Cardiovascular disease (CVD) refers to all the conditions that affect the heart and blood vessels, including coronary artery diseases (CAD) such as angina and myocardial infractions, strokes and transient ischaemic attacks (TIAs), as well as peripheral arterial and aortic diseases. CVD is the biggest contributor to mortality, morbidity and disability worldwide, particularly in high-income countries where these conditions have emerged as a modern epidemic, having resulted in approximately 17.5 million deaths per annum (WHO, 2014). In the UK, CVD has been recognised to be a major public health concern in that it dangerously affects half of the population, it especially impacting upon the elderly, with almost 160,000 deaths being reported every year (British Heart Foundation, 2016).
There are multiple risk factors that have been attributed to this condition including biological, genetic, environmental, lifestyle and psychological factors (Murray & Lopez, 1996) these being categorised as either non-modifiable (I.e., gender, family and ethnic background) or modifiable (i.e., diet, obesity, physical inactivity, alcohol and smoking status); they explaining approximately 80% of cases (Liu et al., 2012; Atlantis et al., 2012). In recent years, this field has witnessed a rapid surge of research and publications, with this increasing evidence base identifying associations between psychological stressors that seem to act as both potential aetiological as well as prognostic elements for the development of the medical conditions that are categorised as CVD.
Speculations about the link between psychological factors and CVD have been long present in medicine, with William Harvey having first described the circulatory system in 1628 and noted that the heart is affected by our emotions. Psychological stressors and the most common mood based disorders of anxiety and depression have been implicated in research for over forty years where they are commonly noted in context of general medical diseases including CVD (Rod, Andersen & Prescott, 2011). Research has suggested that about 15% of patients describe symptoms of major depression after undergoing acute myocardial infraction (AMI), and this figure climbs to 40% for patients describing symptoms of mild depression (e.g., Wayne, 1967). This figure is two to three times the predicted lifetime prevalence of depression found in the general population.
Epidemiological studies have recommended a significant, although variable, relationship between depression and the incidence of cardiac events, amongst healthy individuals and those with CVD (e.g., Gunn et al., 2012; Panagiotakos et al, 2002), in that 20% of the population will experience an episode of depression in their lifetime, and this rising to 50% of the population amongst those suffering from heart diseases (Nicholson, Kuper & Hemingway, 2006). Although, the direction of causality between depression and CVD remains unclear, the relationship between them is evidence, hence it is likely that an increase in the severity of depression is associated with higher subsequent risk for developing CVD (Nancy-Frasure-Smith et al, 1999). Nicholson, Kuper and Hemingway (2006) conducted a systematic review that demonstrated that healthy individuals with depression had a relatively elevated risk of 1.60 for a later index of a CVD event. This being later confirmed in the Whitehall (II) study which assessed 5,936 individuals over a period of 6 years and reported a hazard ratio of 1.93 for related cardiovascular events (Nabi et al., 2010).
Both CVD and depression have an overwhelming impact on the overall quality of life, particularly for patients experiencing heart failure (Rutledge et al., 2006), with associated increasing costs of medical care and use of health services, a general loss of productivity (Jiang, 2001), as well as increased mortality rates (Colquhoun et al., 2013), thus implying that patient with CVD who are also suffering from depression, have a much worse outcome than those who are not depressed. Such reports, particularly those on the scope, prevalence and patient burden of co-morbid CVD and depression highlight the importance of targeted interventions to manage these conditions.
A health based intervention aimed at an entire population with the intention of reducing the risk for CVD by improving symptoms of depression may be an effective way of reducing costs of hospitalisation, as well as long-term medical care and rehabilitation of patients. However, despite the efforts made so far, there have proved to be various obstacles that have created difficulties in the chronic and acute management of CVD and depression, with lifestyle-related risk factors playing an exceptionally important role.
1.2 Importance of diet in CVD risk
The primary areas that have been emphasised in the literature in terms of the development and management of CVD are physical activity and dietary and smoking habits, these areas now being placed at the forefront of both primary and secondary intervention programmes. These modifiable risk factors have hence been subject to innumerable epidemiological and clinical investigations within the international arena (e.g., Stampfer et al., 2000). The landmark INTERHEART case-control study by Yusuf et al., (2004) consisted of 15,152 patients and 14,820 controls from Asia, Africa, Australia, Europe, South and North America, and the Middle East, investigated the association between these potential risk factors and the probability of developing an AMI. The results identifying the two most important risk factors for AMI which together predicted 66% of the global risk for heart attack, are smoking status and abnormalities in blood lipid ratio (apolipoprotein B to apolipoprotein A1). Other risk factors being diabetes, obesity (waist-to-hip-ratio), psycho-social factors (e.g., depression, stress and anxiety), hypertension, an unhealthy diet lacking in daily fruit and vegetables, as well as physical inactivity and alcohol consumption.
