FL: Fluoride and the Renal System (2010)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

The present study was designed to evaluate the role of fluoride in urolithiasis in humans.

Inclusion Criteria:
  • Two areas were selected for this purpose, a fluoride endemic area (EA) and a fluoride non-endemic area (NEA)
  • Location: Udaipur region of India (Rajasthan).
Exclusion Criteria:

Not reported.

Description of Study Protocol:

Recruitment: Not reported

Design: Cross-sectional study design. 18,706 tribal people were personally interviewed to ascertain the prevalence of urinary stone disease. Only patients with a proven history were considered as stone formers. Recurrence of the disease was defined as the appearance of fresh stones after the diagnosis of the first episode in either kidney or the appearance of a new stone after previous removal or spontaneous voiding.

Blinding used: Not available

Intervention: Not available

Statistical Analysis: Student's T test was applied to evaluate the significance, with an alpha set at 0.05.

Data Collection Summary:

Timing of Measurements

Spot collection of urine and blood was done approximately four hours after the morning meal.

Dependent Variables

  • Urine chemistries: calcium, magnesium, oxalate, citrate, fluoride, thiobarbituric acid reactive substance (TBAR) levels
  • Serum TBAR.

Independent Variables

Endemic and non-endemic variables.

Control Variables

  • Endemic and non-endemic variables, with fluorosis and without fluorosis
  • In the EA understudy, fluoride levels ranged from 3.5ppm to 4.9ppm, and NEA is 0.5ppm.
Description of Actual Data Sample:

Initial N

18,706.

Attrition (final N)

Seventeen stone formers were detected in NEA and 62 were detected in EA.

  • Number of people surveyed
    • NEA, n=10,436 (6,067 males, 4,369 females)
    • EA, n=8,270 (4,270 males, 4,000 females)
    • Subjects surveyed for fluorosis, n=1,704 (854 males, 850 females)
      • With fluorosis, n=840 (431 males and 409 females)
      • Without fluorosis, n=864 (423 males and 441 females).
  • Number of patients
    • NEA, n=17 (16 males and one female)
    • EA, n=62 (44 males and 18 females)
    • With fluorosis, n=nine (seven males and two females)
    • Without fluorosis, n=five (three males and two females).

Age

30 years to 37 years.

Ethnicity

Asian Indian Tribals.

Other relevant demographics

The number of children younger than 12 years was 2,325 and 2,080 in the respective populations.
Seventeen stone formers (nine renal, five ureteric and three vesical) were detected in NEA and 62 (43 renal, 14 ureteric and five vesical) were detected in EA.

Location

Udaipur, Rajasthan, India.

Summary of Results:

Prevalence of Urinary Stone Disease in a Fluoride Endemic Area (EA) and a Fluoride Non-Endemic Area (NEA)

Category
Prevalence per 100,000
Male:Female Ratio
Recurrence (%)
Total
Male
Female
NEA
163
264
23
11.5:1
17.6
EA
750
1030
450
2.3:1
35.5
Subjects surveyed for fluorosis          
With fluorosis
1071
1624
489
3.3:1
-
Without fluorosis
578
709
453
1.6:1
-

  • Prevalence was higher in males than females in all categories
  • The recurrence of the disease was nearly double in EA compared to NEA.

Urine Chemistry and Serum TBAR Levels Along with Statistical Comparison of Normal Subjects and Stone Formers Residing in an Endemic Area (EA) and a Non-Endemic Area (NEA)

  EA NEA
 
Normal Subjects
Stone Formers
Normal Subjects
Stone Formers
Calcium, mg per gCr
101±42 (15)
139±80 (14)
99±38 (16)
132±18c (17)
Magnesium, mg per gCr
54±24 (80)
55±19 (20)
c87±28 (60)
70±41b (24)
Oxalate, mg per gCr
36.1±15.0 (80)
44.2±24.6a (20)
d23.8±16.5 (60)
34.3±13.5b (24)
Citrate, mg per gCr
565±156 (80)
435±188c (20)
558±176 (60)
b305±206d (24)
TBAR,µmol per gCr
4.4±1.8 (80)
4.6±1.5 (20)
4.2±1.5 (60)
4.2±1.7 (24)
Fluoride, mg per gCr
2.5±1.6 (80)
2.3±1.2 (20)
d0.9±0.8 (60)
d1.1±0.6 (24)
Serum TBAR nmol per mL
2.3±1.04 (26)
3.8±1.5b (8)
1.9±1.1 (40)
3.2±1.3c (8)

Statistical significance, P value: a<0.1, b<0.05, c<0.01 and d<0.001. P value on the right side indicates a statistical comparison between normal subjects and stone formers from the same area (EA and NEA). P value on the left side indicates a statistical comparison between normal subjects (EA) and normal subjects (NEA) and between stone formers (EA) and stone formers (NEA). The number of subjects is given in parenthesis.
  • In the EA, normal subjects had higher fluoride and oxalate excretion and low magnesium excretion than normal subjects from NEA
  • Serum TBAR levels (oxidative stress levels) were higher in stone formers of both the groups compared with normal subjects, and stone formers from EA tended to the higher levels (3.8±1.5 nmol per mL) than did stone formers from NEA (3.2±1.3 nmol per L).
  • The above data shows excess fluoride in water (intakes) increases oxidative stress and increases oxalates to form stones in the kidneys.

