Nephrolithiasis, commonly known as kidney stones, is a prevalent medical condition influenced significantly by environmental factors, particularly diet. The formation and recurrence of kidney stones are closely linked to nutritional habits, making dietary adjustments a cornerstone in their medical management. Dietary advice typically aims to reduce the majority of lithogenic risk factors, thereby reducing the supersaturation of urine, mainly for calcium oxalate, calcium phosphate, and uric acid. Current guidelines emphasize increasing fluid intake, maintaining a balanced calcium intake, reducing dietary intake of sodium and animal proteins, and increasing intake of fruits and fibers.
Prevalence of Kidney Stones
Nephrolithiasis is a common medical condition characterized by high prevalence all over the world. During the last decades, the incidence of nephrolithiasis is rising in both genders, with resulting increased economic burden for health systems. Calcium nephrolithiasis, in combination with oxalate or, less frequently, phosphate is by far the most common form, representing 75% of all kidney stone phenotypes. Conversely, the prevalence of uric acid nephrolithiasis does not exceed 10%.
In the USA’s National Health And Nutrition Examination Survey (NHANES) 2007-2010, the rate of having had a kidney stone was 10.6% for men and 7.1% for women.
Understanding Kidney Stones
A kidney stone is a hard deposit that forms inside the kidneys from various minerals and salts. Most stones are calcium oxalate stones (about 70% or more). The pathogenic pathway of calcium oxalate stone formation includes several processes starting from nucleation, crystal growth, and crystal aggregation. Many factors influence urine supersaturation for calcium oxalate, being classified as promotors or inhibitors. Low urine volume and high urinary excretion of calcium, oxalate, and urate are considered as promotors. Besides, citrate, magnesium and potassium and other organic substances (nephrocalcin, urinary prothrombin fragment-1, osteopontin) are known to inhibit stone formation.
Many factors involved in the pathogenesis of renal stones are influenced by the diet. Nutritional exposure is probably one of the most important factors involved in the increased frequency of nephrolithiasis among the general population. Furthermore, genetic predisposition should also be considered. More than 30 genetic variants with Mendelian inheritance are known for causing kidney stones, and polygenic involvement in idiopathic stone formers is even more frequent.
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In addition, stone disease is associated with other comorbidities such as arterial hypertension, diabetes mellitus, obesity, metabolic syndrome, and increased likelihood of developing chronic kidney disease. Besides, patients affected by urolithiasis have an increased odds of multi-organ complications such as metabolic bone disease, cardiovascular events, and vascular calcifications.
The Vegan Diet: A Closer Look
Vegan and vegetarian diets are becoming more and more popular among the general population. A vegan diet is defined as a diet with no intake of animal derivatives, where only vegetal derivatives are allowed. On the other side, vegetarian diet is a diet where only meat and fish are prohibited, while eggs and milk derivatives are allowed. These diets have gained great popularity in Western countries in the last decades, especially for cultural or religious reasons.
These diets emphasize plant foods-the primary sources of dietary fiber-while discouraging meat consumption. A diet with low intake of animal protein can alkalize the urine and therefore can reduce the risk of stones.
Plant-based diets focus on whole grains, fruits, and veggies, and may benefit kidney health. They can help manage weight, blood pressure, and diabetes.
The Role of Oxalate
Oxalate is a molecule many in the vegan community are familiar with for preventing the absorption of calcium. Oxalate is a small molecule found in large amounts in many plants foods but not found in animal foods. Our bodies make oxalate as an end product of metabolism (primarily the metabolism of the protein amino acids glycine and serine, but also of vitamin C and possibly fructose). However, our bodies do not use oxalate in any way, nor degrade it, and it must be excreted through the urine or feces. Oxalate is made of two carbons and four oxygen with a charge of -2, making it attracted to other molecules with a charge of +2; especially calcium and to a lesser extent magnesium.
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In food, oxalate is either soluble or insoluble. Insoluble oxalate is bound to another molecule that makes it much harder to absorb; normally calcium but also sometimes magnesium.
An increased level of oxalate in the urine is a risk for calcium-oxalate kidney stones. Oxalate is generally not found in animal products while many plant foods are moderate or high, and some are extremely high (such as spinach, beets, beet greens, sweet potatoes, peanuts, rhubarb, and swiss chard).
Oxalate is mainly found in plants, which use it to eliminate excessive calcium present in water. In fact, it accumulates in leaves, fruits, and seeds. When those parts are detached, the calcium excess is eliminated together with oxalate. For this reason, large quantities of oxalate are usually ingested every day, although the exact amount is difficult to estimate.
