Dietary Intervention for Calcium-Based Kidney Stones

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Renal calculi, or kidney stones, are crystal aggregates composed of organic and inorganic materials, several of which are typical solutes in urine composition.  Stone formation generally occurs when the urine becomes supersaturated with a specific solute such as calcium, uric acid, or struvite.  Calcium-based stones are the most common, comprising about 80% of cases in the United States (1).            

High incidence of calcium-based stones in affluent societies is directly linked to certain dietary patterns, which likely contribute to the increasing prevalence of kidney stones (2).  These factors include low intakes of dietary fiber, fluid, citric acid and dietary calcium, as well as high intakes of dietary oxalates (2).

Fiber Intake           

Overall, vegetarians have a lower incidence of stone formation, as well as meat eaters with high fruit and vegetable intake (1).  Bran supplementation and increased consumption of whole grains also result in lower urinary calcium levels (1).

Fluid Intake

A low fluid intake results in a decreased urine output and a slower urine flow, both of which encourage stone formation.  Therefore, increasing fluid intake, specifically water, is the recommended approach for decreasing urine supersaturation.  Studies show that a water intake of 2 L/day or urine excretion of 2.5 L/day lowers kidney stone recurrence by 60% (1).

According to Gul and Monga (2014), coffee, tea, wine, beer, and fruit juices are considered acceptable beverages, but tomato, grapefruit, and cranberry juice should be avoided.  Tomato juice is high in sodium whereas grapefruit and cranberry juices are rich in oxalate.  Additionally, the high fructose levels of popular sports drinks have recently been linked to kidney stones (1).  Soft drinks may also play a role in stone formation but appear to only affect those who consume phosphoric acid-containing drinks as opposed to citric acid-containing drinks (2).

Citric Acid

Hypocitraturia affects 60% of patients with calcium stones, indicating an association with low dietary citric acid (2).  Also, a higher acid load facilitates renal citrate reabsorption, thus decreasing its excretion.  Fruit juice consumption may reduce stone formation due to an overall fluid increase and high potassium and citric acid content (2).  Citrate found in fruits and fruit juice bind with urinary calcium and with calcium oxalate crystals and thus prevent stone formation.  Consuming four ounces of lemon juice a day has been shown to reduce urine citrate levels without increasing oxalate levels (2).  Melon juice and orange juice are also rich sources of citric acid.

Dietary Calcium

Increased dietary calcium may decrease stone formation due to its ability to bind to dietary oxalate in the intestine (1, 2).  Dietary sources of calcium include dairy products; calcium-fortified foods such as orange juice, soy milk, tofu, and selected cereals; sardines with bones; and almonds. The target intake for calcium is 1200 mg/day, and dietary calcium is preferred to supplementation due to the risk of stone formation (2, 3).  However if supplementation is needed, calcium citrate is recommended versus calcium carbonate (2).

Dietary Oxalate

Dietary oxalates may contribute as much as 80% of urine oxalate, and those with recurrent kidney stones may absorb higher levels of dietary oxalates (1).  Hyperoxaluria is a known risk factor for calcium-based stone disease.  Therefore, a low-oxalate diet, defined as less than 50 mg of oxalate per day, is recommended for people with hyperoxaluria in order to reduce the level of oxalic acid excreted in urine (2).  This is not recommended for otherwise healthy people due to the “heart healthy” components found in high-oxalate foods.  Additionally, adequate dietary calcium (1200mg/day) reduces intestinal oxalate absorption (1).

As previously mentioned, the increasing prevalence of calcium-based stones, especially in Western societies, indicate a possible link to diet-related factors.  Though genetic and other physiological factors play a role in stone formation, the importance of dietary intervention in the prevention and treatment of stone formation, especially in susceptible individuals, is well recognized.  

References

1. Pizzorno, J. E., & Murray, M. T. (2013). Textbook of natural medicine (4th ed.). Philadelphia, PA, United States: Elsevier/Churchill Livingstone.

2. Gul, Z., & Monga, M. (2014). Medical and dietary therapy for kidney stone prevention. Korean Journal of Urology, 55(12), 775–779. http://doi.org/10.4111/kju.2014.55.12.775

3. Copstead, L.-E. C., & Banasik, J. L. (2013). Pathophysiology (5th ed.). Amsterdam: Elsevier Health Sciences.