This large scale international study suggested that these nine modifiable risk factors account for almost 90% of CVD cases. The possibility of reducing the risk for CVD in both men and women was hence highlighted as regular physical activity and smoking cessation in addition to a healthier eating habit within guidelines for preventing and reducing the risk for CVD. Presently, there is a vast amount of evidence based on specific eating habits recommended to help reduce the risk of developing CVD, in particular diets like the Mediterranean have been recommended as a much heart healthier alternative to those typically found in the West. A Mediterranean diet is well known for including large quantities of fibre cereals, fruits and vegetables, in addition to being low in saturated mammalian fats, dairy products and sodium, which overall result in a healthier heart and help to successfully lower the risk of developing CVD (Antonogeorgos et al., 2012).
Although CVD prevention programs require long-term adherence to a healthier lifestyle, recent surveys indicate a high prevalence of unhealthy eating habits and poor adherence to dietary recommendations amongst patients with established CVD (Euroaspire, I. I., 2001),
A prominent theory accounting for the resistance of patients with CVD to modifying their diet and adhering to healthier eating habits is the presence of depression. Bonnet and colleagues (2005) found that when depressive symptoms are present, even of modest intensity, they can create a great barrier for patients in making the necessary changes to their eating habits, and actually predict unhealthier diet choices such in the consumption of fatty foods and a lack of vegetables (Konttinen et al, 2010).
1.3. The association between diet and depression for CVD risk
Depression is a disorder that is associated with symptoms of sadness, low mood (anhedonia), loss of interest in normally pleasurable activities, as well as changes in sleep and diet. When depression occurs in the context of CVD, patients are noticed to experience difficulties in adhering the necessary lifestyle alterations required for managing and improving their condition, therefore resulting in poor dietary quality in comparison to their recommended guidelines (Pagoto et al., 2009; Euroaspire, I. I., 2001).
Poor dietary habits are well recognised in patients who suffer from depression (Kronish et al, 2012), for instance, when we are investigating the diet of depressed people, it is often noticed that not only is their nutritional intake far from adequate, but they also make poor food choices by selecting foods that might actually increase their likelihood of developing depression (Pagoto et al., 2009). Beydoun and colleagues (2009) recently examined depression as the exposure variable of interest and reported that an increase in subjects’ depressive symptoms was significantly associated with their likelihood of eating an unhealthy diet, thus suggesting unhealthy diets to be a common correlate of depression.
Dietary changes typically noted in depressed individuals include either a loss of appetite or an increased activity of eating which are suggested to be associated with other symptoms of depression such as low energy levels and poor motivation, as well as feelings of sadness and worthlessness. Low energy levels and poor motivation found in depressed individuals can potentially exacerbate the effects of poor eating habits, and this may be especially the case for older people who don’t find themselves with an interest in cooking or with the energy to prepare meals for themselves.
On the other hand, the feelings of sadness and worthless that accompany depression can result in emotional eating, also referred to as ‘comfort eating’ which describe a common event in which the need to eat is a response to emotional hunger (rather than physical hunger) (Dallman et al., 2003). This is a common event which involves eating in response to ones’ emotions in order to be soothed by food and the feeling of fullness and satiety it offers, which is found to be more comfortable than an empty stomach and thereby improving mood through positive association with happier times.
Dietary approaches for managing and reducing the risk for CVD and depression have repeatedly investigated the association between consumption levels of carbohydrates and these two conditions. Carbohydrates can be described as naturally occurring polysaccharides that are vital in structure and function of organisms, and in humans they are noted to affect mood and behaviour. Consumption of diets low in carbohydrates have been suggested to typically precipitate depression due to the production of serotonin and tryptophan which promote feeling of well-being being triggered by foods rich in carbohydrates (Rao et al., 2008).
