Review for 10.1186/s12876-025-04029-0 (2025)

Title: Association Between Blood Lead Levels and Constipation in Adults: Insights from
NHANES Data (2005–2010)
Version: 3 Date: 11 May 2025
Reviewer's report:
Here is may assessment to the manuscript:
Strengths:
• Novel Finding and Importance: The study examines an underexplored question – the
relationship between blood lead levels and constipation – using a large, nationally representative
sample of U.S. adults (NHANES 2005–2010, n # 14,000). The discovery of a significant
inverse association (higher lead associated with lower constipation prevalence) is intriguing
and challenges conventional expectations, offering new perspectives for research into
gastrointestinal effects of environmental exposures.
• Robust Analysis: The statistical approach is thorough. The authors used multivariable
logistic regression with stepwise adjustment for a wide range of confounders (demographics,
socioeconomic status, health conditions, and dietary factors). They further employed restricted
cubic spline (RCS) analysis to explore non-linear trends, confirming the inverse relationship
across the range of lead levels. No significant subgroup interactions were found, and a sensitivity
analysis (excluding a potential mediating variable, serum creatinine) yielded consistent results.
These analyses enhance confidence in the robustness of the findings.
• Contextualization and Cautious Interpretation: The manuscript is well-written and organized.
The discussion provides a solid context by comparing the results with prior studies (noting
differences in pediatric populations and specific exposure scenarios). The authors offer several
plausible biological mechanisms (neurological effects on motility, gut microbiome changes,
endocrine influences, oxidative stress) to explain the unexpected association, but importantly
they clearly state that these explanations are speculative and not proven. This careful,
hypothesis-generating discussion shows appropriate caution. The manuscript also transparently
acknowledges the study’s limitations (cross-sectional design, self-reported outcomes, and
potential confounders), demonstrating the authors’ understanding of the study’s constraints.
Weaknesses:
• Cross-Sectional Design: As an observational analysis of cross-sectional data, the study cannot
establish causation or directionality. The authors observed an association but it remains unclear
whether lower constipation is a result of higher lead exposure or if other factors are at play. The
possibility of reverse causation (e.g. individuals with chronic constipation changing behaviors
that incidentally lower lead levels) exists. While this limitation is acknowledged, it inherently
tempers the strength of any causal interpretation.
• Outcome Measurement: Constipation was defined based on self-reported bowel habits
(infrequent bowel movements or Bristol stool scale type 1–2). Such a definition, while practical
for a large survey, is relatively broad and subject to recall bias. It may misclassify some
participants’ true constipation status (for example, those using laxatives might not report
infrequent stools). This could dilute the association and introduces some uncertainty compared
to clinical diagnostic criteria.
• Potential Residual Confounding: Despite adjusting for many covariates, there remain
unmeasured factors that could influence the results. Notably, the analysis did not include
certain medication or supplement uses (e.g. opioid painkillers, iron supplements, laxatives,
antidepressants) that affect gastrointestinal function and could be unevenly distributed across
lead exposure levels. Lifestyle factors like specific dietary components or occupational
exposures were also not fully captured. These unmeasured variables leave room for residual
confounding, meaning the inverse association might partly reflect differences in lifestyle or
health status unrelated to lead itself.
• Counterintuitive Association: The finding that higher lead levels correlate with lower
constipation risk is counterintuitive given lead’s known toxicity. This paradox raises questions
about underlying causes. The authors hypothesize biological mechanisms and note that
individuals with high lead were more often coffee/alcohol consumers (factors linked to less
constipation), suggesting a possible explanation. However, without experimental or longitudinal
data, the interpretation remains speculative. Readers may need to be cautious in drawing
conclusions, as the result could be specific to this dataset or require further validation.
• Generalizability: The study population (U.S. adults from 2005–2010) may limit broader
applicability. Environmental lead exposure patterns and population characteristics can differ
by region and over time. Additionally, previous research in children found opposite trends
(higher lead linked to more constipation), highlighting that age and context matter. Thus, while
the current findings are compelling for adult public health, they may not directly translate to
pediatric populations or other settings without additional study.
Recommendation: In sum, the authors have produced a substantially improved manuscript
with well-addressed prior critiques. The study’s strengths – notably the large sample, rigorous
analysis, and careful discussion – make it a valuable contribution despite the inherent limitations
of a cross-sectional design. The remaining issues are minor and largely editorial. Therefore, I
recommend Minor Revision before acceptance. This would allow the authors to resolve a few
small details (for example, ensuring consistency in reported p-values and further emphasizing
interpretative caution) but no major additional analyses or rewrites are needed. With these minor
adjustments, the manuscript should be suitable for publication.