Anthony Kai-ching Hau*,,
Tze Hoi Kwan and
Philip Kam-tao Li*
* Department of Medicine and Therapeutics, Prince of Wales Hospital, Chinese University of Hong Kong, and Department of Medicine, Tuen Mun Hospital, Hong Kong
Correspondence: Prof. Philip Kam-tao Li, Department of Medicine and Therapeutics, Prince of Wales Hospital, Chinese University of Hong Kong, Hong Kong. Phone: 852-2632-3616; Fax: 852-2648-9864; E-mail: philipli{at}cuhk.edu.hk
The toxicity of melamine caught the attention of physiciansas a result of a recent spate of renal injury after exposureto melamine-tainted milk in China. Melamine is an organic nitrogenouscompound used in the production of plastics, dyes, fertilizers,and fabrics. In the current incident, melamine was added tomilk to elevate falsely assay results for protein content. Avariety of toxic effects from melamine, including nephrolithiasis,chronic kidney inflammation, and bladder carcinoma, all havebeen studied in animals. We review here the epidemiology, clinicalfeatures, and investigative findings concerning the only outbreakof melamine poisoning in humans. We also examine the renal toxicitiesof melamine and cyanuric acid—a by-product of its synthesis—andthe associated risk factors on exposure and provide guidanceon levels in foods.
Melamine was an unknown substance to nephrologists until today.Although studied as diuretics 50 yr ago,1 it never came intoclinical practice with the development of other potent and safermeans for naturesis. This compound became headline news recentlyafter the occurrence of an outbreak of urinary stones in infantsand children consuming melamine-tainted milk in China. The nephrotoxiceffects of melamine now warrant the attention of nephrologists,pediatricians, urologists, and radiologists.
Melamine is an organic base commercially synthesized from ureawith an intermediate step producing cyanic acid (Table 1). Thereaction also results in the formation of other byproducts,including cyanuric acid, ammeline, and ammelide (Figure 1).Melamine is 66% nitrogen by molecular weight. It is combinedwith formaldehyde by industry to produce melamine resin, a verydurable thermosetting plastic, and melamine foam, a polymericcleanser. Other commercial products containing melamine includecountertops, dry erase boards, fabrics, glues, housewares, andflame retardants. Melamine is also one of the major componentsin pigment yellow 150, which is a colorant for inks and plastics.It is also a derivative of arsenical drugs, and Melarsoprolis one such drug used for the treatment of African trypanosomiasis.Beginning in 1958, melamine has been used in fertilizers andis occasionally offered as a nonprotein nitrogenous source forfeeding cattle. Subsequently, however, it was shown to be anineffective nonprotein nitrogen source for animals because ofits slow hydrolysis in ruminants (Table 1).
Figure 1. Chemical structure of melamine: There are six atoms of nitrogen in a molecule of melamine, constituting 66% of molecular weight. International Union of Pure and Applied Chemistry name: 1,3,5-triazine-2,4,6-triamine; other names: cyanurotriamide, cyanurotriamine, or cyanuramide.
Melamine was recently added to milk to elevate falsely its proteincontent by the Kjeldahl method (Figure 2). The first step ofthe Kjeldahl method detects not only nitrogen in protein butalso nitrogen in all organic nitrogenous compounds, includingmelamine. In essence, nitrogen in the assay reacts with sulfuricacid to form ammonium sulfate. Small amounts of sodium hydroxideare then added, and the ammonium salt is converted back to ammonia.Ammonia is reacted with sulfuric acid again, and the remainingacid is quantified by adding sodium carbonate with a methylorange pH indicator to quantify the amount of ammonium salt(Figure 2).
Figure 2. Kjeldahl method, a universally used quantitative method for assessing protein content in food.
The addition of 1 g of melamine to 1 L of milk falsely increasesthe protein content by 0.4%. When melamine is dissolved at theroom temperature, 3.1 g of melamine can be dissolved in waterwithout forming precipitate, and protein content will falselyincrease by 1.2%. This can roughly lead to an overestimationof the protein content in liquid milk by 30%. In case of milkpowder, the amount of melamine added can be greater becauseof its greater solubility at higher temperature when addingwarm water.
