Salt – the myths v the facts
Salt, or sodium chloride, is an essential mineral for the maintenance of pressure within the cells and in determining extracellular fluid volume. Sodium also has a vital role in energy metabolism and enables the transmission of nerve impulses and muscular contractions. Chlorine is essential for acid-base balance and has a role in digestion, as hydrochloric acid in gastric contents.
Salt homeostasis in companion animals is an extremely well regulated system and the risk of dietary salt toxicosis is highly unlikely when fresh clean drinking water is provided. Acute sodium toxicity cases have been reported- due to ingestion of toxic sources, not from diet2. Sodium deficiency however, has been reported due to dietary content, and the National Research Council (NRC) do specify a minimum recommended amount5. The level of sodium within the diet has been used in certain conditions very effectively to promote water intake and diuresis.
Recently there have been reports regarding the possibility of salt manipulation in prescription diets having adverse effects on patients, particularly those with comorbidities. Concerns include the link between hypertension and salt intake in some species, and also the risk of renal or cardiac disease progression in patients with concurrent or early-stage disease.
Salt in dietary management of LUTD
Lower Urinary Tract Disease (LUTD – Dog and Cat) includes cystitis, urolithiasis and urethral obstruction and affects both dogs and cats. Following an incident of LUTD, increasing water intake will be part of the treatment plan, regardless of diagnosis. Flavouring the water and use of drinking fountains can help, but often dietary management will be more effective.
For cats in particular, dietary management is the mainstay in dissolution of struvite stones and avoidance of recurrence of both struvite and oxalate stones 2. Struvite stones in dogs are linked to bacterial cystitis, and treatment for this in combination with dietary modification is necessary.
Dietary management aims to both decrease the pH of the urine and increase volume and dilution. Increasing water intake by the use of wet food or by increasing sodium content is effective management in all lower urinary tract disease6. Calcium oxalate prevention in particular is via dilution of the urine to prevent saturation of solutes.
Increased sodium within the food has been found to be an effective and safe method for urinary dilution, provided free access to drinking water is maintained2. Concerns regarding increasing calcium excretion and therefore a higher risk of calcium oxalate crystals have so far been disproven in studies2.
Ingestion of salt in humans can increase the risk of hypertension, however there is no strong evidence to extend this risk to dogs and cats.
Organ impairment can affect sodium regulation, which has given rise to concerns regarding the increase in salt in Urinary S/O diets and their suitability in cardiac and renal cases.
Sustained hypertension in humans has been associated with damage to organs including the eyes, brain and heart2. Hypertension is linked to sodium ingestion with increasing sodium levels expanding extracellular fluid, and thereby increasing peripheral blood pressure2.
This relationship between dietary salt intake and blood pressure cannot be extrapolated to companion animals2.
Control of blood pressure is achieved by a combination of neural, hormonal and intrinsic factors involving multiple organs8. In a healthy individual with free access to drinking water, sodium excretion will adjust rapidly to match intake. Serum sodium levels remain relatively constant2. This regulation relies on the Renin-Angiotensin-Aldosterone system (RAAS), a system vital also in cardiac and vascular function. This system can be compromised in individuals with renal, cardiac or adrenal insufficiency7.
Cardiovascular disease
Sodium restriction in cases of congestive heart failure (CHF) has been a historic recommendation based on human medicine4,7. Several studies have since assessed low sodium diets in early CHF and the concern that the RAAS is already activated in early congestive heart failure7. Dietary sodium restriction may exacerbate the effect on this neuroendocrine system4,7. The long term consequences of this stimulation are not fully understood but increasing levels of aldosterone due to RAAS activation have been associated with an inflammatory response and oxidative stress2.
Moderate salt restriction at this stage is preferable4.
Medications commonly used to ease preload (including ACE inhibitors and furosemide, prescribed when clinical signs of CHF are seen) may risk electrolyte disturbances developing including hyponatraemia and hyperkalaemia7. Low sodium diets should be used only when prescribed and under veterinary supervision.
