Nephrolithiasis: Nutritional Considerations
Kidney stones are increasingly common. Their incidence increases with animal protein intake, obesity, and poor fluid intake. Oxalates appear to be problematic primarily in persons with low calcium intake, and a vegetarian diet may offer significant protection against stone formation.
In observational studies, the following factors are associated with reduced risk:
Reducing Intake of Animal Protein and Sodium
A high animal protein intake causes a significant increase in the urinary excretion of calcium, oxalate, and uric acid, 3 of the 6 main urinary risk factors for calcium stone formation.3 Compared with individuals eating 50 grams or less of animal protein per day, those eating the most (77 g or more) have a 33% higher risk for kidney stones.4Compared with a standard (low–calcium) diet used for prevention of calcium oxalate stone formation, a diet restricted in animal protein and sodium reduces the risk for stone recurrence by half.5
Although calcium intake was previously considered a culprit, current evidence indicates that urinary oxalate is the most important determinant of calcium oxalate crystallization.6 Individuals who tend to form stones of this type experience a significant increase in urinary oxalate excretion with even small increases in oxalate intake, whereas persons who do not form these stones do not experience a similar increase in urinary oxalate excretion with increased oxalate intake.7 Patients should be advised to avoid high–oxalate foods, such as rhubarb, spinach, strawberries, chocolate (especially dark), wheat bran, nuts, beets, and tea. Alternatively, certain food preparation methods may be used to reduce oxalate content. Boiling, for example, reduces soluble oxalate content by 30% to 87%, compared with steaming, which achieves a 5% to 53% reduction.8
Calcium at Mealtime
Calcium intake from foods lowers the risk for calcium oxalate stones,9 presumably because calcium binds oxalates within the intestinal tract. Oxalate absorption decreases as calcium intakes increase over a range of 200 to 1,200 mg per day.10 Individuals consuming the greatest amount of calcium from foods have about a 30% lower risk for stone formation, compared with persons consuming the lowest amounts.11 Research findings are mixed regarding the potential for calcium supplements to affect risk for urolithiasis. In the Nurses Health Study, women taking calcium supplements experienced a 20% increase in risk for stone formation, compared with not using supplements.9 But individuals taking doses of > 500 mg per day have been associated with decreased risk.12 Whether taking calcium supplements contributes to stone formation may depend on timing, with between–meal dosing associated with the risk for stone formation, while supplements taken with meals may reduce risk by binding dietary oxalates in the digestive tract.9
Limiting Colas, Coffee, and Tea
Although further research is required, evidence indicates that cola consumption significantly increases urinary calcium13 and oxalate excretion.14 Patients who avoid colas and other phosphoric acid–containing beverages have been found to have a 15% lower rate of stone recurrence than those who continue to consume these beverages.15 The association of coffee and tea intake with risk for stone formation appears to be less controversial, with findings of an inverse association with risk.16,17 Overall fluid consumption appears to be most important. Individuals who consume the highest amount of fluid each day (~2.6 L) have a 30% to 40% lower risk for stone formation than those consuming the least amount (~1.4 L).4,16
Vegetarian diets are associated with low excretion of calcium, oxalate, and uric acid3 and may lower the risk for urolithiasis in a number of ways. These include the absence of animal protein and provision of higher amounts of magnesium and potassium, both of which are associated with lower risk for stone formation.12,18 Vegetarian diets also provide ample amounts of whole grains high in phytic acid, a plant constituent that is associated with about a 40% lower risk for stone formation in persons eating the most (about 900 mg/day), compared with those eating the least amount (about 600 mg/day).19 Nationwide surveys have determined that the risk for stone formation is 40% to 60% lower in individuals following vegetarian diets.20 Compared with individuals following self–selected or Western diets, those on a vegetarian diet also have a decreased risk for uric acid crystallization.21
In a prospective study, each 8–oz serving of wine decreased the risk for stone formation by 39% to 59%.16,17 However, the potent diuretic effects of alcohol give it the potential to cause dehydration and increase stone risk if adequate consumption of other fluids is not maintained.
For patients with suspected or documented oxalate stones, consult with registered dietitian for instructions on how to follow a low–oxalate diet.
Consume ≥2.5 L of fluid per day.
What to Tell the Family
The risk of kidney stones is influenced by dietary factors, particularly high intake of animal protein and salt, along with low fluid consumption. Persons with recurrent stones may benefit from avoiding animal protein and oxalate–containing foods, getting adequate intake of calcium–rich foods at mealtime to reduce oxalate absorption, and drinking adequate fluids.
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