Melamine is not metabolized by animals and is rapidly eliminatedin the urine. More than 90% of ingested melamine is excretedwithin 24 h. The half-life of melamine excretion in animal studiesranged from 2.7 to 4.04 h.2,3 The levels of melamine in blood,liver, or plasma are similar.2 The volume of distribution ofmelamine in pigs is 0.61 ± 0.04 L/kg and is not extensivelydistributed to most organ tissues.3 This study in pigs alsoconfirms the excretion of melamine best fits a one-compartmentmodel in which melamine has a half-life of 4.04 ± 0.37h and a renal clearance of 0.11 ± 0.01 L/h per kg (approximately27 ml/min).3
Studies concerning the toxicity of melamine taken orally inhumans are nonexistent. Toxicity data mainly come from studiesin sheep, cat, dog, mice, and rat. Toxicity can be classifiedas acute or chronic. The most common toxicity is renal toxicity,which is also the area of most concern to nephrologists.
Acute Toxicity in Animals
Melamine has a low acute toxicity; the LD50, the lethal doseof a compound that would result in death in 50% of the testedanimals, for melamine in rats is 3.161 g/kg body wt.4 Acutedermal toxicity in rabbits presents when the exposure is >1g/kg body wt.5
Direct contact results in skin irritation and eye irritation,and inhalation causes respiratory tract irritation. Oral ingestionaffects the digestive tract, presenting as nausea, vomiting,and diarrhea.6
Acute renal toxicity is best illustrated by a sheep study donein 1953.7 When sheep were fed a single 100-g oral dose of melamine,all of them died by the 11th day. When a daily dose of 25 to50 g of melamine was given for 7 to 9 d, again all of the sheepdied. They experienced acute renal failure with elevation inblood urea nitrogen and creatinine followed by oliguria precedingdeath. Postmortem examinations revealed crystals in the kidneytubules, nephrosis, and hemorrhagic cystitis. When the exposurewas reduced to 10 g/d melamine for 16 to 31 d, two thirds ofthe sheep died. Again, they experienced loss of appetite, oliguria,and elevation of blood urea nitrogen and creatinine before theirdeath. Postmortem analyses under these conditions also revealedcrystal deposition in the kidney.
Chronic Toxicity in Animals
Long-term exposure to melamine reduces fertility and resultsin fetal toxicity in animal studies. The classical descriptionof urinary changes relating to chronic exposure to melaminedates back to 1953, when Hazleton Laboratories performed anexperiment in dogs by feeding them 3% (30,000 ppm) melamineby weight in food for 1 yr.8 Distinctive urinary changes werenoted, including reduction in specific gravity, increases inurine output, melamine crystalluria, and proteinuria with microscopichematuria. The most commonly reported chronic renal toxicityis stone formation. Uncertainty exists as to whether melamineresults in any chronic damage to kidney other than aggressivestone formation.
Urinary Stone Formation
Most animal studies concerning subacute or chronic melamineexposure reveal stone formation. Incidence ranges from 5 to100% depending on the dosage of melamine, gender, and amountof water intake.4,9 The composition of stones is either a combinationof melamine and uric acid or melamine in a matrix of protein,uric acid, and phosphate.10
The incidence of stone formation increases with daily exposureto melamine. The lowest possible daily dosage of melamine thatresults in bladder stone formation is as low as 750 ppm for13 wk.4 The dose-response curve for the induction of urolithiasisin weanling rats is extremely steep (Figure 3). This suggestsformation of calculi occurs in supersaturated urine but notin urine that is undersaturated.11 A study performed on B6C3F1mice with exposure to 13 wk of melamine showed that male miceare much more affected than females despite similar body weights.Relative risk of stone formation in males is twice as greatas in females.4
Figure 3. Chance of bladder stone formation in male F-344 rats after 13 wk of exposure to melamine. Figure drawn using data from Melnick et al.4
A study of exposure to melamine for 36 wk in rats demonstratedthe incidence of stone formation was reduced by increasing theamount of fluid intake.9 In this study, one group of rats, thecontrol group, was fed diets containing 1% melamine. The othertwo groups of rats received feed containing 1% melamine togetherwith 5 or 10% of sodium chloride, respectively. The increasein salt content in the diet resulted in doubling and triplingthe amount of fluid intake in groups taking 5 and 10% sodiumchloride when compared with control. Incidence of stone formationwas reduced from 37% in control group to 11% in the group taking5% sodium chloride and to 5% in the group taking 10% sodiumchloride.
