Nutritional Management

5. Nutritional Management

Stimulating Diuresis

The easiest way to produce undersaturated urine is to promote diuresis. Increasing urinary flow reduces the concentration of lithogenic substances, which outweighs the disadvantage of diluting crystallization inhibitors. High urine volumes also increase the frequency of urination, which helps remove any free crystals that form in the urinary tract (Borghi et al., 1999). To stimulate diuresis, drinking must be encouraged. This can be done either by feeding canned diets that contain 70 to 80% water, by adding water to the diet or by slightly increasing the sodium chloride content of dry diets. Increased dietary sodium chloride has been shown to increase water intake as well as urine production, and to decrease urine supersaturation in dogs and cats (Stevenson et al., 2003b, Lulich et al., 2005) (Figure 16).

Figure 16. The effect of dietary sodium content on calcium oxalate relative supersaturation and urine volume in miniature schnauzers (Stevenson et al., 2003b). Moderate levels of dietary sodium, which will promote diuresis, will not affect blood pressure in healthy dogs and those with moderate renal disease.

There has nonetheless been some controversy about the use of sodium chloride to stimulate thirst and diuresis, because it could potentially affect urinary calcium excretion and blood pressure (Lulich et al., 1999; Osborne et al., 2000).

In humans, high salt intake has been associated with increased urinary calcium excretion, and similar observations were initially made in dogs (Lulich et al., 1999; Devois et al., 2000; Biourge et al., 2001). This led to the assumption that salt-enriched diets could promote calcium oxalate urolithiasis and that diets for management of LUTD should thus be salt-restricted (Osborne et al., 2000; Allen et al., 2000).

However, later studies failed to observe an effect of dietary salt on urinary calcium excretion in dogs (Stevenson et al., 2003b). Epidemiological studies in dogs furthermore showed that increasing dietary sodium actually reduced the risk of calcium oxalate urolithiasis, because the dilutional effect of higher dietary sodium offsets the tendency towards hypercalciuria (Lekcharoensuk et al., 2001; 2002).

Several studies have shown that moderately increased dietary salt intakes (up to 3.2 g Na/1000 Kcal ME) do not alter blood pressure in healthy dogs, and dogs with induced renal disease (Greco et al., 1994; Biourge et al., 2002; Kirk, 2002; Burankarl et al., 2003; Luckschander et al., 2004).

Changing Urine pH

Changing urine pH, via dietary manipulation or medical means, can be very effective in the management of some but not all uroliths. Urine acidification markedly increases struvite solubility and is essential in the medical dissolution of these uroliths. In contrast, urine alkalinization is important in increasing the solubility of urate and cystine uroliths (Figure 15). Diet efficacy is generally increased if it also reduces urinary excretion of the crystalloids that participate in urolith formation (Lulich et al., 2000).

Most other urolith types are less amenable to dissolution based upon pH changes. It is furthermore advisable to aim for a urine pH that prevents further precipitation and potentiates excretion of other minerals that may co-precipitate or act as inhibitors.