Heart disease is a major cause of morbidity and mortality in cats and dogs, with an estimated 10 percent of dogs suffering from some type of heart disease (Atkins et al., 2009). Myxomatous mitral valve disease (MMVD) is the most prevalent heart disease worldwide, accounting for 75 percent of heart disease in dogs, while dilated cardiomyopathy (DCM) is recognised as the second most common canine heart disease (Keene et al., 2019; McCauley et al., 2020). Cardiomyopathies are common in cats, and cardiovascular disease is among the 10 most common causes of death in the species. The true prevalence of heart disease in cats is unknown, but recent studies show that the prevalence of hypertrophic cardiomyopathy (HCM) reaches 15 percent of the feline population (Luis Fuentes et al., 2020).
Prevalence of cardiac disease
MMVD (also known as endocardiosis) is more common in small dog breeds. It has a strong genetic predisposition in certain breeds, including the Cavalier King Charles Spaniel, in which the prevalence of this anomaly reaches 90 percent in individuals over 10 years of age (Borgarelli et al., 2004; Chetboul et al., 2004). The prevalence of MMVD increases with advanced age in small-breed dogs: up to 85 percent show evidence of valve damage at the age of 13 years, and it is about 1.5 times more common in males than females (Keene et al., 2019). The disease develops into four stages labelled A to D according to the presence or absence of symptoms, signs of cardiac remodelling and response to treatment (Figure 1) (Keene et al., 2019). The subclinical phase is further divided into stages B1 and B2. The latter is defined as those with no clinical signs of heart failure but where structural heart enlargement puts them at greater risk of progression.
In addition to MMVD, DCM is also of significant clinical relevance in dogs. This heart disease leads to the progressive enlargement of the left ventricle and results in heart failure (Tidholm and Jönsson, 2005). Typically, DCM is a disease that affects large and giant breeds and is very common in Deerhounds, Dobermanns, Irish Wolfhounds, Great Danes, Boxers and Newfoundlands. It can, however, occasionally be found in medium breeds such as English and American Cocker Spaniels. Prevalence increases with advanced age, affecting mainly middle-aged to elderly dogs. As with MMVD, there is a predisposition in male dogs (Dukes-McEwan et al., 2003). In addition to genetic predisposition, other factors such as nutritional deficiencies, metabolic disorders, concurrent diseases, infections and toxins can cause DCM in otherwise unaffected breeds (McCauley et al., 2020).
Heart disease is a concern not only for dogs but also for cats. Feline hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. Although it occurs in all breeds, there is a known genetic mutation in some, such as Maine Coon, Persian, Ragdoll, British Shorthair, Sphynx, Chartreux, and others. It is also about three times more common in male cats. Larger cats and those with a higher body condition score are more affected, and prevalence increases with advanced age to almost 30 percent at the age of nine years (Payne et al., 2015).
The importance of nutrition in the management of cardiac disease
Treatment of heart disease in dogs and cats varies depending on the type, underlying cause and severity. In addition to drug treatment, nutritional support can contribute to the management of several conditions involved in heart disease, such as vascular balance, cardiac cachexia and electrolyte levels. It also plays a role in the additional support of cardiac contractility and the fight against free radical species derived from oxidative stress. Such nutritional support can also complement inflammation reduction (Freeman and Rush, 2007).
Protein restriction could be deleterious, as cardiac patients are predisposed to the loss of lean body mass
The loss of lean muscle mass, also known as cardiac cachexia, affects many dogs with congestive heart failure, especially those with DCM (Freeman and Rush, 2007), and can also be seen in cats with cardiomyopathy, mainly in stage D of the disease (Luis Fuentes et al., 2020). Such weight loss is due to many factors and results in the loss of lean muscle mass rather than body fat reserves. Cardiac cachexia is a poor prognostic indicator and a predictor for disease progression. There is no evidence that it is necessary to restrict protein intake in dogs and cats diagnosed with congestive heart failure. Furthermore, protein restriction could be deleterious, as cardiac patients are predisposed to the loss of lean body mass (Freeman et al., 2006).
Nutrients and cardiac health
One of the nutrients of particular importance in the management of heart disease is sodium, as heart disease results in the activation of the renin–angiotensin–aldosterone system and abnormal sodium excretion. Dietary sodium can lead to increased fluid retention (oedema and effusion, such as ascites) in dogs with heart failure. Low-to-moderate dietary sodium intake reduces sodium and water retention, thus reducing congestion. In dogs with chronic heart failure (secondary to MMVD or DCM), dietary sodium restriction has been shown to have beneficial effects in reducing cardiac size parameters (Rush et al., 2000) when compared to moderate sodium intakes.
It is unnecessary to institute severe sodium restriction in the early stages when a heart murmur is first detected or before clinical signs are present (Freeman et al., 1998), although owners should be counselled to avoid high-sodium diets, treats and table foods. However, as heart disease progresses and cardiac heart failure ensues, sodium restriction becomes more important, though newer and more effective medications have diminished the need for severe sodium restriction in many patients.
