Calcium-Based Kidney Stones and the Low-Sodium Diet, a Literature Review

Updated: Jan 13, 2020

Stacie Savage

Cox College, 2018


Abstract

Nephrolithiasis can be costly to an individual in terms of medical expenses and work absenteeism. Quality of life can also be impacted as stones are associated with obesity and poor diet. And stones can cause significant pain and distress to the individual when trying to pass a stone as well as procedures to eradicate. Both original and review articles were found at the University of Central Missouri research database which including MedLine and PubMed. These resources were integrated with the author’s knowledge and experiences surrounding nutrition and nephrolithiasis. A quality improvement project is suggested by the author to encourage the integration of nutrition education into urology clinical practice involving kidney stone management, diet, and prevention of recurrence. This review supports with evidence the significance of the low-sodium diet and its effectiveness to reduce risk of kidney stones as well as a proposed low-sodium educational tool.


Keywords: calcium-based kidney stones, low-sodium diet, hypercalciuria, nephrolithiasis

Nephrolithiasis, also known as kidney stones, has an 8.8% prevalence in the United States according a self-report study; more specifically, in men at 10.6% and women at 7.1% (Scales, C., Jr., Smith, A., Hanley, J., & Saigal, C., 2012). Kidney stones can also be undetected within the kidney for many years, and often start out attached to the inside wall of a nephron. The stone then detaches and will attempt to be passed in urination. Microscopically, stones are jagged and crystalized and can cause great pain when passed within the ureter. They may become stuck in the ureter or they may be so small that they are passed in the urine without notice.

Stones can vary in composition: calcium-oxalate, calcium-phosphorus, cystine, struvite, and uric acid. Each different type of stone has a different etiology from diet to infection to genetic predisposition. Prevalence of kidney stones have been known to increase with obesity, in diets low in fruits and vegetables and high in animal protein and sodium (Vezzoli, G., et al, 2015). Herein this literature review will be a focus on calcium-based stones and the associated effects of a low-sodium diet.


Significance of Topic

Economic impact

According to the Economic impact of urinary stones, a 2014 journal article by Elias Hyams and Brian Matlaga, first time kidney stone patients have a 50% recurrence rate. Kidney stones can be costly; patients with acute kidney stone obstruction often go to emergency departments of hospitals, need expulsive therapy, and also experience work absenteeism and personal pain and distress that may last days until they pass their stone or receive a medical intervention. Over 1.3 million working adults with kidney stones is estimated to cost $4.5 billion in the United States per year which includes medical care and does not account for lost pay from work absenteeism (Hyams, E., & Matlaga, B., 2014).


Current medical management

From the prospective of an average kidney stone patient, it is not uncommon to arrive in the emergency room of a hospital in severe and destressing pain in the back below the ribs. Pain may come in waves and radiate to the lower abdomen. The patient will then undergo helical computed tomography (CT) scan which a radiologist will evaluate. The patient is then diagnosed with a calculi for the first time. The CT scan evaluation provides details of how many current calculi are in the kidney and the location of the obstructive stone often within the ureter. All kidney stone patients should initially undergo this radiologic method unless contraindicated. A urinalysis is also requested of the patient for evaluation, and the patient is then discharged from the hospital with a referral to the urologist.


The first diagnosis of a kidney stone includes a basic metabolic workup involving a complete blood count, serum levels, and 24hr-urinalysis while patient follows usual diet and a discontinuation of any lithogenic medication. The 24hr-urinalysis is a significant component in assessing kidney stone formation risk. Those risk factors include hypercalciuria, hypernatriuria, hyperoxaluria, low urine volume, or pH disturbances (Shadman, A. & Bastani, B., 2017). Also significant in assessing kidney stone formation is an analysis of the actual stone composition if able to capture stone upon expulsion.