Conversely, the associated risk of carbohydrate consumption with cardiovascular diseases risk is not as evidently established. Traditionally, diets high in carbohydrates were proposed to be associated with lower rates of CVD, with multiple control trials demonstrating that low-fat and high-carbohydrate diets lead to a reduction in risk of CVD (in addition to helping control of hypertension and preventing type 2 diabetes (major risk factors for CVD) (Grundyet al., 1968; Avenell et al., 2012). However, more recently this association has not been met with adequate support to suggest that this nutritional content may benefit CVD risk of reduce mortality rates (Howard et al., 2006). The most recent evidence instead suggest that it is the quality rather than quantity of carbohydrate that is more important and that low-fat, high-carbohydrate diets may lead to reductions in levels of high-density lipoprotein cholesterol (HDL) which are in fact associated with increased risk for CVD (Mensink & Katan, 1992).
Another popular and rising secular trend for the incidence of depression has been based on the increased levels of fat found in Western diets, these have been further associated with increased risk of coronary heart diseases, although lower levels of risk are reported in populations with reduced intakes of saturated and total fats (Astrup et al., 2011). Such findings can be interpreted via the rationale that the relative ratio by which different dietary fats are consumed is closely linked to the concentration of blood lipids HDL and LDL. High LDL and low HDL plasma concentrations are considered to cause deleterious effects on the cardiovascular system whereas cardioprotective effects are observed with high HDL and low LDL levels (Gaziano & Manson, 1996; Buttar, Li, & Ravi, 2005).
A systematic review by Hooper et al., (2001) based on 27 studies conducted on 30,909 individuals assessed the effects of dietary fat intake and prevention of CVD and suggested that reduction or modification of total fat, saturated fat and dietary cholesterol were associated with 16% reduction in cardiovascular events and 9% in cardiovascular mortality. In addition, randomised placebo-controlled dietary fat trials have reported up to 24% reduction in cardiovascular events. Based on these findings, it can be suggested that significant reductions in CVD can be achieved by continuous reduction of dietary fat and cholesterol intake or via modification of the proportions of both the nutritional components. People should also be encouraged to follow dietary reference intakes set by regulatory health authorities for fat forming only 20-35% of their daily energy in order to ensure a heart-conscious diet is being carried out (Buttar, Li & Ravi, 2005).
From summarising the information discussed so far, one may suggest that by modifying or altering unhealthy nutritional factors like, fat, carbohydrate and calorie intake, we may be able to significantly reduce the risk of depression and as a result have more positive effects on preventing the risk of developing CVD or other major heart problems in the future.
From our understanding, so far only a few studies have investigated the relationship between CVD and depressive symptoms with nutritional intake as a mediator, therefore nutritional content has been established as a consistent but poorly understood predictor of CVD risk (Barth et al, 2004; Vogelzangs et al., 2010) and it’s potential mechanism underlying the relationship between depression and CVD have not yet been widely studied.
1.4. Aims and Hypothesis
This research was set out to explore the relationship between depression and cardiovascular disease, with a particular focus on the specific eating habits and nutritional intake of individuals in order to identify the mediating factors between the two conditions. A mediation analysis was carried out using nutritional data from a randomised control trial of people identified at high risk of CVD in order to establish the existence of an indirect effect of diet, to look at whether this mediated the relationship between depression and CVD.
The main objective of this study was to ascertain whether the risk of CVD for individuals with depressive symptoms is mediated via their unhealthy diet, i.e. high fat, and calorie intake compared to low carbohydrate intake. In order to be able to test this, we had to establish whether: a) there is a causal effect of depression on CVD; b) whether depression has an effect on diet; c) whether diet has an effect on CVD, and d) whether the diet mediates the effect of depression on cardiovascular disease. In terms of diet, we examined the three most widely studied nutritional contents with regards to both depression and CVD, these were fat, calorie, and carbohydrates intake.
The overall goal of this study was to discern the population of people most at risk of developing CVD, in addition to identifying those who would benefit the most from dietary guideline recommendations and interventions like the Mediterranean diet. Our findings hold important implications in identifying a subgroup of people that respond well to various treatment or are more vulnerable to certain risk factors, as they can inform policies, interventions/prevention strategies.
H1: It is hypothesised that the relationship between depression and CVD is not direct but operates through an increase in unhealthy dietary consumption of fat and calories as well as a reduction in carbohydrate intake on a daily basis.