The Association Between Dietary Intake of Iodine and Kidney Stones: A Study of the National Health and Nutrition Examination Survey 2007-2018

NHANES data on 9,443 adults hints at a kidney stone risk bump at mid-range iodine levels — but the numbers in the abstract don't match the results table

Journal: Medicine | Published: 2026-06-12 | Type: Cross-sectional study | PMID: 42299521 Authors: Mao Xinhong, Li Changjiu, Lv Chentao, Chen Weiyu, Chen Chao (Hangzhou First People's Hospital / Zhejiang Chinese Medical University, China) Funding/COI: None declared

Summary

Using six cycles of NHANES data (2007–2018), this paper found that participants with urinary iodine concentrations (UIC) in the 67.7–121.2 μg/L range had a nominally higher risk of kidney stones than those below 67.7 μg/L — but higher iodine levels showed no association at all. That U-shaped non-finding is biologically odd and the paper doesn't explain it convincingly. More troubling: the abstract reports OR = 1.47 (95% CI: 1.05–2.06) for the key association, but the actual results table for the same fully adjusted model (Model 3) reads OR = 1.41 (95% CI: 0.98–1.96) — a confidence interval that crosses 1.0, meaning the primary result is not statistically significant by the paper's own numbers.

Claims

• 718 of 9,443 participants (7.60%) had a history of kidney stones
• Mean UIC across the cohort: 262.69 μg/L
• Q2 (UIC 67.7–121.2 μg/L) vs. Q1 (<67.7 μg/L): OR = 1.47, 95% CI: 1.05–2.06 per the abstract; OR = 1.41, 95% CI: 0.98–1.96 per the results table (Model 3, fully adjusted)
• Q3 (121.2–215.5 μg/L) and Q4 (≥215.5 μg/L): no statistically significant association in any model
• Restricted cubic spline analysis identified a nonlinear relationship with an inflection point at UIC = 134.9 μg/L
• No linear trend detected across quartiles (P > .05 after full adjustment)
• Kidney stone cases were older, more often male, more often non-Hispanic white, and had higher rates of diabetes and hypertension

Study Quality

Cross-sectional design from NHANES, which is nationally representative and methodologically rigorous at the data-collection level. Urinary iodine was measured by ICP-DRC-MS — a validated lab method. The authors adjusted for a reasonable covariate list (age, sex, race, education, income, BMI, alcohol, smoking, diabetes, hypertension, sodium, calcium, zinc, albumin, creatinine).

The analytic approach is otherwise standard for this type of NHANES secondary-analysis paper. Trend analysis and RCS curves are appropriate for detecting nonlinearity. The sample size of 9,443 is adequate for detecting moderate associations. However, UIC from a single urine collection is a poor surrogate for habitual dietary iodine intake — iodine excretion varies substantially day-to-day and with hydration status. Kidney stone history is self-reported with no clinical verification.

Red Flags

• Abstract contradicts results table. The abstract states the fully adjusted OR for Q2 is 1.47 (95% CI: 1.05–2.06). The results section Table 2 states Model 3 OR = 1.41 (95% CI: 0.98–1.96). One of these is wrong. The table version does not reach statistical significance.
• Biological implausibility of the pattern. Risk is elevated at moderate iodine levels (Q2) but not at high levels (Q3, Q4). No credible mechanistic explanation is offered for why mid-range intake would be harmful while double that intake is not.
• Cross-sectional design cannot establish causation, as the authors acknowledge but minimize.
• Single urine sample is an unreliable measure of long-term dietary iodine exposure.
• Self-reported kidney stone history with no imaging or medical record confirmation.
• Contradicts prior literature — the authors note at least one prior study found an inverse association — but dismiss the discrepancy in a single paragraph.
• The paper describes itself as "pioneering" and, in the Discussion, uses the word "breakthrough" — language the evidence does not support.
• Age restriction to 20–60 limits generalizability (most kidney stone burden falls in older adults).

Strengths

• Large, nationally representative sample across six NHANES cycles
• Validated laboratory method for urinary iodine measurement
• Comprehensive covariate adjustment including dietary minerals (Ca, Na, Zn)
• RCS analysis is an appropriate tool for capturing nonlinear dose-response relationships
• Sensitivity analyses reported (though not shown in the provided text)
• No declared funding conflicts

Verdict

The headline finding — that mid-range urinary iodine is associated with kidney stones — rests on a statistically marginal result that the paper's own results table suggests does not clear the significance threshold, contradicting the abstract. Even if you accept the abstract's numbers, the biological story makes no sense: higher iodine isn't riskier, only moderate iodine is. This is the signature of a data dredging artifact, not a meaningful exposure-outcome relationship. The NHANES platform is solid; the analysis on top of it is not. File this under "hypothesis-generating at best, noise at worst" and wait for a prospective study with repeated iodine measurements before taking the association seriously.