Today’s Dietitian
Vol. 28 No. 2 P. 14
Kidney stones may be tiny, but their impact is anything but. Roughly 8% to 12% of Americans will experience at least one stone in their lifetime,1 and the vast majority—about 80%—are calcium oxalate stones.2 Other less common stone types include calcium phosphate, uric acid, and struvite stones. Despite the high prevalence of calcium oxalate stones, nutrition recommendations often remain oversimplified and, in some cases, misguided.
Patients are often told to avoid high-oxalate foods or cut back on dairy or other calcium-rich foods. Both ideas seem logical but may contain flaws in deeper biochemical understanding. Restricting calcium can actually raise urinary oxalate and increase stone risk.3
Renal nutrition expert Melanie Betz, MS, RD, CSR, FNKF, FAND, founder and CEO of The Kidney Dietitian, sees the confusion firsthand and says there is no one-size-fits-all kidney stone diet. “People can be making the exact same type of kidney stone for different reasons. We need to know what is off in a patient’s urine before we can put together a personalized stone prevention plan.” She adds, “There is up to a 75% lifetime recurrence rate for kidney stones. Prevention is where it is at.”
Why Do Calcium Oxalate Stones Form?
Calcium oxalate stones develop in response to several biochemical factors, including (but not limited to) supersaturation, urine pH, and urinary citrate. Understanding these drivers clarifies why diet matters and why some common recommendations may be missing the mark.
Supersaturation
The most important driver of stone formation is urine supersaturation. When urine becomes highly concentrated with calcium and oxalate, crystals are more likely to form and stick together.2,4 Increasing urine volume helps dilute these minerals and lower risk.
The American Urological Association recommends most patients produce about 2.5 liters of urine per day, which typically requires about 3 liters (12 cups) of fluid. In hotter climates or during heavy sweating, needs are higher.
“It’s not exciting,” says Betz, “but drinking enough fluid to dilute your urine is by far the most effective way to prevent stones.”
Urine pH
Urine pH—how acidic or alkaline the urine is—also affects stone risk. Acidic urine (low pH) encourages calcium oxalate crystals to form. Diet plays a major role: high intake of animal protein, sodium, and refined carbohydrates increases dietary acid load, while fruits and vegetables contribute alkali, helping raise urine pH.4
More alkaline urine also supports higher urinary citrate, which brings us to the next key player.
Citrate Levels
Citrate is a compound the body naturally produces and excretes in urine, and it acts as a powerful inhibitor of calcium oxalate stone formation.4
Citrate helps protect against calcium oxalate stones in two important ways. It binds to calcium, reducing its ability to pair with oxalate, and it raises urine pH, creating a less favorable environment for crystal formation.
Diet plays a role in urinary citrate levels. Diets high in animal protein increase acid production, which causes the kidneys to conserve citrate rather than excrete it in urine. As a result, urinary citrate levels fall. In contrast, fruits and vegetables—especially citrus—reduce dietary acid load and promote higher urinary citrate levels.
The counseling message is simple: more plants, more citrate, fewer calcium oxalate stones.
The Role of the 24-Hour Urine Test
The 24-hour urine test is a critical tool for personalizing MNT. This allows providers to identify which biochemical factors are driving a patient’s stones and tailor interventions accordingly. According to Betz, tests are typically covered by insurance, with out-of-pocket costs ranging from $150–$300, depending on the lab.
Betz says, “It measures things like urine volume, pH, calcium, oxalate, citrate, sodium, and urea nitrogen (which reflects protein intake)—along with other important markers. Most labs (Litholink is the most common) also calculate a ‘supersaturation’ value for calcium oxalate, calcium phosphate, and uric acid. That number is gold because it estimates someone’s risk of forming that type of stone.”
Debunking Myths
MYTH #1: People with calcium oxalate stones should avoid calcium.
Fact: Calcium is protective when consumed with meals because it binds oxalate in the gut and reduces absorption.
Low-calcium diets can lead to higher urinary oxalate and increase stone risk.3 In a landmark study, a normal-calcium diet combined with reduced sodium and moderate protein intake reduced stone recurrence by approximately 50% compared with a low-calcium diet.3
“People avoid dairy because they think calcium causes stones,” Betz says. “In reality, getting calcium with meals is one of the most effective prevention strategies we have.”