Quantitative Composition of Urinary Stone

Parameters
EA, n=8
NEA, n=35
Protein, mg per 100g
6.7±2.0
6.3±3.1
Calcium, mg per 100g
15.1±2.7
12.1±8.5
Magnesium, mg per 100g
2.5±0.7
1.9±1.9
Oxalate, mg per 100g
27.0±4.0
20.3±10.3
Phosphate, mg per 100g
7.0±1.5
6.3±4.6
Ammonium, mg per 100g
6.5±1.4
8.0±8.6
Uric acid, mg per 100g
5.8±1.0
5.8±4.2
Fluoride, mcg/100mg
295±72
86±41*
*P <0.001
  • Urinary stones from EA had significantly higher levels of fluoride than from NEA
  • The above data indicate that the stones from endemic stone formers had significantly higher fluoride and calcium content.

Other Findings

  • The prevalence of uroliathiasis was 4.6 times higher in EA than in NEA
  • The prevalence was almost double in subjects with fluorosis than without fluorosis in the endemic area.
    • In the NEA the prevalence of urinary stone disease in the tribal population was 163 per 100,000
    • Males were 11.5 times more often affected than females
    • In EA the prevalence was 750 per 100,000
    • The prevalence was 1,071 per 100,000 in patients with fluorosis and 578 per 100,000 in patients without fluorosis
    • No relationship was observed between urolithiasis and the duration of fluorosis.
    • The fluoride levels in drinking water ranged from 3.5ppm to 4.9ppm in EA and subjects from this area excreted more fluoride
    • A comparison of normal subjects from EA and NEA revealed that endemic subjects tend to have slightly higher mean serum TBAR levels and excrete more oxalate and fluoride than did their non-endemic counterparts
    • The urinary stone formers from the two areas showed a similar tendency, although again the difference was not significant
    • Citrate excretion in stone formers was almost normal in the EA, but NEA stone formers had significantly lower excretion levels
    • Urinary stones from endemic patients had higher fluoride, oxalate and calcium levels than those from non-endemic patients
    • In vitro studies suggested that fluoride did not influence the heterogonous mineralization of calcium oxalate.
Author Conclusion:

The human data suggest that fluoride slightly increases the oxidative environment in situ. Fluoride may also be synergistically acting with oxalate to potentiate the oxidative environment.

The authors believe that chronic intake of fluoride increases the risk of urinary stone formation.

Fluoride may be indirectly increasing oxaluria by enhancing the absorption of oxalate from the intestine because of low availability of calcium due to part of the intestinal calcium is precipitated as calcium fluoride.

 The data suggest that fluoride in vivo may behave as a mild promoter of urinary stone formation by

  • Excretion of insoluble calcium fluoride
  • Increasing oxalate excretion
  • Mildly increasing the oxidative burden.
Funding Source:
Other: not reported
Reviewer Comments:

Strengths

  • Large sample size
  • Comparison of low and high fluoride levels and its effects on urine markers
  • Fluoride exposure increases fluoride, oxalates, TBAR levels and calcium content. The increase in these markers suggests that fluoride exposure may increase urolithiasis.

Limitations

  • Other etiological factors need to be considered. There was no follow-up. This is a prospective observational study design in tribals only. This is not a representative sample of Asian Indian population.
  • Study design is not discussed in detail
  • Further details about the demographics, tribal categories, age, education and duration of the exposure to fluorine in water are required. Duration of fluorosis in males and females and its effect on urolithiasis by sex is required.
  • More clinical and radiological information is required on urolithiasis in this population and its association with other contributing risk factors
  • More information is required to assess the effect in adults and children
  • It is not clear whether all the subjects from the original sample were accounted for the analysis
  • It is also not clear whether the investigators were blinded about the data outcomes. Information about the intensity and duration of exposure to fluorine is required.
  • Details on the survey questionnaire are also needed. Statistical analysis is very weak in this study. Further statistical analysis is required.
  • The analysis should include Wilcoxon’s rank-sum tests (for differences in median values), or X2 tests. Pearson correlation coefficients are needed to evaluate associations among various biomarkers. It is necessary to use conditional logistic regression to estimate the odds ratio (OR) and 95% confidence intervals for levels of biomarkers predicting fluorosis based on the exposure of people to fluorine content and also the duration of fluorosis, conditioning on matching factors such as age, fasting status and races in tribals.
  • Spot evaluation will help for qualitative assessments, but for quantitative assessments radiological information and other standard tests are required. Regression models to estimate the significance of trend in ORs across increasing biomarker quintiles are needed. Adjustment of other contributing risk factors such as nutritional factors and nutritional status of the subjects need to be considered.
  • Study bias and limitations are not identified. Funding source is not reported.
  • Further controlled human trials are required in different countries and a comparison study between rural and urban areas in India is required.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) N/A
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) N/A
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? No
2. Was the selection of study subjects/patients free from bias? No
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? No
  2.2. Were criteria applied equally to all study groups? No
  2.3. Were health, demographics, and other characteristics of subjects described? No
  2.4. Were the subjects/patients a representative sample of the relevant population? No
3. Were study groups comparable? No
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) No
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? No
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? No
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) No
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? No
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? N/A
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) No
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? No
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? No
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? No
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? No
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? No
  6.6. Were extra or unplanned treatments described? No
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? N/A
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? No
  7.5. Was the measurement of effect at an appropriate level of precision? No
  7.6. Were other factors accounted for (measured) that could affect outcomes? No
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? No
  8.2. Were correct statistical tests used and assumptions of test not violated? No
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? No
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? No
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? No
9. Are conclusions supported by results with biases and limitations taken into consideration? No
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? No
  10.2. Was the study free from apparent conflict of interest? Yes