Normally, intestinal oxalate absorption is low and highly variable (around 10%-15%). In individuals without malabsorption syndrome, bowel oxalate uptake may increase only when intestinal ionized Ca is reduced, often due to high dietary consumption of phytate (calcium-binding molecule) and/or low-calcium diet.
High oxalate foods include broadleaf vegetables such as spinach, green cabbage, beets, but also nuts, tea, chocolate, and rhubarb. Oxalate is anyway broadly present in foods, so it is difficult to significantly limit its intake.
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Oxalate Absorption and Factors Affecting It
In a 2006 study from the University of Bonn compared oxalate absorption between 120 kidney stone formers and 120 non-formers on low-oxalate diets with equal amounts of oxalate (63 mg/day) and calcium (800 mg/day). None of the participants had a history of digestive disorders (which can increase oxalate absorption). They found a small, but statistically significant difference in oxalate absorption between stone formers (10.2%) and controls (8.0%). There were no gender or age differences. Of the oxalate absorbed within 24 hours, 80-85% of it was absorbed within the first 6 hours after ingestion. Absorption values greater than 20% only occurred in the stone formers.
Based on 5 individuals, researchers from Wake Forest University estimated the daily intake of oxalate to be an average of 152 ± 83 mg, ranging from 44 - 352 mg/day.
A 2008 study was conducted by the Bonn researchers on 8 healthy volunteers (3 men, 5 women) with no history of kidney stones. The researchers were surprised that the higher oxalate vegetarian diet resulted in less oxalate absorbed and excreted through the urine than the low-oxalate vegetarian diet (see Table 1). They attributed it to the fact that the highest oxalate meal (251 mg from spinach) in the high-oxalate diet contained a cream sauce providing 212 mg of calcium, which could have negated the higher oxalate load. This seems reasonable, but there is still a question as to why more oxalate was excreted in the urine and the supersaturation was higher on the low-oxalate diet. The authors were also concerned about the binding of calcium by phytate in high-oxalate vegetarian meals as this could reduce the available calcium for binding with oxalate.
| Diet | Urinary Oxalate (mg/day) |
|---|---|
| High-Oxalate Vegetarian Diet | Lower |
| Low-Oxalate Vegetarian Diet | Higher |
Soluble vs. Insoluble Oxalate
Oxalate in food is either soluble or insoluble. Insoluble oxalate is bound to another molecule that makes it much harder to absorb; normally calcium but also sometimes magnesium. For example, in a 2008 study from the University of Wyoming, subjects without a history of kidney stones were given just over 1 teaspoon of cinnamon (3.5 g) or 3.2 g of turmeric, each providing 63 mg of oxalate, broken into 3 doses per day for 4 weeks with meals. In turmeric, 91% of the oxalate was soluble, while only 6% of the oxalate in cinnamon was soluble.
A number of studies have found that boiling plants allows much of the soluble oxalate to escape into the boiling water which can then be discarded. University of Wyoming researchers measured the oxalate levels in a variety of vegetable using different cooking methods. Boiling reduced total oxalate levels by a decent amount, steaming to a lesser extent (Table 2). The authors write, “There was a significant loss of soluble oxalate in almost all test vegetables by boiling, ranging from 30 to 87%. One caveat to this study is that many of these vegetables were cooked for a much longer time than might be typical.
| Vegetable | Cooking Method | Oxalate Reduction |
|---|---|---|
| Various | Boiling | 30-87% |
| Various | Steaming | Lesser extent |
A 2003 study from Lincoln University in New Zealand measured the oxalate absorption from spinach with calcium added to the diet. The spinach contained 957 mg of oxalate, of which 737 mg was soluble. The spinach was grilled to prevent loss of soluble oxalate into cooking water. The spinach contained 90 mg of calcium, of which 69 mg was bound to oxalate. The mean bioavailability of soluble oxalate in the grilled spinach was 0.75 ± 0.48% over a 6-hour period after intake and was 1.93 ± 0.85% measured over a 24-hour period. Adding 117 mg of calcium from sour cream reduced the 6-hour oxalate absorption to .52%. Adding 480 mg of calcium from sour cream and milk reduced the absorption to .14%.