The appropriate time to institute sodium and chloride restrictions and the optimal level of restriction at each stage of heart disease in small animals is still unclear. Currently, the author does not recommend severe sodium restriction for animals with cardiac disease without chronic heart failure, such as dogs with chronic valvular disease displaying no clinical signs or asymptomatic cats with hypertrophic cardiomyopathy. Instead, the author recommends counselling the owner to avoid high-sodium diets (under 0.40 percent dry matter), treats and table food. When heart failure first arises, additional sodium and chloride restriction is recommended (sodium under 0.30 percent dry matter for cats and dogs). Often, this can be accomplished with a diet designed for “senior” pets; however, it is important to examine each individual diet to make sure it meets this restriction.
Severe sodium restriction in dogs with early chronic valvular disease can lead to a marked early activation of the renin–angiotensin–aldosterone system, thereby accelerating the progression of heart disease
It is advisable to start with mild to moderate sodium restriction in patients with MMVD at stages B2 and C, and further restrict sodium intake in patients with MMVD at stage D to treat refractory fluid retention (Keene et al., 2019). As previously stated, a less severe level of sodium restriction is recommended in the early stages of MMVD. This is because severe sodium restriction in dogs with early chronic valvular disease can lead to a marked early activation of the renin–angiotensin–aldosterone system, thereby accelerating the progression of heart disease.
Effect on renal function
There has been some concern about the impact of dietary sodium restriction on renal function. However, research demonstrates that dietary sodium restriction does not cause deterioration of kidney function or a deleterious effect on blood pressure in dogs, even with moderate renal impairment (Greco et al., 1994). Thus, sodium restriction is thought to be impactful only in dogs with overt renal failure.
Magnesium and potassium balance
The electrolytes magnesium and potassium can become unbalanced after treatment for heart disease. Furosemide, a loop diuretic used routinely to treat congestive heart failure, can cause loss of magnesium through urine and hypomagnesaemia, which in turn can result in undesirable symptoms. Using diuretics can cause potassium levels to rise or fall in the bloodstream, and abnormalities can cause arrhythmias. For this reason, it is recommended to provide adequate magnesium, modulate potassium concentration and monitor electrolyte levels when diuretics are used (Roudebush et al., 1994; Pedersen and Mow, 1998; Keene et al., 2019).
Other important nutrients
In addition to providing optimal concentrations of nutrients such as proteins, sodium, magnesium and potassium, an increased intake of certain nutrients, including taurine, arginine, carnitine, omega-3 fatty acids and antioxidants, may be beneficial for animals with cardiac disease (Freeman et al., 2006).
Taurine is essential to the contractile strength of the heart muscle. Unlike cats, dogs can synthesise taurine from cysteine and methionine, which they get from their diet (Satoh and Sperelakis, 1998). Despite this, taurine deficiency is responsible for some cases of dilated cardiomyopathy, particularly in certain breeds (Kittleson et al., 1997). There is a correlation between low-protein vegetarian diets and, possibly, grain-free diets in dilated cardiomyopathies related to taurine deficiency (Sanderson et al., 2001; Tôrres et al., 2003). Due to its relevance to cardiac disease, canine cardiac diets should be supplemented to ensure an adequate supply of taurine. For cats with a DCM phenotype, enquiries about dietary history and measurement of plasma taurine concentrations are recommended, with supplementation and dietary change as necessary (Luis Fuentes et al., 2020).
Carnitine is found in high concentrations in the heart and skeletal muscle. Although carnitine is synthesised by the body and, therefore, not considered nutritionally essential under most conditions, it is required for fat oxidation in the heart and helps ensure the supply of energy to the heart cells. In certain breeds, including Boxers and Doberman Pinschers, carnitine deficiency can occur and can be associated with cardiomyopathy (which is managed with carnitine supplementation). Carnitine can also contribute to improving the contractile strength of the heart. For this reason, cardiac diets should be formulated with adequate carnitine supplementation (Freeman et al., 1998).
Arginine, an essential amino acid for dogs and cats, is a precursor of nitric oxide (NO). NO is crucial for normal cardiovascular function, contributing to the maintenance of normal vascular tone and serving as a physiological messenger (Bocchi et al., 2000). Small amounts of NO are synthesised by the enzyme NO-synthase (NOS) and released by the endothelial cells that form the inner lining of blood vessels. In humans, the administration of L-arginine may be beneficial (Rector et al., 1996), but there is still no evidence in dogs. Nevertheless, arginine supplementation is considered beneficial in canine cardiac diets (Freeman et al., 2006).
Fatty acids and free radicals
Eicosapentaenoic acid and docosahexaenoic acid are both omega-3 fatty acids found in fish and algae oil. They are potent anti-inflammatory modulators that have effects on many tissues.
In humans, antioxidants have displayed protective effects on heart conditions, and their use is also recommended for the formulation of cardiac diets in pets
In general, heart disease has an inflammatory component, and this increased inflammation increases oxidative stress, giving rise to free radicals. Free radicals have negative inotropic properties, which means they reduce cardiac contractility and contribute to the progression of heart disease (Freeman et al., 2014, 2017; van Hoek et al., 2006, 2020). In humans, antioxidants have displayed protective effects on heart conditions (Urquiaga and Leighton, 2000), and their use is also recommended for the formulation of cardiac diets in pets.
As discussed above, specific nutrients play an important role in dogs and cats with heart disease and should be regarded as a crucial complement in their management. However, you must choose an appropriate diet for each patient according to their clinical signs and stage of heart disease.