For animals with cardiovascular disease and hypertension, current recommendations are to avoid high dietary salt content, but not to restrict2. Moderate rather than low levels of salt in the diet are recommended particularly in early stages of cardiac disease2,4,7.
Renal disease
Renal disease will frequently result in hypertension due to decreased ability to excrete sodium in the distal tubules8. A moderate to high dietary salt content has historically been contraindicated, as the sodium excretion mechanism is impaired and will aggravate the hypertension1. However, a study on both normal and renally-impaired cats saw no changes in arterial blood pressure associated with sodium content1. The low sodium diet given in this study was associated with the lowest glomerular filtration rate and significant hypokalaemia- indicating further research is required to determine whether a low sodium diet is in fact deleterious and contraindicated in renal disease1.
Reynolds et al6 looked at healthy ageing cats over a two-year period and found an increase in sodium content in the food had no adverse effect on renal function or blood pressure. CKD was not detected, even by early indicators such as proteinuria and plasma parathyroid hormone. Measurements taken also included blood urea nitrogen and creatinine, urine specific gravity and systolic blood pressure. This study found no evidence to suggest a moderate salt content causes kidney impairment.
This is also consistent with previous findings in studies over a shorter time period9.
Food safety regulation
Safety standards in pet foods encompass deficiencies and excesses of nutrients, not simply contamination risk. Recommended amounts of nutrients such as salt have been outlined by the National Research Council (NRC)5. Salt homeostasis does rely on efficient renal and cardiac function and must also be considered in light of free access to drinking water.
Salt in excess can have potentially negative consequences on the cardiovascular system in humans but it is important to note that the results of studies have been inconsistent and suggest sensitivity to salt varies between individuals and over time3. It does not follow that sodium must be severely restricted in cases of cardiac or renal failure in our companion animals- further research on these species are required to determine the optimal nutritional requirements.
Contrary to humans, salt does not affect blood pressure in healthy cats and dogs3.
All Royal Canin Veterinary Urinary S/O products fall within the safe upper limit for sodium in dogs and cats as recommended by the NRC5. All diets must be offered alongside free access to drinking water.
These diets are prescribed by a veterinarian in charge of the case and the recommendation is to seek veterinary advice in cases of renal or cardiac impairment.
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References;
- Buranakarl, C., et al (2004) Effects of dietary sodium chloride intake on renal function and blood pressure in cats with normal and reduced renal function. American Journal of Veterinary Research
- Chandler, M (2008) Pet Food Safety, Sodium in Pet Foods Topics in Companion Animal Medicine 23:148-153
- Chetboul, V., et al (2014) Cardiovascular effects of Dietary Salt Intake In Aged and Healthy Cats: A 2-year Prospective Randomized, Blinded and Controlled Study Plosone Open access
- Freeman, L.M. (2006) Effects of dietary modification in dogs with early chronic valvular disease Journal of Veterinary Internal Medicine 20:1116-1126
- National Research Council (2006) Nutrient Requirements of Dogs and Cats. Washington DC: The National Academies Press. Available [online] https://doi.org/10.17226/10668
- Reynolds, BS et al (2013) Effects of dietary salt intake on renal function: a 2-year study in healthy aged cats J Vet Internal Medicine 27:507-515
- Rush, J.E. et al (2000) Clinical, Echocardiographic, and neurohormonal Effects of a Sodium restricted Diet in Dogs with Heart Failure Journal of Veterinary Internal Medicine 14:513-520
- Syme, H (2011) Hypertension in Small Animal Kidney Disease Veterinary Clincis:Small Animal Practice available [online] https://doi.org/10.1016/j.cvsm.2010.11.002
- Xu, H et al (2009) Effects of dietary sodium chloride on health parameters on mature cats Journal of Feline Medicine and Surgery 11:435-441
Postdate: 13th may 2020