**2.1. Fluid intake**

Metaphylaxis of urinary stones may primarily involve regulating fluid intake and diet. Although hydration has been shown to decrease stone recurrence, the effectiveness of nutri‐ tion is controversial [19]. Increased fluid intake increases urine volume and inhibits crystal supersaturation and crystallization. In children, the fluid intake required for adequate uri‐ nary output must be calculated over 1.5 l/m2 of body area [7]. Sweet‐flavored liquids should be avoided, since fluid containing glucose and fructose increase excretion of calcium and oxalate [20]. In addition, fluid intake must be distributed over the entire day. Consequently, in metaphylaxis of stones, water intake is the approach that has the optimal cost‐benefit ratio [21]. However, it is not possible to monitor the fluid intake of children in school, and chil‐ dren's compliance in hydration is poor in general [22]. Because the need for liquids varies with temperature and activity level, parents may be recommended to monitor hydration based on urine color and urine density, if possible. Urine densities that repeatedly measure higher than 1010 indicate inadequate fluid intake [22].

#### **2.2. Nutrition**

Eating fast foods and processed foods can potentiate increased salt intake, causing hyper‐ volemia, which leads to decreased absorption of water and sodium through the proximal tubules. This, in turn, decreases the absorption of calcium by sodium in the proximal tubules, which increases the level of calcium in the urine. The concentration of sodium in the urine is in proportion to dietary salt intake. In turn, dietary salt intake is in direct proportion to the calcium excreted in the urine [23]. In a Western‐type diet, salt intake is almost 9 g/day in adults, nearly 77% of which comes from processed foods [24]. In contrast, daily salt intake should be below 3 mEq/kg [25]. Therefore, children's consumption of fast foods should be limited to decrease dietary salt intake.

Only 10–20% of oxalate in the urine is associated with diet [26]. However, oxalate absorption through the bowels is related to dietary calcium intake [27], and restricting dietary calcium increases urinary excretion of oxalate. However, restricting salt decreases oxalate, and more significantly, the calcium excreted in the urine of those with calcium‐oxalate stones [28].

Oxalate is the product of vitamin C metabolism and is excreted in urine [29]. Therefore, increased vitamin C intake may lead to hyperoxaluria. Children with hyperoxaluria should decrease consumption of high‐oxalate foods [6]. In addition, oxalobacter formigenes, a probi‐ otic, decreases urinary oxalate and is effective against hyperoxaluria [30].

Animal proteins increase the acid load, the excretion of calcium, and decrease the excretion of citrate [25]. In addition, purine metabolism increases uric acid [25], which increases the risk of calcium stones through epistaxis [31]. Despite all this, the protein restrictions recommended for adults are not suitable as part of stone metaphylaxis in children and adolescents, except in cases of definite indications [6].

Urinary excretion of citrate is affected by the system's acid‐base balance. Although acidosis decreases renal excretion of citrate and increases its reabsorption, the opposite is true in alka‐ losis [32]. A Western‐type diet that includes decreased consumption of fruits and vegetables and increased consumption of animal products causes metabolic acidosis, resulting in lower urine pH and hypocitraturia [33]. Hypokalemia also lowers urine pH, and low potassium intake decreases urine potassium and citrate in hypokalemia and increases urinary excretion of calcium [32, 34]. Systemic alkalization increases excretion of citrate, decreases excretion of calcium, and raises urine pH [35]. Oranges, lemons, limes, and some types of mandarins are natural sources of citrate [35]. Alkaline fruits, including melons, cause urinary alkalization [36]. Grapefruits increase excretion of both citrate and oxalate [37]. The lithogenic effects of grapefruit juice and apple juice are controversial [38, 39].

Metaphylaxis benefits may be provided by increasing hydration and citrus intake and decreasing intake of sodium, oxalate, animal protein, and fructose [32]. Cranberry juice is high in oxalate and, therefore, increases urinary calcium and oxalate and decreases uric acid concentration. However, cranberry juice acidifies the urine, resulting in increased formation of calcium‐oxalate and urate stones but decreased formation of calcium‐phosphate stones [40]. Since cranberry juice acidifies the urine, it may be useful for infection stones that have limited options for medical treatment [41].

A study of 42,859 adults showed that high coffee and tea intake decreased the risk of symp‐ tomatic stone formation [39]. A more recent study of 6033 adults suggested that coffee intake decreased urine oxalate and uric acid, increased urine calcium and potassium, and also decreased the supersaturation of calcium oxalate by increasing urine volume [42]. However, dietary intake of coffee and tea cannot be recommended for pediatric patients because of the lack of studies of these substances in children.

Potential renal acid load (PRAL) is used to calculate the acid load of foods in adults. However, renal net acid excretion (NAE), which is based on body area, is recommended for use in pediat‐ ric patients [43]. PRAL is calculated by the formula (mEq/d) = 0.49 × protein (g/d) + 0.037 × phos‐ phorus (mg/d) − 0.021 × potassium (mg/d) − 0.026 × magnesium (mg/d) − 0.013 × calcium (mg/d) [43]. As the formula indicates, dietary protein and phosphorus have acidic effects, whereas potassium, magnesium, and calcium have alkaline effects.
