(UroToday.com) Saturday morning’s plenary session featured Dr. Kristina Penniston, who challenged some of the most common myths in the dietary management of kidney stones. Drawing on her clinical nutrition research at the University of Wisconsin-Madison, Dr. Penniston argued that generic dietary prescriptions, while well-intentioned, may be ineffective and even harm patient trust. She organized her talk around five common myths, dismantling each with clinical evidence, before offering a framework for individualized, phenotype-driven nutrition therapy.
The Problem
Dr. Penniston opened with a patient vignette familiar to any urologist: a 57-year-old woman with recurrent calcium oxalate stones who strictly followed a low-oxalate, low-protein diet for two years, yet continued to form stones. This scenario, she argued, encapsulates three interconnected failures: 1) dietary advice, if given at all, is generic, 2) dietary myths are persistent and capable of causing harm, and 3) the resulting harms are potentially serious yet entirely avoidable.
Myth 1: Dietary Stone Prevention Is the Same for All
Dr. Penniston traced the first myth to historical simplification, incomplete mechanistic understanding, and a public health model that prioritizes broad guidelines over individualized metabolic evaluation. The evidence shows that generalized dietary restrictions can worsen risk and delay precision-based prevention. Dr. Penniston stated, when patients follow generic advice and still form stones, it can create mistrust in the care team.
The key point, Dr. Penniston emphasized, is that kidney stones represent heterogeneous systemic disorders with distinct risk factors. Different stone types (calcium oxalate, struvite, calcium phosphate brushite, cystine, and calcium phosphate apatite, uric acid) carry different etiologies beyond just the diet. This includes environment, medications, genetics, comorbidities, anatomical abnormalities, and infections. Urinary risk factors differ accordingly, and a single dietary prescription cannot address this heterogeneity.
Myth 2: Dietary Oxalate Is the Primary Driver of Calcium Oxalate Stones
The instruction to “avoid spinach, nuts, and chocolate” is so deeply embedded in patient education and some clinical practice guidelines that it has become dogma. Dr. Penniston dismantled this myth by redirecting attention to the actual sources of urinary oxalate.
In typical stone formers, approximately 70% of urinary oxalate derives from endogenous synthesis via glycine, hydroxyproline, and ascorbate pathways, while only 30% comes from dietary absorption (Holmes et al., Kidney Int, 2001; Taylor & Curhan, Clin J Am Soc Nephrol, 2008). This ratio varies substantially by patient and depends on calcium consumption. Intestinal oxalate absorption is highly variable among stone formers, as Dr. Penniston explains, and the gut microbiome plays a key modulatory role. She then presented her data on medical nutrition therapy in managing hyperoxaluria. In her study, 144 adult patients with stone formation were treated with diet changes alone or calcium citrate supplementation and diet. The results showed that medical nutrition therapy with ample calcium distributed with meals is effective in managing hyperoxaluria.
Dr. Penniston then outlined best practices grounded in this evidence: 24-hour urine testing should precede dietary restriction, not the reverse. Oxalate restriction should be reserved for confirmed high absorption on metabolic evaluation, as a blanket restriction causes unnecessary dietary limitation, erodes patient trust, and misses the true cause.
Myth 3: Calcium Intake Should be Limited
Dr. Penniston stated plainly that this myth was physiologically incorrect. Dietary calcium binds oxalate in the GI tract, reducing oxalate absorption and lowering urinary oxalate. Restricting calcium removes this protective effect, raises PTH and calcitriol, and paradoxically increases urinary calcium. Curhan et al. demonstrated that dietary calcium intake is inversely associated with stone risk in most patients (N Engl J Med, 1993), and Bataille et al. confirmed that calcium restriction raises urinary oxalate (J Urol, 1983).
Dr. Penniston illustrated this myth with a patient who had adopted a plant-heavy diet, rich in flaxseeds, hempseeds, nuts, leafy greens, and chocolate, to resolve chronic constipation. His 24-hour urine oxalate was markedly elevated at 137 mg/day, with 100% calcium oxalate stone composition. The true driver was low calcium intake - when dietary calcium is insufficient, GI oxalate binding is reduced, oxalate absorption rises, and urinary oxalate increases.
Figure 1. Plots of urinary oxalate excretion (mg/d) for 2 treatment groups before and after treatment for hyperoxaluria with dietary recommendations alone (triangles, n = 10) or dietary recommendations plus calcium citrate supplementation with meals (circles, n = 12), 2009.
