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Table of Contents
ORIGINAL ARTICLE
Year : 2018  |  Volume : 29  |  Issue : 6  |  Page : 303-306

The impact of body mass index on the stone composition of 191 patients who received percutaneous nephrolithotomy in a single hospital


1 Department of Urology; Department of Urology, School of Medicine, College of Medicine; Graduate Institute of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
2 Department of Urology; Department of Urology, School of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan
3 Department of Urology; Department of Urology, School of Medicine, College of Medicine; Graduate Institute of Medicine, College of Medicine, Kaohsiung Medical University; Department of Urology, Kaohsiung Armed Forces General Hospital, Gangshan Branch, Kaohsiung, Taiwan
4 Department of Urology; Department of Urology, School of Medicine, College of Medicine; Graduate Institute of Medicine, College of Medicine, Kaohsiung Medical University; Department of Urology, Kaohsiung Municipal Ta-Tung Hospital, Kaohsiung, Taiwan

Date of Web Publication22-Nov-2018

Correspondence Address:
Ching-Chia Li
Department of Urology, Kaohsiung Medical University Hospital, No. 100, Tzyou 1st Road, Kaohsiung 807
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/UROS.UROS_16_18

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  Abstract 

Objective: We aimed to analyze the renal stone composition and evaluate the epidemiology of body mass index (BMI) and renal stones. Materials and Methods: We conducted a retrospective, single-center study of patients with large stones (ɮ cm) who underwent percutaneous nephrolithotomy for renal stones between 2010 and 2015. We performed stone analysis using stereomicroscopy and infrared spectroscopy to determine the chemical composition of these stones. Chi-square analysis was used to determine the relationship between BMI and renal stone formation. Results: We examined stones from 191 procedures. Among these stones, we classified 58.6% as having “pure” composition and 40.8% as having “mixed” composition. Most stones (68.1%) were composed of calcium oxalate monohydrate (COM), followed by carbonate apatite (50.8%), calcium oxalate dihydrate (COD) (36.6%), uric acid (14.1%), struvite (8.9%), ammonium hydrogen urate (2.1%), and brushite (1.0%). Chi-square analysis revealed that stones in obese patients (BMI >27 kg/m2) were more likely to contain COM (hazards ratio [HR]: 1.66, 95% confidence interval [CI]: 1.10–2.49, P = 0.008) and less COD (HR: 0.77, 95% CI: 0.60–0.99, P = 0.049) than stones in nonobese patients (BMI ≦27 kg/m2). Conclusion: COM is the most frequently occurring compound in renal stones. Obese patients were significantly more likely to develop COM-containing renal stones. One must consider these factors when choosing a treatment modality.

Keywords: Percutaneous nephrolithotomy, renal stones, stone analysis


How to cite this article:
Kang LM, Lu YM, Cheng WT, Chien TM, Chou YH, Wu WJ, Li CC. The impact of body mass index on the stone composition of 191 patients who received percutaneous nephrolithotomy in a single hospital. Urol Sci 2018;29:303-6

How to cite this URL:
Kang LM, Lu YM, Cheng WT, Chien TM, Chou YH, Wu WJ, Li CC. The impact of body mass index on the stone composition of 191 patients who received percutaneous nephrolithotomy in a single hospital. Urol Sci [serial online] 2018 [cited 2018 Dec 13];29:303-6. Available from: http://www.e-urol-sci.com/text.asp?2018/29/6/303/240951


  Introduction Top


Urolithiasis is a common disease, and the overall probability of stone formation differs considerably worldwide. Adults in the Western Hemisphere experience higher rates of urolithiasis than those in the Eastern Hemisphere (Asia [1%–5%], Europe [5%–9%], the United States [13%–15%], and Canada [12%]).[1] Stone recurrence rates are approximately 10% within 1 year, 35% within 5 years, and 50% within 10 years.[1] Lee et al. reported an incidence rate of 8.9% and a stone recurrence rate of about 63.3% in the Taiwanese population.[2] The highest risk has been reported in some Asian countries.[3] In the pathogenesis of urolithiasis, living environment, socioeconomic status, and working conditions may play key roles.[2]

Stone composition analysis is an important step in the metabolic evaluation of kidney stones. Infrared spectroscopy has been used to identify chemical compounds since the beginning of the 20th century; Beischer first applied it to urinary calculi in 1995.[4]

Meanwhile, we know little about the relationship between obesity and urolithiasis and have no insight into the relationship between increasing degrees of obesity and the prevalence of urolithiasis.[5] Consumption of an unbalanced diet (excessive protein, fat, calcium, and oxalate intake), the presence of an endocrine metabolic disorder, or influence of an exposure–response relationship, which are factors commonly associated with obesity, also influence stone disease. A better understanding of the relationship between obesity and urolithiasis may ultimately improve therapies for patients that develop stones.