MYTH #2: High-oxalate foods are the main cause of kidney stones.
Fact: Only a small number of stone formers have high urinary oxalate. Because urinary oxalate is derived from both dietary sources and endogenous metabolism, focusing on oxalate restriction alone often has limited benefit.5
According to Betz, “About 20% of people with stones have high urine oxalate, and even if you do, it is much more effective and healthful to get that urine oxalate down with adequate calcium rather than a strict low oxalate diet, which makes it nearly impossible to get in things like fiber, magnesium, potassium, and other alkali precursors.”
MYTH #3: Everyone with calciumoxalate stones should follow the same diet.
Fact: Kidney stones develop for different biochemical reasons, and individualized nutrition depends on urine chemistry.4
Stone formers vary widely in terms of their chemistry. Some have high urinary calcium, some have low citrate or high sodium (which increases calcium excretion), others have very acidic urine or low urine volume. A 24-hour urine test identifies these factors and guides targeted MNT.
“The 24-hour urine test is essential,” Betz says. “It tells us exactly which chemistry issues to target instead of guessing.”
Putting It All Together: The Stone-Smart Diet
A prevention-focused dietary pattern for calcium oxalate stones centers on a few core strategies that RDNs can adapt to individual needs.
- Hydration. Encourage patients to aim for roughly 3 liters of total fluid per day, with the goal of producing at least 2.5 liters of urine.4 Water is best, but milk, coffee, unsweetened tea, juice, and citrus-infused water also count.
- Calcium with meals. Help patients distribute 1,000–1,200 mg/day across meals and snacks. Pairing calcium with oxalate-containing foods reduces absorption and supports prevention.3 Calcium is also important for bone health.
- Lower sodium. Most patients should aim for no more than 2,300 mg/day, or 1,500 mg/day of sodium for higher-risk individuals. Further personalization and adjustment may be necessary for those who experience higher sweat losses.
- Moderate animal protein. Aiming for about 0.8–1.0 g/kg/day helps minimize acid load and support urinary citrate.4
- Moderate oxalate intake, not elimination. Rather than long lists of “forbidden” foods, RDs can help patients limit only very high-oxalate foods if urinary oxalate is elevated, and pair moderate-oxalate foods with calcium-containing foods.5
- Limit added sugar. High fructose intake increases urinary calcium and lowers urine pH, creating conditions that can promote stone formation.6
- More fruits and veggies. Fruits and vegetables increase dietary alkali and support higher urinary citrate, improving urine pH.4 DASH- and Mediterranean-style patterns—which include oxalate-containing foods—are associated with a reduced risk of kidney stones.7
— Liz Weiss, MS, RDN, shares recipes and healthful living advice on LizsHealthyTable.com and her podcast, EAT, DRINK, LIVE LONGER. Weiss is a cooking instructor, frequent lifestyle guest on TV shows across the country, and a recipe developer and cookbook author.
References
1. Scales CD Jr, Smith AC, Hanley JM, Saigal CS. Prevalence of kidney stones in the United States. Eur Urol. 2012;62(1):160-165.
2. Khan SR, Pearle MS, Robertson WG, et al. Kidney stones. Nat Rev Dis Primers. 2016;2:16008.
3. Borghi L, Schianchi T, Meschi T, et al. Comparison of two diets for the prevention of recurrent stones in idiopathic hypercalciuria. N Engl J Med. 2002;346:77-84.
4. Taylor EN, Curhan GC. Diet and fluid prescription in stone disease. Kidney Int. 2006;70(5):835-839.
5. Fontenelle LF, Sarti, TD. Kidney stones: treatment and prevention. Am Fam Physician. 2019;99(8):490-496.
6. Taylor EN, Curhan GC. Fructose consumption and the risk of kidney stones. Kidney Int. 2008;73:207-212.
7. Taylor EN, Fung TT, Curhan GC. DASH-style diet associates with reduced risk for kidney stones. J Am Soc Nephrol. 2009;20:2253-2259.