Despite advancements in diagnosis and management the prevalence of heart failure is increasing (Al-Mohammad 2011). A recent epidemiological analysis found that in 2010 more than 41 million people were living with heart failure globally (Forouzanfar 2013). The majority of heart failure cases were attributable to ischaemic heart disease (Forouzanfar 2013; Mosterd 2007). UK estimates suggest that 13% of males and 12% of females aged over 75 years have heart failure (BHF 2012). The increasing burden of disease management is predicted to affect both primary and secondary care, with the majority of costs being attributable to the hospitalization of individuals with heart failure (Stewart 2002; Stewart 2003).
While pharmacological management is widely accepted as the mainstay of treatment, several international guidelines emphasize the importance of lifestyle changes as an adjunct to pharmaceutical therapies (BHF 2012; Canada 2012; Dickstein 2008; SIGN 2007; HFSA 2010). For example, the UK National Institute for Health and Care Excellence (NICE) advises beneficial changes relating to diet, physical activity, weight loss, smoking, alcohol consumption and annual vaccinations (NICE 2010).
Several guidelines worldwide also advocate the importance of dietary advice in individuals with heart failure, specifically advice to reduce sodium intake both in the hospital and community (outpatient) setting (Gupta 2012). The rationale for such advice is described below.
Description of the condition
Heart failure is characterized by the impairment of the heart to maintain an adequate circulation of blood throughout the rest of the body. Broadly speaking it can be divided into two types: 1) heart failure with reduced ejection fraction; and 2) heart failure with preserved ejection fraction. Both types result in similar clinical symptoms, such as breathlessness and fatigue, and signs of fluid retention (e.g. oedema) (NICE 2010). The onset of symptoms can be acute, although more often this is seen on a background of chronic long-term symptoms.
The American College of Cardiology Foundation (ACCF)/American Heart Association (AHA) describe four stages of heart failure, a system that complements the New York Heart Association (NYHA) functional classification of heart failure (Table 1 and Table 2) (Yancy 2013). The emphasis of the ACCF/AHA system is on the presence of structural disease and symptoms, whereas the NYHA functional classification emphasizes the impact on physical activity and symptoms.
|A||At high risk for HF but without structural heart|
disease or symptoms of HF
|B||Structural heart disease but without signs or|
symptoms of HF
|C||Structural heart disease with prior or current|
symptoms of HF
|D||Refractory HF requiring specialized interventions|
|Class||New York Heart Association functional classification|
No limitation of physical activity
Ordinary physical activity does not cause undue fatigue, palpitation, dyspnea or anginal pain
Slight limitation of physical activity
They are comfortable at rest. Ordinary physical activity results in fatigue, palpitation, dyspnea or anginal pain
Marked limitation of physical activity
They are comfortable at rest. Less than ordinary activity causes fatigue, palpitation, dyspnea or anginal pain
Inability to carry out any physical activity without discomfort
Symptoms of heart failure or the anginal syndrome may be present even at rest. If any physical activity is undertaken, discomfort increases
There are several causes of heart failure of which ischaemic heart disease and hypertension are the most common (Dickstein 2008). Other causes include cardiac arrhythmias and myopathies, certain drugs and toxins, and endocrine and connective tissue disorders (NICE 2010). The classification of severity is commonly based on the NHYA system, which ranges from class I (asymptomatic with no limitations on daily living) to class IV (severe symptoms persistent at rest). Greater severity of symptoms is associated with poorer prognosis, and most individuals with heart failure experience morbidity and eventually mortality. Estimates suggest that nearly half of those diagnosed with heart failure die within four years, and those with a higher number of hospital admissions having a particularly poor prognosis. (Dickstein 2008).
Description of the intervention
The mainstay for the management of individuals with heart failure is largely pharmacologic through combinations of drugs such as diuretics, beta-blockers and angiotensin-converting enzyme (ACE) inhibitors (NICE 2010). However, like many chronic diseases, pharmacological management is underpinned by self-care and lifestyle advice, such as recognizing the worsening of symptoms, monitoring weight (as a marker of fluid overload), maintaining physical activity levels where possible, minimizing alcohol intake, stopping smoking and reducing the intake of dietary salt (Dickstein 2008; NICE 2010).
Such is the drive to reduce the dietary salt intake of the overall population that the World Health Organization (WHO) member states have agreed to target a 30% relative reduction in mean population salt intake by 2025 (WHO 2010; WHO 2013). Recent guidance has advocated that adults should consume less than 5 g of salt per day (WHO 2013). In the UK, the Department of Health has taken this further by setting a target limit of 3 g per day for adults by 2025 (NICE 2013).
For the purpose of this review we are using the term 'salt' to refer to sodium chloride, where 1 g of standard sodium chloride contains 0.38 g of sodium (SACN 2003).
How the intervention might work
The rationale for advocating reduced salt intake in individuals with heart failure is not fully understood but is based on several possible mechanisms. One mechanism is that reduced dietary salt has a positive impact on sodium-fluid homeostasis, thus preventing fluid overload (He 2010). Low-salt diets have also been shown to lower blood pressure in individuals with hypertension, which may also have benefit in those with heart failure (He 2013a). There is also evidence that salt depletion can directly lead to a reduction in left ventricular mass (Ferrara 1984).
Why it is important to do this review
Existing Cochrane reviews have looked at the effect of reduced dietary salt on blood pressure, concluding that even modest reductions can reduce blood pressure in both normotensive and hypertensive individuals (He 2013a; He 2013b). Broader Cochrane reviews have examined the effect of reduced dietary salt on preventing cardiovascular disease (Hooper 2004; Taylor 2013). However, neither review could draw any conclusions regarding the impact of low-salt diets in individuals with heart failure. Our review is therefore important as we will include only trials of participants with existing heart failure that have examined the impact of low-salt diets on participant health.
Despite the rationale for advocating low-salt diets in individuals with heart failure, the evidence base is weak. A recent review highlighted a lack of clarity on this management option despite widespread advocacy (Gupta 2012). The narrative review went on to highlight variations in results between observational and randomized controlled trials (RCTs) while also pointing out areas in which there is little evidence at all (e.g. reduced dietary salt in individuals with heart failure with preserved ejection fraction). A recent systematic review and meta-analysis included six RCTs examining the role of dietary salt restriction in individuals with heart failure. All six trials showed that a low-salt diet, compared with a normal-salt diet, significantly increased morbidity and mortality in individuals with heart failure (DiNicolantonio 2013a). Such a conclusion goes against current consensus (Gupta 2012), and led to a growing call for a re-evaluation of guidelines and current practice (Jun 2013; Lucan 2013). However, the review was retracted in its entirety after concerns were expressed over the integrity of the data (Dinicolantonio 2013b). Our preliminary searches have identified at least two further RCTs showing a beneficial role of dietary salt restriction in individuals with heart failure that were not included in the above systematic review (Colin 2004; Colin 2010). The lack of evidence in this area has been reflected in a recent executive summary from the US National Heart, Lung, and Blood Institute and the National Institutes of Health Office of Dietary Supplements calling for better-quality evidence for diet-related therapies in heart failure (Van Horn 2013). Hence, a systematic review examining the role of reduced dietary salt in individuals with heart failure is needed.