  Materials and Methods Top


Between May 2010 and August 2015, we studied 191 patients with renal stones (age, 27–90 years) who underwent percutaneous nephrolithotomy (PCNL) operations. Our inclusion criteria and surgical indications were based on the European Association of Urology guidelines, which recommend PCNL for the treatment of renal stones >2 cm in size and lower pole stones >1.5 cm in size.[6] We analyzed stones by performing stereomicroscopy and infrared spectroscopy. We classified stones composed of a single component as “pure” stones and those composed of more than one component as “mixed” stones. We also performed Chi-square and Student's t-test analyses to determine whether there is a relationship between body mass index (BMI) and renal stone formation. The cutoff value of obesity was recommended by previous report using the Taiwanese definition (BMI <24, 24–27, and >27).[7]


  Results Top


We analyzed the stones removed from 191 patients with urolithiasis by performing stereomicroscopy and infrared spectroscopy. The average age of the patients was 57.8 ± 14.0 years and 75.0% of the patients were older than 50 years; there were 116 (60.7%) men. Sixty (31.4%) patients were diagnosed with hypertension, 36 (18.8%) were diagnosed with diabetes mellitus, and 42 (22.0%) were diagnosed with dyslipidemia before the operation. The average BMI of patients was 26.3 ± 4.1 kg/m2 (range, 17.6–35.6 kg/m2), and 45.0% of the patients exhibited a BMI <27 kg/m2. Of these patients, 112 (58.6%) harbored pure stones, and 78 (40.8%) harbored mixed stones [Table 1]. Among renal stones, the component distributions were as follows: 68.1%, calcium oxalate monohydrate (COM); 50.8%, carbonate apatite; 36.6%, calcium oxalate dihydrate (COD); 14.1%, uric acid; 8.9%, struvite; 2.1%, ammonium hydrogen urate; and 1.0%, brushite [Table 2].
Table 1: Composition of pure and mixed stones by body mass index groups

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Table 2: Basic demographic data and composition of renal stones (n=191)

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The ratio of pure-to-mixed renal stones was 62.0%–37.1% in nonobese patients (BMI ≤27 kg/m2) and 54.7%–45.3% in obese patients (BMI >27 kg/m2). Therefore, the ratio of pure-to-mixed stones was not different between nonobese and obese patients (P = 0.251) [Table 1]. Our study confirms that kidney stones are formed mainly of COM. In the obese group (BMI >27 kg/m2), the proportion of renal stones composed of COM was 77.9%, whereas in the nonobese group (BMI ≦27 kg/m2), this proportion was 60.0% (hazards ratio [HR]: 1.66, 95% confidence interval [CI]: 1.10–2.49, P = 0.008). In contrast, stones in obese patients contained less COD (HR: 0.77, 95% CI: 0.60–0.99, P = 0.049) than stones in nonobese patients. A Student's t-test analysis revealed no significant relationship between increased BMI and renal stone formation.

Our analysis revealed a significant, positive relationship between high BMI and the formation of COM-containing renal stones. Obese patients were significantly more likely to develop COM-containing renal stones than were nonobese patients (P = 0.008).


  Discussion Top


The etiology of stone formation is universally believed to be multifactorial, including gender, race, age, geography, climate, occupation, weight, and hormonal factors. It has been established that interactions among these etiological factors lead to the formation of renal calculi, due to excessive excretion of stone-forming components into an environment that enhances precipitation and crystal accumulation.[8]

In our study, the percentage frequency of stone compounds in nonobese patients was approximately equal (60%) to the global distribution of 50%–56% COM-containing stones. A higher percentage of COM (77.9%) was noted in obese patients. The prevalence of uric acid stone is not different in obese patients than that of in the general stone-forming patient population (7%–10%)[9] [Table 3].
Table 3: Renal stone composition by body mass index groups

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Lee et al. used infrared spectroscopy to analyze 728 urinary tract calculi and established that in mixed renal stones, the most common component of pure urinary calculi was calcium oxalate, followed by calcium phosphate, uric acid, and struvite.[4] The prevalence is similar comparing to our results.

The BMI is a value derived from the weight and height of an individual. It is defined as the body weight in kilograms divided by the square of the body height in meters and is universally expressed in units of kg/m2 The BMI can be used to classify individuals as underweight, overweight, or obese. There is some debate as to whether there is a possible need to develop different BMI cutoff points for different ethnic groups. However, the adjusted 2004 World Health Organization recommendations for commonly accepted BMI ranges (kg/m2) are as follows: underweight, under 18.5; normal weight, 18.5–25; overweight, 25–30; and obese, over 30. The definition of obesity in Taiwan by BMI (ᡓ kg/m2) is different from that in Western countries.[7]

Obesity has been linked to some types of stones. The prevalence of urolithiasis correlates directly with weight and BMI. The prevalence of COM-containing stones and uric acid stones are much higher in obese patients in previous report.[8] We confirmed that a higher percentage of COM was noted in obese patients but not in uric acid stones.