Hyperoxaluria
Hyperoxaluria is a condition in which the amount of oxalate in the urine becomes very high, so high that it can cause severe kidney damage. Primary hyperoxaluria is a genetic disease in which the liver produces too much oxalate. Enteric hyperoxaluria is when too much oxalate is absorbed from the digestive tract. In cases of hyperoxaluria, it is possible that the build-up of oxalate in the body can become so great that it doesn’t just damage the kidneys, but can be deposited in other parts of the body. If you suspect that you are suffering from hyperoxaluria, you should talk to a health professional.
Current practice (as of October 2013) is that if a low-oxalate diet is adhered to but does not succeed in lowering urinary oxalate levels, the low-oxalate diet can be abandoned because dietary oxalate is not the cause.
Patients with Crohn’s are more likely to have hyperoxaluria, and this hyperoxaluria is more common in Crohn’s patients who have had a bowel resection (part of their digestive tract removed). A 2012 study from Germany found that 28% of Crohn’s patients had calcium-oxalate kidney stones. The authors of the study note a case in which a young woman with Crohn’s suffered from end-state renal failure at age 34. Her first kidney transplant failed due to hyperoxaluria. Her second transplant succeeded due to extra pre-transplant hemodialysis and vitamin B6, and post-transplant fluids, citrate, and O. The authors conclude that “[T]he high prevalence of [kidney stones]/oxalosis in patients with Crohn’s disease is based on secondary, enteric hyperoxaluria.
Vitamin B6 deficiency could possibly play a role in hyperoxaluria.
Calcium Intake: A Balancing Act
Meet the RDA for calcium. Nutritional lore goes like this: Since most stones are made from calcium and oxalate you should restrict foods high in calcium and oxalate in order to have fewer kidney stones. Oxalate is found in vegetables…so restrict vegetables.
It could be presumed that reducing dietary calcium intake is an appropriate therapeutic strategy. However, it should be considered that calcium intake rarely exceeds 1.2 g/day with diet.
Significant insights on the role of calcium in stone disease were obtained in 1993, when an analysis of 45,619 40-75 years old male individuals without stone disease at recruitment (the Health Professionals Follow-up Study cohort) showed that lower calcium intakes were associated with higher risk of kidney stone events by more than 50% compared with higher dietary intakes (797 ± 280 vs. 851 ± 307 mg of dietary calcium, respectively). This was confirmed in a randomized trial comparing a normal calcium (1200 mg/day), low-salt, and low-animal-protein diet to a low-calcium diet (400 mg/day) in a group of 120 men with recurrent calcium oxalate stones and hypercalciuria. In this study, the normal calcium diet group demonstrated a significant reduction in the risk of stone recurrence of approximately 50% after five years compared with the low-calcium diet group (relative risk for recurrence 0.49, 95% CI 0.24 to 0.98). In addition, urinary oxalate excretion was higher in patients on low dietary calcium intake (60 µmol/day increase) and lower in patients under normal calcium, low-salt, and low-animal-protein diet (80 µmol/day decrease). This phenomenon is explained by the fact that calcium in the intestine acts as a chelator for several substances, including oxalate. In case of low-calcium diet, there is an increased intestinal absorption of free oxalate, which increases oxaluria and the urinary supersaturation for calcium oxalate, favoring the nucleation process. Furthermore, balanced dietary calcium intake seems to have protective effects on kidney stone events independent of its origin, from both dairy and nondairy sources.
In a 1993 report, the Health Professionals Follow-Up Study (men) found an increased risk of kidney stone in the lowest one-fifth of calcium intake of < 605 mg/day compared to all other fifths, the highest category having a median intake of 1,326 mg per day. Animal protein intake was also associated with a higher risk of kidney stones, whereas potassium and fluid intake was associated with a lower risk.
A 1997 report from the Nurse’s Health Study I, conducted on older women, found that the highest one-fifth intake of dietary calcium (not including supplements, median intake of 1,303 mg/day) was associated with a significantly lower risk of kidney stones than the lowest intake (median 391 mg/day). Supplemental calcium didn’t fare as well; it increased risk (1.20, 1.02-1.41).
Other Dietary Factors
Animal Protein
In a study done in 1979, men with the highest meat consumption were more likely to be recurrent stone formers due to the increase urinary excretion of calcium, oxalate and uric acid. This study was again repeated in 1982 and vegetarians had a 40-60% decreased risk of kidney stones.
Available scientific evidence agrees on the harmful effects of high meat/animal protein intake.