The toxicity of melamine came to our attention in Hong Kongbecause of a recent outbreak of urinary stones in children whoconsumed melamine-tainted milk in China. On September 11, 2008,Xinhua News Agency in China reported an outbreak of urinarystones in children younger than 3 yr.12,13 The outbreak wasinitially linked to consumption of melamine-tainted milk powderfrom the Sanlu Group, a leading dairy producer in China. Allof the children affected had an exposure to the tainted milkfor approximately 3 to 6 mo before the onset of stones. Later,the contamination was found in other brands of dairy product.The highest content of melamine was in Sanlu milk powder, upto 2.563 g/kg powder, whereas melamine in the other brands rangedfrom 0.090 to 619 mg/kg.14
The official data released by the Ministry of Health of thePeople's Republic of China on September 21, 2008, stated thata total of 52,857 children had received treatment for melamine-taintedmilk13; 99.2% of the children were younger than 3 yr, althoughmore children who were older than 3 yr were reported afterward.Some of the children were asymptomatic; however, most symptomsincluded irritability, dysuria, difficulty in urination, renalcolic, hematuria, or stone passage.15 Hypertension, edema, oroliguria also occurred in more severe cases. Urinalyses of exposedchildren revealed microscopic hematuria with or without proteinuria.Melamine can be detected in the urine of the affected childrenby biochemical test or the presence of fan-shaped crystals.
Analyses of stone composition mainly demonstrated melamine anduric acid.15,16 Stones were radiolucent, and plain x-ray filmsfailed to show their presence. Ultrasonography or computed tomographyscan can reveal the presence of stones and detect whether thereis any obstruction. In view of the large radiation dose of computedtomography scans for children, ultrasonography is the preferredfirst-line investigation.
Melamine stones in humans are characteristic. They usually occurbilaterally, and multiple stones are often present.15 The acousticshadow of stone can be absent. They are soft in nature and canbe broken up easily. Most of the stones are usually <1 cmin diameter. Stones that are <4 mm in diameter can spontaneouslypass to the bladder with adequate hydration. When stones are>4 mm or show evidence of obstruction, medical treatmentwith hydration and follow-up ultrasonography is the first-lineattempt at treatment. If conservative medical therapy fails,surgical drainage and removal will be needed. Acute renal failureoccurred in 2.5% of the cases.16 Mortality was recorded in fourcases.17
Uncertainty exists as to whether melamine causes direct renaltoxicity on long-term exposure; however, in studies in whichno chronic toxicity was observed, the dosage of melamine wasusually lower.18 Two studies specifically investigated the toxicityof melamine by looking at postmortem sections of kidneys inanimals. The first study from Melnick et al.4 observed a significantincrease in chronic inflammation in the kidneys of female ratsexposed to melamine for 103 wk compared with controls (82% inrats fed a diet containing melamine at 9000 ppm versus 34% inthose on diets containing melamine at 4500 ppm versus 8% incontrol; P 0.01). The observed chronic inflammation in thisstudy could not be attributed to stone formation, because nostones were detected in the urinary tract. The male rats showeda slightly increased incidence of chronic inflammation, butthis was not statistically insignificant.
The second study from Ogasawara et al.9 demonstrated ischemicchanges in the renal cortex of rats (focal lesion of fibrosis,inflammatory cell infiltration, and renal tubular regeneration)after feeding on melamine for 36 wk. Ischemic changes occurredin 100% of rats fed diets containing 3% melamine by food weightand occurred in only 5% of rats when fed diets containing 1%melamine.
Melamine Toxicity Combined with Cyanuric Acid
A widely known epidemic of melamine poisoning is the infamous"pet food–induced nephrotoxicity in North America" in2007.19 In March 2007, numerous cases of acute renal failurein dogs and cats were associated with the ingestion of a varietyof dog and cat pet foods. One of the contaminants was melamine,which was added for the same reason as in the recent milk exposurein humans: To give a falsely high protein content. In this petoutbreak, not only was melamine present, but also another toxiccompound—cyanuric acid–was a contaminate, givingrise to a very high mortality in these animals.
Cyanuric acid (s-triazine-2,4,6-triol) is structurally relatedto melamine (Figure 4A). It is used as a stabilizer in outdoorswimming pools and hot tubs to minimize the decomposition ofhypochlorous acid by light.20 How cyanuric acid got into thepet food is unknown. It could have been added intentionallyor remained as a contaminant during melamine synthesis, becausecyanuric acid is a byproduct.