In some patients, a kidney stone is removed by surgical means. For most patients, the first mode of kidney stone treatment is to pass the kidney stone through usual urination while drinking ample water volume. If the stone does not pass, medications may also be prescribed for expulsion therapy. Tamsulosin or nifedipine, smooth muscle relaxants, can be used to help pass calculi (Shadman, A. & Bastani, B., 2017). If the stone is not passing on its own, shockwave lithotripsy is a common expulsion therapy that sends extracorporeal shockwaves to the stone in attempts to break it up into smaller, more passible pieces. Ureteroscopy is another option of stone removal which involves inserting a flexible scope in the urinary tract and then is able to extract the stone with a tiny wire basket or a laser can then be used to break up the stone.


Follow up is recommended for eight to 12 weeks for management of stone recurrences and prevention. Urinalyses are essential in evaluating risk markers of kidney stone recurrence. As well as diagnostic imaging to follow progress of current kidney stones which can include plain radiography, ultrasound, or KUB at follow up (Shadman, A. & Bastani, B., 2017). Medical intervention may also include thiazides, which reduces urine calcium, as well as citrates, namely potassium-citrate, which is a known crystallization inhibitor. When urine parameters are normalized, follow ups are recommended once per year (Skolarikos, A., et al, 2015).

Diets are often recommended by urologists depending on stone composition and results of urinalysis. Diets recommended may be low-oxalates, low-sodium, animal protein moderation, increased water intake, increased dairy products, and weight management. Since dietary sodium is linked to lithogenic risk factors such as hypercalciuria, hypernatriuria, and calcium-oxalate supersaturation (Skolarikos, A., et al, 2015), the effectiveness of a low-sodium diet will be reviewed herein.


The role of dietitian

Dietitians are not traditionally involved in the clinical care of patients with kidney stones. Though, dietitians offer many nutrition-focused services that may be of benefit to the urologist and the patient. Dietitians are able to analyze lab values independently, review medical records, and derive nutrition recommendations thereof. They can also support recommendations made by a urologist who has referred the patient to a dietitian. Diet modifications can influence urine levels: pH, citrate, oxalate, uric acid, magnesium, volume, calcium, and sodium - all having lithogenic associations. The focus on this review is the interrelationships between urine calcium, urine sodium, and the low-sodium diet.

Nutrition counseling and education are provided by dietitians which could increase patient adherence to recommended low-sodium diet. Dietitians can help guide patients from the grocery store to the dinner table to dining out to reading nutrition facts labels. Dietitians can also follow up with patients and provide the individualized support they need and answer questions. Effectiveness of the dietitian and patient relationship could be observed by urologist via improved urinalysis markers and decreased risk of kidney stone recurrence such as normalizing urine sodium and calcium through improved diet choices.


Statement of Purpose

The purpose of this literature review is to demonstrate the link between dietary sodium and risk for calcium-based stones as well as support dietary intervention to prevent recurrence. A quality improvement project is proposed for a dietary intervention which includes a dietitian-created low-sodium comprehensive guide customized for the calcium-based kidney stone patient as well as a sodium tracker. The PICOT (Population, Intervention, Comparison, Outcome, Timeframe of interest) statement is as follows: In the outpatient urology clinic for patients >18 yrs at risk for kidney stone recurrence (P), what is the effect of a provided low-sodium diet education tool (<2g Na/day) designed by a dietitian (I), compared to the current standard of providing no nutrition counseling by an RDN (C), on patient understanding of low-sodium diet and intent to follow dietary recommendations (assessed by pre/post-test) (O) over a 4 month time frame (T)?


Target population of interest are current urology outpatient clinic patients at risk for calcium-based kidney stone recurrence. “At risk” includes history of recurrence, 24hr-urinalysis markers such as high urine sodium/calcium, patients treated with thiazides as kidney stone medical management, or patients with a diagnosed calcium-based kidney stone. The overarching goal of the quality improvement project is to promote reduced risk of kidney stone recurrence.


