried out in the Database of Abstracts of Reviews of Effects (DARE) and the Health Technology Assessment (HTA) database. We also searched for retractions of studies included in the review. An information specialist identified titles and abstracts in an initial search, which an evidence scanner then assessed against predefined criteria. An evidence analyst then assessed full texts for potentially relevant studies against predefined criteria. An expert contributor was consulted on studies selected for inclusion. An evidence analyst then extracted all data relevant to the review. Quality issues relating to included studies: study design criteria for inclusion in this review were published RCTs and systematic reviews of RCTs in the English language, and containing greater than or equal to 100 individuals of whom more than 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 less than 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, and dieticians, which may have been supplemented by 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. To avoid confounding and allow judgements on the application of different elements of advice in clinical practice, we 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). Much of the advice given in RCTs included in reviews involved more than one element. 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. Due to the paucity of RCTs reporting cardiovascular outcomes in our population group or interventions of interest, we have occasionally reported data in the Comments section, 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 included RCTs and systematic reviews of 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).

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
3 (at least 2285)	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 unclear importance longer term (13–60 months)
4 (970)	Cardiovascular risk factors	Advice to increase fruit and vegetable intake alone v no advice	4	0	0	–2	0	Low	Directness points deducted for heterogeneity of interventions between trials and unclear generalisability of intensive regimen

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)

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