Introduction

Patients depend on us to prescribe tested and effective treatments. In the current era, as public skepticism about science and medicine is rising, patients’ trust in us is more important than ever. This applies to medical and nutritional management of kidney stone disease. Therapeutic dietary changes may reduce stone recurrence risk provided they 1) address (an) identified etiological factor(s) contributing to the patient’s stone disease, 2) are achievable and sustainable over time, 3) cause no or limited unwanted side effects and 4) have ample evidence to support their use. A low oxalate diet, including when prescribed within a blanket regimen of dietary therapies to all patients with stones, fails these tests and is thus not appropriate in the current era stone management.

What Is a Low Oxalate Diet?

There is no consensus definition. In theory, it limits oxalate-rich foods to below some cutoff expressed in mg per day. This cutoff is variable, depending on providers’ beliefs, but is typically 50–100 mg per day. There are problems with this approach. For example, it is possible for individuals to consume the same foods but to calculate different oxalate content. Factors related to this variability are shown in table 1.^1–3 For these reasons, cutoffs for a low oxalate diet are practically meaningless. The low oxalate diet is a treatment with no standard definition and varies widely in its application and practice.

Etiology

Determining the etiological factor(s) in any disease is the first step in medical management. While patients with kidney stones may present with similar physical symptoms and renal imaging (flank pain, lower urinary tract symptoms, visualized calculi in kidneys or lower urinary tract), causative factors can differ. This is not unique among medical conditions. In anemia, for instance, the etiology for the generally shared sequelae (fatigue, shortness of breath, low hemoglobin or hematocrit, low red blood cells) must be correctly identified in order to develop an effective response—iron supplementation does not address anemia caused by vitamin B12 or folate deficiency. Similarly, patients whose calcium oxalate (CaOx) stone disease is not caused by high urine oxalate will not be helped by a low oxalate diet. There are many potential contributors to hyperoxaluria; oxalate intake is not always primary. Consider the patient who consumes virtually no or only a little plant material. Recommending a low oxalate diet to this patient, whose oxalate intake is already low, is meaningless therapy. Moreover, in such a case, the recommendation of a more appropriate strategy, ie one that correctly addresses the etiology, is obviated.

Achievability and Sustainability

To be successful, nutrition therapy must be achievable. Challenges in undertaking a specifically defined low oxalate diet were already addressed. Very diligent patients frequently end up eliminating more foods than necessary in pursuit of the goal: “I haven’t eaten a tomato in 20 years.” This patient did not believe me when I told her that available evidence suggests that tomatoes are not actually high for oxalate. But she was given the information decades ago by her urologist (“He has treated all my stones,” she said), who she insisted knew more about nutrition than I. Even after pointing out she had continued to form primarily CaOx stones all these 20 years and that her 24-hour urine results demonstrated other treatable risk factors (low citrate and magnesium), she could not be convinced to eat a tomato. By avoiding tomatoes all these years, not to mention many other fruits and vegetables, the patient had not achieved a lower oxalate diet.

What about sustainability? Patients who don’t like to eat fruits, vegetables and whole grains find a low oxalate diet quite sustainable. But if you’ve ever heard a patient ask, “What can I eat?” or express some other frustration about following dietary recommendations, sustainability must be questioned. Dietary restrictions of any kind have a poor history of sustainability.⁴ For those who want to incorporate a healthy variety of plant-based foods into their diets, a low oxalate diet is not sustainable.

Side Effects

A diet low in plant-based foods, particularly when practiced zealously, is insufficient for antioxidants, which are primarily plant-derived phytochemicals conferring many health benefits, including those related to cardiovascular disease, inflammation, degenerative diseases and many cancers. The benefits of fiber, nearly exclusively provided by foods of plant origin, are lost. Potential digestive tract effects of low fiber consumption include altered bowel function (eg constipation, diarrhea, loss of colonic lubrication) and dysbiosis. Patients with diabetes on a lower fiber diet sacrifice a dietary strategy to control blood glucose. Table 2 lists other side effects. In sum, the risk-to-benefit ratio for a low oxalate is high, rendering it of questionable therapeutic value.

Evidence

The kind of evidence we typically require prior to implementing a clinical therapy is lacking for a low oxalate diet. One controlled nutrition study among CaOx stone formers demonstrated a reduction in urinary oxalate excretion on a low oxalate diet.⁵ However, this same study revealed higher CaOx stone risk due to lower urine citrate and magnesium and higher urinary CaOx supersaturation. A short-term study demonstrated lower urine oxalate⁶ and a longer-term study demonstrated lower recurrence rates⁷ among CaOx stone formers on low oxalate diets, but there are no high-quality, well-designed, appropriately controlled, long-term trials to confirm these findings. In contrast, evidence for reducing the bioavailability (absorption) of dietary oxalate is robust.^2,8,9 In clinical nutrition, the bioavailability of nutrients and other food-derived components is frequently manipulated. While the aim is usually to enhance bioavailability (eg by pairing consumption of nonheme iron with ascorbic acid, dietary fat with carotenoids and fat-soluble vitamins), the opposite can be accomplished, such as by pairing fat, fiber and/or protein with meals to reduce glucose absorption.

Few other widely practiced “therapies” in medicine have so little evidence as the low oxalate diet for stone prevention. In patients whose CaOx stone risk is diet-related, evidence supports 1) balanced dietary patterns that are ample for fruits, vegetables and fiber,¹⁰ and, if needed, 2) calcium consumption sufficient to reduce oxalate bioavailability.