Figure 4. (A) Molecular structure of cyanuric acid, noted for its structural similarity with melamine. (B) Melamine can interact with the isomeric form of cyanuric acid to form melamine cyanurate, explaining the increase in stone formation and toxicity.
Studies on the toxicity of cyanuric acid are limited, but itis likely to behave as melamine as a result of its structuralsimilarity. The subacute feeding of sodium cyanurate at 700or 2200 mg/kg in rats resulted in bladder calculi and some associatedbladder epithelial changes, respectively.20 No other adverseeffects were noted.
Melamine, however, can interact with the isomeric form of cyanuricacid to create crystals (Figure 4B). Melamine combined withcyanuric acid results in acute renal failure in cats within48 h after ingestion. The toxic dosage in their diet is as lowas 0.2% melamine and 0.2% cyanuric acid in contrast to no evidenceof renal failure in groups taking either melamine or cyanuricacid alone up to 1% in their diet for 10 d.21 Urine analysesfrom affected cats revealed the presence of amorphous or, insome cases, fan-shaped (Figure 5A) birefringent crystals. Thecrystals were a combination of melamine and cyanuric acid.22Cross-sections of the kidney demonstrated severe renal interstitialedema and hemorrhage at the corticomedullary junction. Histopathologicfindings were limited to the kidneys, and numerous crystalswere found within the distal nephrons associated with tubularepithelial necrosis and regeneration (Figure 5B).22 In chroniccases, lymphoplasmacytic or granulomatous tubulointerstitialinflammation and fibrosis were found and the associated crystalswere larger.23,24 This toxicity was not limited to cats. Similarnephrotoxicity was observed when a combination of melamine andcyanuric acid was given to fish and pigs.25 The toxicity issize dependent, with cats affected more than dogs and smalldogs affected more than large dogs.23,25 The mortality of combinedmelamine and cyanuric acid is as high as 74% in dogs and 61%in cats.19
Figure 5. Histologic diagnosis of melamine-associated renal failure based on renal crystal characteristics. (A) Dilated distal cat tubule contains clusters of round green melamine/cyanuric acid crystals with radiating spokes and concentric striations (arrow). Surrounding proximal tubules appear unaffected (hematoxylin and eosin; bar = 45 µm). (B) Dilated distal cat tubule contains fragmented or globular dense green melamine/cyanuric acid crystals (long arrows). Note attenuation of the lining epithelium with wide separation of nuclei (short arrow) and mitotic figure (arrowhead) indicative of tubular epithelial necrosis and regeneration (hematoxylin and eosin; bar = 45 µm). Reprinted from Brown et al.,22 with permission from the American Association of Veterinary Laboratory Diagnosticians.
There are no data concerning the carcinogenicity of melaminein humans. The carcinogenicity in animals was determined fromstudies in rats and mice. Exposure produces urinary bladderand ureteral transitional cell carcinomas in male rats but onlyurinary bladder hyperplasia in male mice. Female rats or micedid not have carcinoma, but transitional cell papillomas werefound in female rats.4 The occurrence of urinary bladder tumorsin male rats correlates well with stone formation and exposureto high dosages. A similar dosage-dependent relationship wasconfirmed in another study using male rats.9 The administrationof sodium chloride to increase fluid intake and urinary outputreduces the prevalence of stone and tumor occurrence.
There is no evidence that melamine undergoes biotransformation.Mutagenesis of melamine was not observed in studies of exposureto Salmonella typhimurium and Drosophila melanogaster.26,27The urinary bladder tumors seen in male rats exposed to highdosages of melamine seem to be produced by a non–DNA-reactivemechanism involving epithelial hyperplasia secondary to thepresence of melamine-containing bladder stones. These studiesconcluded that bladder tumors would not occur in rodents unlessexposed to dosages that result in bladder stones.
Melamine has been classified as a group 3 carcinogenic riskby World Health Organization,28 meaning that melamine is notclassifiable as to its carcinogenicity in humans, referringto the fact that the evidence of carcinogenicity is inadequatein humans and inadequate or limited in experimental animals.