The nutrition diagnosis was low calcium consumption in the setting of a higher oxalate diet. The intervention targeted restoring calcium-rich foods and beverages at meals. Dr. Penniston referenced her group’s work (Penniston & Nakada, Urology, 2009) showing that patients with high urine oxalate advised to pair calcium-rich foods with meals, and those with malabsorption who additionally used calcium supplements, achieved substantial reductions in urinary oxalate excretion.
Myth 4: Protein Restriction Prevents All Kidney Stones
The assumption that reducing dietary protein lowers urinary calcium and uric acid overlooks critical nuance, as Dr. Penniston explained. She stated, “Not all kidney stones are driven by protein-related pathways,” and individual metabolic responses vary considerably. Protein needs differ by age, lifestyle, body habitus, and health status. When stones are driven by low urine volume, high oxalate absorption, or genetic conditions, protein is only a minor contributor. Compensatory mechanisms and bone buffering responses vary widely; not all proteins are equal, and not everyone overeats protein to begin with. She stated that reducing protein may be at the detriment of other conditions, such as osteopenia or osteoporosis.
Myth 5: High Fluid Intake Is Sufficient for Everyone
Dr. Penniston acknowledged the partial truth here: higher urine volume is the most broadly supported dietary intervention for stone prevention. However, it is not sufficient as a standalone strategy. Fluid type matters, as citrate-containing beverages raise urinary citrate and reduce risk, while sugar-sweetened drinks increase risk. For uric acid stones, volume alone does not correct the low urinary pH that drives crystallization. Cystinuria requires ≥ 3 L/day, well above standard guidance. Volume targets must be individualized, and prescriptions should account for occupation, patient preferences, continence, extra-renal losses, and quality of life.
The Cost of Dietary Myths
Lastly, Dr. Penniston catalogued the real-world harms of unchallenged dietary myths across four domains: 1) nutritional harm, or introducing micronutrient deficiencies without clinical benefit, 2) diagnostic delay, 3) bone density loss and sarcopenia, and finally 4) erosion of trust, as patients who follow general advice and still form stones lose confidence in their care team.
Returning to her opening vignette, Dr. Penniston revealed that a formal nutrition assessment found the 57-year-old’s diet to be insufficient in both protein and calcium and not actually high in sodium or oxalate. Her true stone risk factors were low urine volume, low citrate, low magnesium, and untreated bone turnover related to her osteoporosis. The diet she had been prescribed had not only failed to prevent stones; it had actively harmed her nutritional health.
The Science of Individualization
Dr. Penniston closed with evidence-based principles for individualized stone nutrition. Dietary oxalate restriction is not universally warranted. Calcium and protein restriction are contraindicated without a specific metabolic diagnosis. Fluid targets must be individualized. Diet is frequently not the primary cause, and systemic or genetic etiologies must be excluded; individualized, phenotype-driven nutrition therapy is the optimal approach.
When 24-hour urine data are unavailable, Dr. Penniston recommended counseling patients that diet is not always a contributor and may not be the primary drive. Clinicians should foreshadow the potential need for pharmacologic therapy and encourage high urine output, plenty of fruits and vegetables, adequate calcium timed with meals, and minimization of processed foods. “Individualized, phenotype-driven nutrition therapy is the best approach,” she concluded. With the conclusion of the lecture, she called the audience to move beyond dietary myths toward precision-based stone prevention grounded in metabolic evaluation.
Presented by: Kristina Penniston, PhD, RD, Department of Urology, University of Wisconsin-Madison School of Medicine and Public Health, Madison, WI, USA
Written by: Helen Gao, Medical Student at Robert Wood Johnson Medical School, Leadership and Innovation Fellowship Training (LIFT) Scholar at Department of Urology, University of California, Irvine. @helengao295 on X during the 2026 American Urological Association Annual Meeting, May 15 - 18, 2026, Washington, DC
References:
- Holmes RP, Goodman HO, Assimos DG. Contribution of dietary oxalate to urinary oxalate excretion. Kidney Int. 2001;59(1):270-276. doi:10.1046/j.1523-1755.2001.00488.x
- Taylor EN, Curhan GC. Oxalate intake and the risk for nephrolithiasis. J Am Soc Nephrol. 2007;18(7):2198-2204. doi:10.1681/ASN.2007020219
- Penniston KL, Nakada SY. Effect of dietary changes on urinary oxalate excretion and calcium oxalate supersaturation in patients with hyperoxaluric stone formation. Urology. 2009;73(3):484-489. doi:10.1016/j.urology.2008.10.035
- Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med. 1993;328(12):833–838.
- Bataille P, Charransol G, Gregoire I, et al. Effect of calcium restriction on renal excretion of oxalate and the probability of stones in the various pathophysiological groups with calcium stones. J Urol. 1983;130(2):218–223.