Sodium - Nutrition
What is Sodium?
Sodium is the principal cation (positive ion) of the extracellular fluids, where it is usually in combination with the chloride anion (negative ion) forming sodium chloride (common salt). Sodium does not exist in its free state in nature but is combined with other elements to form salts.
Why is it Important?
Sodium is indispensable for the regulation of acid-base balance and osmotic pressure. Its other main role is maintaining the membrane potential in nerve and muscle tissues thereby facilitating the generation and transmission of electrical impulses.
Roles in the Body
Sodium is distributed in bone (43%) interstitial fluid (29%) and plasma (12%). The concentration of sodium (i.e., sodium chloride) in blood is approximately 0.9%. The main hormone involved in sodium balance is aldosterone, which is released by the adrenal glands in response to a low plasma sodium concentration. Aldosterone acts by increasing the renal retention of sodium. The high level of sodium in extracellular fluids is responsible for establishing a concentration gradient across cell membranes. Sodium attracts water molecules when in solution, and the size of the hydrated sodium ion prevents it crossing cell membranes as easily as other ions such as potassium and chloride. This means that the permeability of the membrane to sodium is generally very low but when there is a momentary change in the membrane’s permeability, sodium is driven into the cell and this dramatically alters the membrane potential from negative to positive. This change – the action potential – is responsible for nervous activity. The extra sodium in the cell is then removed by active transport via a sodium pump, which is energy dependent, and this restores the system to its resting state.
Consequences of Sodium Deficiency
Dog:
As with potassium, naturally-occurring sodium deficiency is rare in dogs but when fed diets very low in salt content adverse effects have been observed.
Recognised Syndromes Related to Sodium Deficiency
The main clinical findings associated with sodium deficiency include increases in heart rate, water intake, urinary water output, haemoconcentration, restlessness, and dry and tacky mucous membranes. These effects were observed in adult beagles fed a diet with a dietary sodium level of 0.016% dry matter (DM)[1]. In contrast, adult German shepherd dogs showed no adverse signs when fed a diet that that was marginally higher in sodium (approximately 22%)[2].
Cat:
The effects of sodium deficiency in cats are broadly similar to those seen in dogs fed diets with similar sodium concentrations. These are confined to studies using purified diets in which the sodium content is deliberately decreased.
Recognised Syndromes Related to Sodium Deficiency
Clinical findings of sodium deficiency were observed in 12- to 15-week old kittens fed a diet with a sodium content of 0.01% DM and included anorexia, excessive thirst, increases in haemoglobin concentration, plasma aldosterone and urine volume, and impaired growth. When dietary sodium was increased to 0.2% DM, normal health was restored[3].
Toxicity
Dog:
The main adverse effect of excessive sodium intake is an increased potassium excretion resulting in a negative potassium balance. A sodium content of 2.9% DM fed to adult dogs caused vomiting and food aversion[4]. However, given an adequate supply of water, dogs can cope with a wide range of sodium intakes and dietary levels up to 1.5% DM are deemed acceptable[5].
Cat:
There is little if any information on the adverse effects of high sodium intakes in cats. Yu et al.[6] reported that, although kittens showed an aversion to a diet containing sodium at 1% DM when given a choice, they consumed it in normal amounts with no adverse effects when it was the only diet available. Burger[7] reported that adult cats receiving dietary sodium up to 1.5% DM showed no abnormalities. These values are similar to those reported for dogs and, as with dogs, assume adequate amounts of water are available.
Dietary Sources
Although sodium is found widely in nature, the main sources are derived from animal materials such as meat meal, meat and bone meal, fish meal and blood meal. These will generally be sufficient to supply the nutritional requirement in dog and cat foods. Nevertheless, there are several mineral salts that are commonly used to supply sodium such as the sodium salts of chloride, carbonate, bicarbonate, phosphate and sulphate.
References
- ↑ Drochner, W, Kersten, U, Meyer, H (1976). “Auswirkungen einer Na-depletion und anschlie Benden Repletion auf den stoffwechsel von beaglehunden”. J. Vet. Med. Series 23A:739-753.
- ↑ Morris, M, Patton, R, Teeter, S (1976). “Low sodium diet in heart disease: How low is low?”. Vet. Med. 71:1225-1227.
- ↑ Yu, S, Morris, J (1997). “The minimum sodium requirement of growing kittens defined on the basis of plasma aldosterone concentration”. J. Nutr. 127:494-501.
- ↑ Zentek, J, Meyer, H (1995). “Normal handling of diets – Are all dogs created equal?”. J. Sm. Anim. Pract. 36:354-359.
- ↑ Boemke, W, Palm, U, Kaczmarczyk, G, Reinhardt, H (1990). “Effect of high sodium and high water intake on 24-h potassium balance in dogs”. J. Exper. Anim. Sci. 33:179-185.
- ↑ Yu, S, Rogers, Q, Morris, J (1997). “Absence of salt (NaCl) preference or appetite in sodium-replete or depleted kittens”. Appetite 29:1-10.
- ↑ Burger, I (1979). “Water balance in the dog and cat”. Pedigree Digest 6:10-11.