Previous studies have demonstrated that the prevalence of stones increases as BMI increases.[8] Insulin resistance and low urine pH, both of which are consequences of obesity, suggest an association between hyperinsulinemia and hypercalciuria. Insulin resistance may reduce urinary citrate excretion and increase calcium excretion, and high BMI is associated with increased urinary oxalate excretion. These mechanisms may drive a common pathophysiology underlying calcium oxalate-containing stone formation. Moreover, sodium and calcium excretion are linked. Increased animal protein intake can further increase urinary levels of calcium and oxalate.[10] Obese individuals may ingest excessive protein, salt (sodium), and oxalate, which increases the production of urinary calcium oxalate-containing stones.[10] Fat malabsorption results in saponification of fatty acids with divalent calcium cations, reducing the calcium oxalate composition in the urinary tract.

In our study, renal stone composition analysis indicated an increased rate of COM-containing stone formation in patients with high BMIs. We found that COM-containing stones were most prevalent compared with calcium phosphate-, struvite-, or uric acid-containing stones in patients with high BMIs. Some studies have determined that obesity is associated with high serum and urinary levels of chemical components such as calcium, phosphate, citrate, oxalate, and uric acid.[5]

COM-containing stone hardness values range from 15.3 to 64.2 HV, as they are harder than stones containing COD, hydroxyapatite, magnesium ammonium phosphate, and uric acid.[11] Harder or larger stones may require lengthier PCNL procedures, especially in obese patients, increasing the difficulty and risk of these operations.

COM is a major constituent of renal stones, especially in obese patients. COM comprises hard stones that may require substantial time to remove during an operation. Postoperative assessment of stone composition may inform the selection of a treatment modality and diet control methods for patients with renal stones.


  Conclusion Top


Our analysis revealed a significant, positive relationship between obesity and the formation of COM-containing renal stones. One must consider these factors when choosing a treatment modality.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Ramello A, Vitale C, Marangella M. Epidemiology of nephrolithiasis. J Nephrol 2000;13:65-70.  Back to cited text no. 1
    
2.
Lee YH, Chang LS, Chen MT, Huang JK. The epidemiology of urolithiasis in Southern Taiwan. J Urol ROC 1994;5:1-7.  Back to cited text no. 2
    
3.
Robertson WG, Hughes H. Epidemiology of urinary stone disease in Saudi Arabia. In: Ryall R, Bais R, Marshall VR, Rofe AM, Smith LH, Walker VR, editors. Urolithiasis 2. New York, London: Plenum Press; 1994. p. 453-5.  Back to cited text no. 3
    
4.
Lee YH, Chen MT, Huang JK, Chang LS. Analysis of urinary calculi by infrared spectroscopy. Chin Med J (Taipei) 1990;45:157-65.  Back to cited text no. 4
    
5.
Semins MJ, Shore AD, Makary MA, Magnuson T, Johns R, Matlaga BR. The association of increasing body mass index and kidney stone disease. J Urol 2010;183:571-5.  Back to cited text no. 5
    
6.
Türk C, Petřík A, Sarica K, Seitz C, Skolarikos A, Straub M, et al. EAU guidelines on interventional treatment for urolithiasis. Eur Urol 2016;69:475-82.  Back to cited text no. 6
    
7.
Pan WH, Flegal KM, Chang HY, Yeh WT, Yeh CJ, Lee WC. Body mass index and obesity-related metabolic disorders in Taiwanese and US whites and blacks: Implications for definitions of overweight and obesity for Asians. Am J Clin Nutr 2004;79:31-9.  Back to cited text no. 7
    
8.
Mosli HA, Mosli HH, Kamal WK. Kidney stone composition in overweight and obese patients: A preliminary report. Res Rep Urol 2013;5:11-5.  Back to cited text no. 8
    
9.
Ngo TC, Assimos DG. Uric acid nephrolithiasis: Recent progress and future directions. Rev Urol 2007;9:17-27.  Back to cited text no. 9
    
10.
Robertson WG, Heyburn PJ, Peacock M, Hanes FA, Swaminathan R. The effect of high animal protein intake on the risk of calcium stone-formation in the urinary tract. Clin Sci (Lond) 1979;57:285-8.  Back to cited text no. 10
    
11.
Bouropoulos N, Mouzakis DE, Bithelis G, Liatsikos E. Vickers hardness studies of calcium oxalate monohydrate and brushite urinary stones. J Endourol 2006;20:59-63.  Back to cited text no. 11
    



 
 
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