Low-sodium diet educational intervention

The vision for the low-sodium educational tool for the kidney stone patient will include a booklet that details an explanation of sodium in the diet and its impact on kidney stones, low-sodium vs. high-sodium foods, table salt, low-sodium goals, how to read a nutrition label, alternative herbs/spices, and how to keep track of dietary sodium intake each day. The booklet is artfully designed by a graphic designer and will be able to adhere to the patient’s refrigerator via magnets fastened to the backside. The educational materials also include a magnetic dry erase board specifically for tracking daily sodium intake and log of total sodium for each day of the week. These items are to be featured on the patient’s refrigerator at the center of the cooking and eating area of the home. This encourages the span of the diet not to be short-lived and preventing the materials from becoming lost on a bookshelf in another room. And is designed to be handy and accessible at all times. Booklet can also be conveniently taken to the grocery store to be utilized as an on-the-go low-sodium resource for the patient.


Objectives

Objectives and effectiveness are measured one month after patient receives educational materials via pre- and post-questionnaire.

· Patient will increase knowledge and understanding of the low-sodium diet.

· Patient will increase intent to follow through on low-sodium diet.

· Patient’s readiness-to-change (Transtheoretical model) will be promoted.

Long-term objective: urologist goal for reducing stone recurrence risk of calcium-based nephrolithiasis will be enhanced.


Review of Literature

Method

Original and review articles were searched for and identified at the University of Central Missouri research library which included many databases such as MedLine and PubMed. Topic of research studies selected was on the association between sodium intake and kidney stone recurrence risk as well as educational interventions to increase diet adherence and address patient perceived barriers. Special interest was given to peer-reviewed journal articles published between 2011 and 2018 in order to examine the most recent evidence on the topic. These resources were integrated with the author’s knowledge and experiences surrounding nutrition and nephrolithiasis. Search terms included calcium-based kidney stones, low-sodium diet, hypernatriuria, and other relevant terms. MeSH (Medical Subject Headings) was analyzed for relevant terms such as “kidney stones” and “nephrolithiasis”. Filters were not used. Free search was found to be the most sensitive approach.


Calciuria and natriuria

Hypercalciuria is a significant factor for calcium-based stone formation within the kidney. Calcium binds readily to other molecules such as oxalate and phosphorus creating crystallization. Excessive salt intake directly correlates with increased urine sodium. Urine sodium and calcium excretion are associated as they are reabsorbed in the renal tubule at common sites. Therefore, dietary sodium and natriuria can influence an elevation in urine calcium creating an environment conducive to kidney stone formation risk. Thus, hypercalciuria is associated with dietary sodium (Nouvenne, A., et al, 2010).


Dietary sodium and kidney stone risk

Measuring sodium intake via dietary recall is notoriously difficult to precisely quantify. The current gold standard in assessing sodium intakes in multiple research study populations is the measurement of urine sodium from a 24-hr urinalysis. The mathematical equation to calculate dietary sodium using a 24-hr natriuria level is as follows whereas 1g salt = 17 mEq urine sodium:

Salt intake (g) = Na+excretion (mEq/24-hr urine) / 17


Results of a study that took place in an outpatient clinic of Renal Lithiasis Metabolism of Botucatu University Hospital and published in 2011 indicated lithiasic patients with hypercalciuria had higher sodium intake (calculated via natriuria) compared to patients with normal calciuria (p = <0.001). And both groups had no difference in calcium intake (Damasio, et al, 2011).


In a randomized control trial published in 2014, two groups of recurrent calcium-oxalate stone formers with hyperoxaluria were compared. One group followed a DASH/low-sodium diet (<2400mg Na/d) while the other group followed a low-oxalate diet and lithogenic risks were compared. While the DASH/low-sodium diet had increased urine oxalate levels compared to the low-oxalate intake group, the DASH/low-sodium group had a trend in decreased urine calcium-oxalate supersaturation (p = 0.08). Sodium intake was found to be positively associated with urine calcium. Since hypercalciuria is a lithogenic risk marker, lowering sodium intake can prove beneficial to lower urine calcium (Noori, et al, 2014).