Recommendations on the safety limits of melamine in food comefrom animal studies. The no observed effect level (NOEL) ofmelamine in studies in rodents is 63 mg/kg body wt per d. Whenthis dosage is converted to a human safety limit, it is referredto as tolerable daily intake (TDI). TDI is usually defined as1% of NOEL in view of the need for a tighter safety limit inhumans; therefore, the TDI in humans may be equivalent to 0.63mg/kg body wt per d, and this is why the recommendation fromthe Food and Drug Administration gives a safety limit of exposureto melamine and its structural analogues to be <0.63 mg/kgper d.29 The European Food Safety Authority, however, recommendsa daily exposure of melamine and its structural analogues be<0.5 mg/kg per d.30 After reviewing the cases in China, childrenwho are younger than 3 yr seem more susceptible to the toxiceffects of melamine. An additional safety factor was thus introducedto protect this most vulnerable group; that is, children whoare younger than 3 yr. Consequently, the latest TDI was furtherreduced to 0.32 mg/kg per d.31 Of course, in determining exposure,one has to take into account the average body weight of thetarget population and the amount of respective food eaten perday to calculate the tolerable exposure of melamine per personeach day.
With the best available evidence in human exposures and animalstudies, we conclude several points regarding the toxicity ofmelamine: High-dosage melamine will result in urinary stones,crystalluria, and acute renal failure in both humans and animals;stone formation is likely enhanced by smaller body size, higherdosage of melamine, and smaller amounts of fluid intake; studiesin animals show that males are more affected than females; toxicityof melamine is further aggravated by the presence of other impuritiesassociated with melamine synthesis, particularly cyanuric acid;tubular damage with obstruction from crystals and chronic inflammationof kidney can occur; and toxicity may not be limited to stoneformation in animal studies if melamine is present in high dosagesor in combination with cyanuric acid.
Lipschitz WL, Hadidian Z: Amides, amines and related compounds as diuretics.
J Pharmacol Exp Ther 81
: 84
–94, 1944[Abstract/Free Full Text]
Mast RW, Jeffcoat AR, Sadler BM, Kraska RC, Friedman MA: Metabolism, disposition and excretion of [14C]melamine in male Fischer 344 rats.
Food Chem Toxicol 21
: 807
–810, 1983[CrossRef][Medline]
Baynes RE, Smith G, Mason SE, Barrett E, Barlow BM, Riviere JE: Pharmacokinetics of melamine in pigs following intravenous administration.
Food Chem Toxicol 46
: 1196
–1200, 2008[CrossRef][Medline]
Melnick RL, Boorman GA, Haseman JK, Montali RJ, Huff J: Urolithiasis and bladder carcinogenicity of melamine in rodents.
Toxicol Appl Pharmacol 72
: 292
–303, 1984[CrossRef][Medline]
Screening Information Data Set (SIDS) for High Production Volume Chemicals, Organisation for Economics Cooperation and Development, United Nations Environment Programme (UNEP) Publications, Nairobi, Kenya, June 2002. Available at http://www.inchem.org/documents/sids/sids/108781.pdf. Accessed December 18, 2008
Jeong W, Do SH, Jeong D: Canine renal failure syndrome in three dogs.
J Vet Sci 7
: 299
–301, 2006[Medline]
Clark R: Melamine crystalluria in sheep.
J S Afr Vet Med Assoc 37
: 349
–351, 1966
Ogasawara H, Imaida K, Ishiwata H: Urinary bladder carcinogenesis induced by melamine in F344 male rats: Correlation between carcinogenicity and urolith formation.
Carcinogenesis 16
: 2773
–2777, 1995[Abstract/Free Full Text]
World Health Organization: Melamine and Cyanuric Acid: Toxicity, Preliminary Risk Assessment and Guidance on Levels in Food. World Health Organization: September 25, 2008. Available at: http://www.who.int/foodsafety/fs_management/Melamine.pdf. Accessed December 18, 2008
Heck HD, Tyl RW: The induction of bladder stones by terephthalic acid, dimethyl terephthalate, and melamine (2,4,6-triamino-s-triazine) and its relevance to risk assessment.
Regul Toxicol Pharmacol 5
: 294
–313, 1985[CrossRef][Medline]
Lipschitz WL, Stokey E: The mode of action of three new diuretics melamine, adenine and formoguanamine.
