Salt has two constituents – namely sodium and chloride – both of which are crucial for the maintenance of body’s fluid balance and blood volume, as well the functioning of nervous tissues. Deficiencies in sodium and chloride result in problems with nervous signal transmission, low blood pressure, restlessness, increased heart rate and pasty or thick mucus.
Research has indicated a diet low in sodium can induce reductions in blood pressure regulating hormones during prolonged sodium deficiency, fatigue , exhaustion , inability to maintain water balance, decreased water intake, retarded growth, and dryness of skin and loss of hair.
Adult dogs require a daily sodium intake of around 13 mg/kg body weight, which corresponds to a minimum dietary level of 0.2 g/1000 kcal. Requirements are 2-3 times higher in puppies and during pregnancy and lactation, and five times higher in very highly active dogs such as greyhounds or sled dogs.
The sodium content of dog foods is in fact similar to that of prey consumed by dogs in the wild, including small deer, rabbits and chickens, which contain between 2.5 and 10 g/kg of dry matter. By contrast, cereals, fruits and vegetables are low in sodium and typically contain less than 1g/kg DM (around 200mg/1000 kcal). Thus dogs as semi-carnivores have evolved to tolerate high levels of dietary sodium. Likewise they show limited ability to detect dietary sodium levels and do not use salt as a driver of food selection and consumption. Omnivores such as man respond to dietary sodium, presumably to enable selection of foods with adequate sodium levels for health.
Commercially available dog foods provide intakes of sodium that are comfortably in excess of minimum requirements and typically have between 0.5 and 2.5 g/1000 kcal or 2-10 g/kg of dry matter. Studies on the sodium requirements of dogs have shown a wide range of tolerance. The minimum requirement for health in adult dogs is 200 mg/1000 kcal and the maximum is approximately 4 g/1000 kcal. Mars pet-foods (Pedigree) are formulated within the Waltham guidelines, which define an even safer range of 0.5 to 3 g/1000 kcal. Dry foods tend to contain less sodium than wet formats, including canned, tray and pouch products, because they contain fewer meat products that are naturally rich in sodium. Sodium levels are similar across brands, with no significant differences between mainstream and premium products in either wet or dry formats.
Processed human foods that are frequently offered to dogs in the form of table scraps – such as bacon, sausages and cheese – have sodium levels well in excess of those of dog foods and hence should be avoided.
Healthy dogs are perfectly tolerant to large amounts of dietary sodium and adapt well to substantial fluctuations in intake. Adverse signs are seen only once intakes are more than twice those found in even the most sodium-rich of dog foods. The recommended upper limit, which includes a margin of safety, is currently set at 15 g/kg dry matter.
There is no evidence that sustained high levels of salt intake in dogs are linked with high blood pressure, renal failure or coronary heart disease in dogs, whereas high salt intakes are implicated in the aetiology of all these diseases in humans. Furthermore, excessive salt intakes do not contribute to disease progression in dogs with either kidney or heart failure.
In fact, increasing dietary salt levels within the NRC range, may have benefits including the risk of calcium oxalate stone formation. Studies have shown that the increased dietary sodium promotes the formation of dilute urine with no net increase in calcium concentrations.
(This article is contributed by Mars India International, with inputs from Dr Tim Watson BVM&S, PhD, MRCVS, Townhead of Aber, Gartocharn, Dunbartonshire, G83 8NQ)