Effectiveness of the low-sodium diet

During a randomized control trial in lithiasic participants, the effects of a low-sodium diet and water therapy compared to the control group of water therapy alone was examined. The water therapy was 2-3 L/day of a provided low-sodium electrolyte water beverage. All participants were calcium-oxalate stone formers with hypercalciuria. The low-sodium diet intervention included recommendations to eliminate added table salt to food and cooking, avoiding foods with high sodium content, and encouraging alternative herbs/spices for flavor. Written materials were included for instructions, high sodium foods, herbs/flavorings, and hidden sources of salt. After 3 months, 62% of low-sodium diet participants had normal urinary calcium compared 34% of the control group of water therapy alone (p = <0.001). The low-sodium diet participants on average had 70% decreased urinary sodium at follow up compared to baseline (Nouvenne, A., et al, 2010).


In a retrospective study, all urolithic patients received an intervention of dietary counseling to reduce risk of kidney stone recurrence. A 24-hr urinalysis was conducted before the intervention and afterwards at a follow-up. The diet counseling was individualized based on specific urinalysis results. Nutrition therapy included moderation of protein, decreased sodium, increased fruits/vegetables, and increased hydration. Patients with hypernatriuria were recommended a <1500 mg/d Na diet. The study found that obesity is associated with increased urine sodium and calcium. But that in the non-obese, obese, and superobese, a low-sodium diet was shown to be an effective intervention to improve urinalysis kidney stone risk markers including a statistically significant reduction in urinary sodium (p = <0.01) (Torricelli, F., De, S., Li, I., Sarkissian, C., & Monga, M., 2014).


A randomized control trial in south Asia examined chronic kidney disease participants’ adherence to the low-sodium diet. Two diet interventions were compared: one diet intervention was individualized with supportive strategies compared to the standard low-sodium diet protocol. The standard nutrition intervention was a general low-sodium handout that was not culturally specific. The intervention of interest was an individualized, culturally-sensitive nutrition education and counseling sessions. The individualized intervention provided a cooking class which included preparing traditional low-sodium Bangladeshi meals. This intervention was dietitian-led and families were welcome for support. Effectiveness was measured via 24-hr urinary sodium excretion before and six months after intervention. Compliance to the recommended diet was found to be improved. The individualized diet intervention had statistically significant results of lowered urine sodium excretion (baseline of 260 mmol/24hr decreasing to 122 mmol/24hr at 6 months) (p = <0.01) (Brito-Ashurst, I., et al, 2013).


Dietary sodium and associated behaviors and barriers

A higher-sodium diet often comprises of processed foods and low intake of fruits and vegetables. A study of Italian women with recurrent calcium-oxalate kidney calculi were examined for dietary habits compared to a control group of healthy women with no history of kidney calculi. The study confirmed through diet history analysis that the women with recurrent stone formation consumed more calories, processed foods, simple sugars, and salt, while consuming less fruits and vegetables. This study supports that the stone formers were more likely to follow a higher-sodium dietary pattern compared to the control group (Meschi, T., et al, 2012).


Behavioral patterns were also found to be associated with increased urine sodium and excess salt intake in a Japanese general-population study. The study found that salt intake increases with discretionary seasoning/condiment use and consumption of packaged noodle soups. The study also found that women who looked at food labels for sodium content when grocery shopping had statistically significant decrease in urine sodium (p = 0.03) (Uechi, K., Asakura, K., Sasaki, Y., Masayasu, S., & Sasaki, S., 2017).


A qualitative study of kidney disease patients and healthcare professionals from a Dutch medical center were split into focus groups. The objective was to explore perceived barriers and support strategies for promoting adherence to a low-sodium diet. Study results demonstrated patients’ desire for more information from healthcare providers in regards to their disease condition and diet recommendation. Also indicated as desirable in the healthcare setting was to help the patient to set personal sodium restriction goals, strengthen intrinsic motivators, and to provide nutrition information on the low-sodium diet while increasing social support and encouraging self-monitoring for the low-sodium diet (Meuleman, Y., et al, 2015).


