Page 916 - Small Animal Clinical Nutrition 5th Edition

948 Small Animal Clinical Nutrition

there is recurrent disease in the face of feeding a dry struvite 0.14% DM). Individual foods intended to reduce the recur-
VetBooks.ir preventive food. rence of both struvite and calcium oxalate urolithiasis should
also be within this range.
Magnesium
Avoiding excess dietary magnesium intake can reduce urinary Phosphorus
concentration of magnesium, which decreases risk for struvite Varying dietary phosphorus levels can alter urinary phosphate
disease. In an epidemiologic study, risk of struvite uroliths was concentrations in cats, thereby influencing likelihood of urinary
increased in cats fed foods higher in magnesium (Lek- struvite precipitates. High-phosphorus foods have been associ-
charoensuk et al, 2001a). Excess magnesium is present in some ated with increased risk of struvite uroliths in cats (Lek-
commercial cat foods because they contain ingredients high in charoensuk et al, 2001a). The recommended range of dietary
magnesium (e.g., high-ash meat and bone, fish and poultry phosphorus for dissolving struvite uroliths is 0.45 to 1.1% DM
meals).The recommended range of dietary magnesium for dis- phosphorus; for struvite prevention the recommended range is
solving struvite uroliths in cats is 0.04 to 0.09% (dry matter 0.5 to 0.9% DM (Table 46-12).
[DM]). For preventing recurrence of struvite uroliths or ure- Urinary phosphate can exist in several states; anionic phos-
thral plugs, the recommended range of magnesium is 0.04 to phate (PO 4 -3 ) is the important form in precipitation and dis-
0.14% DM (Table 46-12). solution of struvite. Urinary concentration of anionic phosphate
Magnesium is considered to be an inhibitor of calcium is reversibly influenced by pH. (See Urinary pH below and Box
oxalate crystal formation. Potential mechanisms for this 46-3.) Thus, as urine becomes more acidic, anionic phosphate
inhibitory effect include increased urinary pH, increased uri- is converted to monobasic and dibasic phosphate, thereby
nary excretion of citrate and formation of magnesium oxalate reducing the concentration of anionic phosphate available for
complexes in urine, which are more soluble than calcium forming struvite precipitates. As urine becomes more alkaline,
oxalate. Formation of magnesium oxalate, in theory, reduces the the reaction proceeds in the opposite direction and concentra-
concentration of oxalate available for precipitation as calcium tion of anionic phosphate increases.
oxalate. Low urinary magnesium concentration has been sug- Compared with foods containing moderate amounts of
gested as a potential risk factor for formation of calcium-con- phosphorus, both low- and high-phosphorus foods are associ-
taining uroliths in cats (Lekcharoensuk et al, 2000, 2001). ated with increased risk of calcium oxalate uroliths in cats.
Clinical studies evaluating effectiveness of magnesium supple- Reduction in dietary phosphorus may cause activation of vita-
mentation on recurrence of calcium oxalate uroliths in people min D, which promotes intestinal absorption of calcium and
have yielded conflicting results (Johansson et al, 1980; Ettinger subsequent urinary calcium excretion. Rats fed a very low-
et al, 1988; Schwartz et al, 2001). Addition of magnesium was phosphorus food (0.07% DM) had marked hypercalciuria
associated with reduced calcium oxalate saturation in urine as (Werness et al, 1981). Feeding this level of phosphorus for one
demonstrated with in vitro studies using synthetic human urine week resulted in urine that was oversaturated with calcium
(Kohri et al, 1988). In vitro studies demonstrated that physio- oxalate and contained large amounts of calcium oxalate crystals.
logic concentrations of magnesium decreased rate of nucleation A possible explanation for increased risk of calcium oxalate
and growth of calcium oxalate crystals (Kohri et al, 1988; Li et uroliths associated with increased phosphorus intake is that
al, 1985). However, excessive dietary magnesium may result in excessive dietary phosphorus could form insoluble salts with
hypercalciuria, a risk factor for calcium oxalate urolithiasis dietary calcium, which in turn could increase availability of
(Fetner et al, 1978). noncomplexed oxalic acid for intestinal absorption and renal
An epidemiologic study of nutritional factors associated with excretion (Lekcharoensuk et al, 2001a).
urolithiasis in cats demonstrated that foods with the lowest The recommended range of dietary phosphorus for decreas-
magnesium content (0.04 to 0.07% DM; 0.09 to 0.18 mg/kcal) ing risk of calcium oxalate urolithiasis is 0.50 to 1.0% DM phos-
were associated with increased risk of forming calcium oxalate phorus (Table 46-12). The recommended range for the phos-
uroliths when compared with foods that had moderate magne- phorus content of foods intended for prevention of both struvite
sium content (0.08 to 0.14% DM; 0.19 to 0.35 mg/kcal) (Lek- and calcium oxalate should be between 0.5 and 0.9% DM.
charoensuk et al, 2001a). Foods with the highest magnesium
content (0.14 to 0.56% DM; 0.36 to 1.40 mg/kcal) also were Calcium
associated with increased risk of developing calcium oxalate Calcium availability from the gastrointestinal tract may be
uroliths when compared with foods containing moderate influenced by non-dietary and dietary factors. Intestinal
amounts of magnesium. In another study of cats with calcium absorption of calcium occurs primarily in the duodenum; trans-
oxalate uroliths, the mean magnesium content (0.19 mg/kcal) port of calcium across the gut is a saturable process that is vita-
of the food being fed at the time of urolith diagnosis was sim- min D-dependent. In general, calcium absorption from the
ilar to that of the magnesium content of a urolith-prevention intestinal tract is inversely proportional to dietary intake. In
e
food (0.2 mg/kcal), which significantly decreased urine calci- other words, absorption is high from low-calcium foods and
um oxalate saturation compared with the regular food (Lulich low from high-calcium foods. Other dietary factors (e.g., vita-
et al, 2004).To minimize risk of calcium oxalate uroliths, foods min D, sucrose, fructose, glucose, xylose, dietary fiber, oxalic
should contain a moderate amount of magnesium (0.07 to acid, phytic acid, protein and phosphorus) reportedly affect cal-