Dietary style and acid load in an Italian population of calcium kidney stone formers
Introduction
Dietary style plays an important role in kidney stone formation [1] and may explain the association of calcium nephrolithiasis with the metabolic syndrome, obesity and cardiovascular disease [2], [3]. Intake of fluids, sodium and animal proteins have been implicated in the development of calcium kidney stones [4]. Therefore, an increase in fluid intake and a reduction of sodium and animal proteins are usually recommended to patients to prevent calcium stone recurrence [5]. Animal protein intake leads to acid production, mainly through the metabolism of sulfuric acid from methionine and cysteine [4], [6]. This may predispose individuals to stones through a tubular effect that enhances calcium excretion and decreases citrate excretion, thus acting on promoters and inhibitors of calcium salt precipitation. In addition, acid load may enhance calcium excretion by stimulating calcium delivery from bones [7], [8], [9]. Despite these findings, the relationship among diet with kidney stones remains controversial. Recent epidemiological surveys confirmed the association between nephrolithiasis and the dietary acid load [10], but no association with animal protein intake [10], [11]. Furthermore, a short-term intervention study in healthy volunteers suggested that factors other than acid load explained the effect of a high-protein diet on calcium excretion [12].
Besides acid load, net acid excretion is influenced by dietary alkali content. Vegetables supply base-forming constituents that may counterbalance the acidifying effect of animal proteins [13], [14]. The possible positive effect of vegetables on stone risk was observed in association studies [11] that also observed a positive correlation between vegetable intake and citrate excretion [15].
Dietary acid load may be calculated with a previously validated algorithm including both acids and bases generated from the diet. The algorithm calculates the potential renal acid load (PRAL) from the intestinal absorption rates of ingested protein, phosphate, magnesium, potassium and calcium [16]. Meat and cheese are the main source of dietary acids and therefore significantly increase PRAL whereas other dairy products like milk and yogurt do not significantly modify PRAL and vegetable foods move it to negative values. A recent study tested this algorithm in stone formers and showed an increased dietary acid load [10].
The present study estimated PRAL in calcium stone formers to analyze the contribution of dietary acid load and animal protein intake to the stone risk. It also investigated their contribution to the excretion of calcium and citrate.
Section snippets
Methods
One hundred and fifty seven stone formers and 144 controls of Italian origin were enrolled (Table 1). Stone formers were recruited at the outpatient stone clinics of the San Raffaele or San Paolo Hospitals in Milan since 2002 to 2012. Patients had a history of urinary emission or surgical removal of calcium-oxalate or -phosphate stones or had a radiologic examination positive for radiopaque stones. Participants to the study did not take drugs affecting calcium and electrolyte metabolism.
Stone formers vs controls
Stone formers had produced the average of 4 stone in their clinical history (range 1–70). Their diet was characterized by a decreased intake of sugars, potassium and fiber in comparison with controls (Table 1). Among sugars, the intake of glucose (3.3 ± 2.45 vs 4.1 ± 2.82 mg/kcal per day, p = 0.007) and fructose (4.9 ± 3.9 vs 6.2 ± 4.67 mg/kcal per day, p = 0.012) was decreased in stone formers, whereas the intake of sucrose (11.5 ± 5.99 vs 11.9 ± 6.36 mg/kcal per day) was not different from
Discussion
The present study found that dietary acid load was increased in stone formers. Net acid load was calculated as PRAL with a standard formula that estimated bases and acids provided by diet [16]. Besides acid load, the intake of potassium, fiber, glucose and fructose was decreased in stone formers. On the contrary, animal protein intake was not different in stone formers and controls. These findings suggest that a low ingestion of vegetables and fruits, that are foods rich of bases, potassium,
Funding
This study was supported by grants from Italian Ministry of University and Research as part of a project about the prevention of kidney stones (PRIN 2005063822).
Disclosures
The authors declare no conflicts of interest.
Acknowledgment
We thank Giovanni Carpani (Blood Transfusion Centre, San Paolo Hospital, Milan, Italy) and Piera Bellinzoni (Department of Urology, San Raffaele Turro, Milan, Italy) who addressed normal subjects and patients to the study. The authors' responsibilities were as follows — GV and LS: study design, statistical analysis and manuscript preparation; ED: study design, data collection, manuscript preparation and data entry; TA, MT, LM, AM, CB, AN and FP: patient management and data collection; TM: study
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Giuseppe Vezzoli and Elena Dogliotti contributed to this work equally and share the role of the first Author.