Limitations

Many studies reviewed herein had both limited sample sizes and diversity in ethnicities. Even so, the variety of studies had similar outcomes. In the study by Nouvenne, A., et al that compared the low-sodium diet and water therapy to the control group of water therapy alone, only one 24-hr urinalysis was evaluated at baseline and one at follow up. Ideally, studies should have two non-consecutive 24-hr urinalyses at baseline and at follow up (Nouvenne, A., et al, 2010).


The study that compared the DASH/low-sodium diet with the low-oxalate diet on calcium-oxalate kidney stone formers defined low-sodium as <2400mg Na/day which may be considered moderate-sodium rather than low-sodium. A low-sodium diet of <1500 to 2000mg Na/day, depending on body mass, may possibly have been more effective at reducing kidney stone risk. More research is needed. Also, the intervention was a DASH diet which included low-sodium but had other independent variables as well such as increased intake of citrate foods (i.e fruit) which has also been shown to decrease kidney stone risk (Noori, et al, 2014).


Discussion

The PICOT quality improvement intervention proposed herein is an educational tool for the patient that includes a low-sodium booklet and daily sodium tracker that surpasses typical handouts in quality and content. The pre- and post-questionnaires are designed to measure patient knowledge and intent to adhere to the low-sodium diet as well as gauge patient readiness-to-change before the intervention and after. The educational tool is intended to expand patient understanding of their condition, the low-sodium diet, and provide information and tips necessary to increase likelihood of diet adherence.


The educational tool is specially designed by a dietitian and developed from the evidenced within this literature review. Results of the qualitative study by Meuleman, Y., et al, indicated patient need and desire for support strategies to aid in adherence to the low-sodium diet. Patients reported attaining limited information from healthcare professionals about condition and diet, and healthcare providers reported lack of time (Meuleman, Y., et al, 2015). The quality improvement educational tool provides both information on kidney stones and the low-sodium diet and the time required is to simply provide the materials to the patient. The materials are a resource the patient takes home with them and encourages self-direction and promotes patient/urologist goals to reduce risk of kidney stone recurrence.


Effective diet intervention in another study included recommendations to eliminate added table salt to food and cooking, avoiding foods with high sodium content, and encouraging alternative herbs/spices for flavor. Written materials were included for instructions, high sodium foods, herbs/flavorings, and hidden sources of salt (Nouvenne, A., et al, 2010). The educational tool can address the same effective diet interventions of this study featuring alternative herbs/spices, low-sodium vs. high-sodium food, and table salt usage. The Urech, et al study found the Japanese women who looked at food labels for sodium when grocery shopping had statistically significant decrease in urine sodium. Because of this finding, the educational tool includes label reading guidance (Uechi, K., et al, 2017).


Even though kidney stone recurrence and diet are strongly link, it is not current practice to involve dietitians in the urology outpatient clinic. Diet intervention for kidney stones is a recognized medical nutrition therapy but is currently underutilized. Effectiveness of the quality improvement intervention would benefit dietitians as it would begin to promote their involvement in urology departments. Dietitians can be an asset to the urology clinic as they are able to analyze results of a 24-hr urinalysis and individualize a diet therapy that meets the patient’s needs.


Urologists may find dietitians supportive to overall end goal of patient care in regards to reducing recurrence rate of kidney stones and increasing low-sodium diet compliance. The PICOT quality improvement project can be implemented at any urology clinic that sees patients with kidney stones. And improvement of kidney stone risk can be monitored via follow up 24hr-urinalysis and recurrence rates.


Most importantly, patients would benefit from diet education and an increased understanding of how their condition is associated with dietary sodium. After a person has experienced a painful kidney stone, diet intervention comes at a time of increased patient motivated to prevent recurrence. The educational tool booklet and sodium tracker is designed as a patient-centered approach to help reduce the risk of kidney stone recurrence.



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