Sodium
Dietary advice aims to reduce the majority of lithogenic risk factors, reducing the supersaturation of urine, mainly for calcium oxalate, calcium phosphate, and uric acid. For this purpose, current guidelines recommend reducing dietary intake of sodium.
Fluids
Drink plenty of water (this decreases the concentrations of minerals in the urine making them less likely to form stones: a simple formula to determine your daily water is ½ ounce per pound of body weight e.g.
Of all dietary interventions aimed to reduce the risk of kidney stones, fluid intake is one of the most important factors, being directly associated with the incidence of nephrolithiasis: for each 200 mL of fluids consumed per day, a 13% reduction in the risk of stone formation was found.
Fructose and Vitamin C
The Health Professionals Follow-up Study found an association between total fructose intake and risk of kidney stones in men (1.28. 1.06-1.55) for highest versus lowest fructose intake of 13.8% vs. 5.6% of calories. The Nurses Health Study I (of older women) and II (of younger women) found similar results. In a 1995 study, researchers from France found that giving fructose intravenously caused an increase in urinary oxalate, while glucose did not. Interestingly, an oral load of fructose had the opposite effect, reducing urinary oxalate.
Oxalate can be a byproduct of vitamin C metabolism. A 2004 report from the Health Professionals Follow-up Study found an increased risk of kidney stones among men getting more than 1,000 mg of vitamin C per day compared to those getting 90 mg/day or less (1.41, 1.11-1.80).
DASH Diet
In another study from 2009, the DASH diet (usually recommended for hypertension) that includes high intake of fruits, vegetables, nuts and legumes, low-fat dairy products, and whole grains, some lean meats and low intake of sodium, sweetened beverages, and red and processed meats, found that this diet cut the risk of kidney stones almost in half.
The Harvard School of Public Health thought that a DASH eating pattern might help prevent kidney stones so they tested it in their three big cohorts (Health Professionals Follow-up Study and the Nurses Health Studies I and II). They found that those eating a DASH-like diet pattern at baseline had about half the risk of kidney stones. Comparing the highest one-fifth to the lowest, in regards to DASH score, the rates were .55 (0.46-0.65) for men, 0.58 (0.49-0.68) for older women, and 0.60 (0.52-0.70) for younger women. The lower risk was independent of age, body size, hypertension, diabetes, thiazide use, and intakes of total calories, fluid, caffeine, and alcohol. Animal protein ended up not varying much across the groups, so that wasn’t the protective factor.
Studies on Vegetarians and Vegans
There is no research on kidney stone frequency in vegans, though anecdotally I know of some who have gotten stones. Of course, meat-eaters also get kidney stones-but is the average vegan at a higher or lower risk?
There are no studies measuring oxalate levels or kidney stone risks in vegans. Twenty-four-hour urines were collected from 22 lacto-ovo vegetarians (10 male and 12 female) and analyzed for 6 risk factors. The vegetarians had a slightly lower risk of forming stones than meat-eating women, a bit more than meat-eating men, and a lot lower than previous stone formers.
In 2002, researchers from the University of Bonn in Germany studied the effects of four different diets on the urine of 10 men with no history of kidney stones. All the men ate each diet for a period of five days each. The risk of calcium oxalate crystallization was highest on the SD and WD diets with no difference between the ND and VD. The VD had lower urinary calcium, which lowers stone risk, but higher urinary oxalate.
General Dietary Recommendations for Stone Prevention
Adherence to dietary and lifestyle modification has a potentially strong impact on stone incidence, as demonstrated in a study in which body mass index (BMI), fluid intake, dietary intake of fruits and vegetables, and calcium intake explained a large proportion of first-time kidney stones.
- Increase Fluid Intake: Aim for at least 2-3 liters of fluid per day to dilute urine and reduce mineral concentration.
- Maintain Balanced Calcium Intake: Aim for the recommended daily allowance, typically around 1000-1200 mg, from both dairy and non-dairy sources.
- Reduce Sodium Intake: Limit processed foods, salty snacks, and added salt in cooking and at the table.
- Limit Animal Protein: Choose lean sources of protein and moderate portion sizes to reduce urinary calcium and uric acid excretion.
- Increase Fruits and Vegetables: Consume a variety of fruits and vegetables, being mindful of high-oxalate sources if you are prone to calcium oxalate stones.
- Consider Citrate: Citrus fruits like lemons and oranges are high in citrate, which can inhibit stone formation.
- Dietary manipulation: might be useful only in case of excessive intake of high-oxalate vegetables, such as spinach, chocolate, and nuts