Nephrolithiasis or kidney stones, as it is popularly known, consist of a common medical condition influenced by several factors, including diet and nutrition. Since nutritional habits play a relevant role in the genesis and recurrence of renal lithiasis, dietary manipulation has become a fundamental tool for the clinical management of nephrolithiasis.
The most common pathogenic pathway for the formation of kidney stones is that which is through calcium oxalate, which includes several processes, from nucleation, crystal growth and crystal aggregation. Many factors influence urinary supersaturation for calcium oxalate, being classified as promoters or inhibitors. Low urinary volume, high urinary excretion of calcium, oxalate and urate are considered promoters. In addition, citrate, magnesium and potassium and other organic substances such as nephrocalcina, fragment-1 of urinary prothrombin, osteopontin are known to inhibit the formation of stones.
Thus, in addition to metabolic and endocrine causes, the literature points to a direct relationship between the formation of kidney stones and unhealthy eating habits, obesity and sedentary lifestyle. However, different components of the diet may alter the composition of urine. Thus, increasing its supersaturation, which is the basis of the formation of calculations.
Oxalate is found mainly in plants, which use it to eliminate excess calcium present in water in the form of calcium oxalate. That is, it accumulates in leaves, fruits and seeds. Therefore, when these parts are taken off, excess calcium is eliminated along with oxalate.
For this reason, large amounts of oxalate are usually ingested every day, although the exact amount is difficult to estimate. A particular example of high variability in the content of oxalate of food is tea: black tea has higher concentration of oxalate compared to oolong or green tea. In addition, other factors such as infusion time, tea quality, preparation, origin and harvest period influence urinary oxalate excretion.
Anyway, only 50% of normal urinary oxalate excretion is of food origin. The remaining amount is due to endogenous liver metabolism. This molecule has no nutritional function and is therefore eliminated by the kidneys. However, in urine, it binds rapidly to calcium, increasing the oversaturation of calcium oxalate.
Intestinal absorption of oxalate is usually low and highly variable (about 10% to 15%). However, in individuals without malabsorption syndrome, intestinal absorption of oxalate may increase only when intestinal ionized calcium is reduced. This is usually due to high dietary intake of phytate (calcium-binding molecule) and/or a low-calcium diet. In addition to hyperoxaluria, a genetic disorder characterized by total or partial deficiency of enzymes involved in glioxylate metabolism that tends to result in overproduction of calcium oxalate.
In the case of hyperoxaluria, the role of anaerobic
Oxalobacter formigen is being investigated
, which ensures health of the intestinal microflora and degradation of the oxalate. As a result of its activity, oxalate absorption decreases, minimizing hyperoxaluria. Therefore, its subsequent use as a probiotic may be beneficial in preventing recurrent calculations.
Nutritionists can make use of 24-hour urine collection or use metabolomic data to understand the intake of sodium, protein, endogenous acid and vegetables. Thus, prescribe dietary and assertive lifestyle modifications based on the interpretation of urinary solute excretions or metabolite levels, avoiding unnecessary broad and difficult indications.
Furthermore, avoiding oxalate-rich foods is only beneficial for patients with hyperoxaluria due to intestinal oxalate hypersorption. In this context, it is recommended to reduce the intake of foods rich in oxalate, such as spinach, rhubarb, parsley, chives, beet leaves, green tea, chocolate and foods rich in vitamin C. Literature also indicates promising data regarding vitamin B6 inducing reduced excretion of oxalate and, therefore, its supplementation being beneficial for such patients.
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Article: Kidney stones and nutrition: Ferraro PM, Bargagli M, Trinchieri A, Gambaro G. Risk of Kidney Stones: Influence of Dietary Factors, Dietary Patterns, and Vegetarian-Vegan Diets.
Nutrients. 2020 Mar 15;12(3):779. doi: 10.3390/nu12030779. PMID: 32183500; PMCID: PMC7146511.