J Pharmacol Exp Ther 83
: 235
–249, 1945[Abstract/Free Full Text]
Burns K: Researchers examine contaminants in food, deaths of pets: Survey, case definition, studies implicate combination of melamine and cyanuric acid.
J Am Vet Med Assoc 231
: 1632
–1644, 2007[CrossRef]
Hammond BG, Barbee SJ, Inoue T: A review of toxicology studies on cyanurate and its chlorinated derivatives.
Environ Health Perspect 69
: 287
–292, 1986[CrossRef][Medline]
Puschner B, Poppenga RH, Lowenstine LJ, Filigenzi MS: Assessment of melamine and cyanuric acid toxicity in cats.
J Vet Diagn Invest 19
: 616
–624, 2007[Abstract/Free Full Text]
Brown CA, Jeong KS, Poppenga, Robert HP, Puschner B, Miller DM, Ellis AE, Kang Kl, Sum S, Cistola AM, Brown SA: Outbreaks of renal failure associated with melamine and cyanuric acid in dogs and cats in 2004 and 2007.
J Vet Diagn Invest 19
: 525
–531, 2007[Abstract/Free Full Text]
Dobson RL, Motlagh S, Quijano M, Cambron RT, Baker TR, Pullen AM, Regg BT, Bigalow-Kern AS, Vennard T, Fix A, Reimschussel R, Overmann G, Shan Y, Daston GP: Identification and characterization of toxicity of contaminants in pet food leading to an outbreak of renal toxicity in cats and dogs.
Toxicol Sci 108
: 251
–262, 2008
Cianciolo RE, Bischoff K, Ebel JG, Van Winkle TJ, Goldstein RE, Serfilippi LM: Clinicopathologic, histologic, and toxicologic findings in 70 cats inadvertently exposed to pet food contaminated with melamine and cyanuric acid.
J Am Vet Med Assoc 233
: 729
–737, 2008[CrossRef][Medline]
Reimschuessel R, Gieseker CM, Miller RA, Ward J, Boehmer J, Rummel N, Heller DN, Nochetto C, de Alwis GK, Bataller N, Andersen WC, Turnipseed SB, Karbiwnyk CM, Satzger RD, Crowe JB, Wilber NR, Reinhard MK, Roberts JF, Witkowski MR: Evaluation of the renal effects of experimental feeding of melamine and cyanuric acid to fish and pigs.
Am J Vet Res 69
: 1217
–1228, 2008[CrossRef][Medline]
Some chemicals that cause tumors of the kidney or urinary bladder in rodents and some other substances. In:
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol 73, Lyon, World Health Organization, 1999
, pp. 329
–338
Cardy RF, Manus AG, Gordon EB, Fizgerald J, Moe JG, Montali RJ, Johnston CD, Rodwin M: Carincogenesis Bioassay of Melamine in F344/N Rats and B6C3F1 Mice (Feed Study), National Toxicology Program, NTP-81-86, NIH Publication No. 83-2501, Bethesda, Maryland, National Institutes of Health, 1983. Available at http://ntp.niehs.nih.gov/ntp/htdocs/LT_rpts/tr245.pdf. Accessed December 18, 2008
Preamble. In:
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Lyon, World Health Organization, 2006
, pp. 20
–22
Interim Melamine and Analogues Safety/Risk Assessment, US Food and Drug Administration, May 25, 2007. Available at: http://www.cfsan.fda.gov/%7Edms/melamra.html. Accessed December 18, 2008
EFSA'S Provisional Statement on a request from the European Commission related to melamine and structurally related compound such as cyanuric acid in protein-rich ingredients used for feed and food, European Food Safety Authority, June 7, 2007. Available at http://www.efsa.europa.eu/es/BlobSever/Statement/efsa_statement_melamine_en_rev1.pdf. Accessed December 18, 2008
C. A. Brown and S. A. Brown Food and Pharmaceuticals: Lessons Learned From Global Contaminations With Melamine/Cyanuric Acid and Diethylene Glycol
Veterinary Pathology,
January 1, 2010;
47(1):
45 - 52.
[Abstract][Full Text][PDF]
0.01). The observed chronic inflammation in this study could not be attributed to stone formation, because no stones were detected in the urinary tract. The male rats showed a slightly increased incidence of chronic inflammation, but this was not statistically insignificant. 
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Abstract
Introduction
