Association Between Blood Cadmium and Vitamin D Levels in the Yangtze Plain of China in the Context of Rapid Urbanization
China’s rapid urbanization over the past three decades has transformed the Yangtze Plain, a critical agricultural and economic hub, into one of the most densely populated and industrialized regions in the country. This transformation has brought environmental challenges, including heavy metal contamination of soil and water. Cadmium, a toxic heavy metal linked to various health risks, has emerged as a significant pollutant in this region. Simultaneously, vitamin D deficiency has become a widespread public health concern in China. This study investigates the relationship between blood cadmium levels (BCLs) and serum 25-hydroxyvitamin D (25(OH)D) concentrations among adults in the Yangtze Plain, shedding light on the interplay between environmental exposure and metabolic health during urbanization.
Study Design and Methodology
The cross-sectional analysis utilized data from the Survey on Prevalence in East China for Metabolic Diseases and Risk Factors (SPECT-China), conducted between February and June 2014. Participants were recruited from 12 villages across three communities in the Yangtze Plain: Fengcheng (Shanghai), Xiaoyue (Zhejiang Province), and Changdong (Jiangxi Province). These areas represent transitioning zones where rural populations have rapidly urbanized.
A total of 3,234 adults (1,245 men, 1,989 women) aged 18–93 years were included after excluding individuals with missing data for 25(OH)D or BCL. Blood samples were analyzed using graphite-furnace atomic absorption spectrometry for BCLs and chemiluminescence assays for 25(OH)D. Demographic, lifestyle, and clinical data, including age, education, smoking status, body mass index (BMI), and diabetes diagnosis, were collected. Diabetes was defined using American Diabetes Association criteria (fasting plasma glucose ≥7.0 mmol/L or HbA1c ≥6.5%).
Key Findings
1. High Prevalence of Vitamin D Deficiency
Vitamin D deficiency (25(OH)D <50 nmol/L) affected 79.2% of participants, with higher rates in women (85.0%) than men (69.9%). This disparity aligns with global patterns where women, particularly in urbanized settings, face greater risks due to indoor lifestyles, clothing practices, and hormonal differences.
2. Elevated Blood Cadmium Levels
Median BCLs were 1.80 μg/L (interquartile range [IQR]: 0.60–3.42) in men and 1.40 μg/L (IQR: 0.52–3.10) in women. While these values fall below the World Health Organization’s safety threshold (5 μg/L), they exceed levels reported in developed countries. For instance, the U.S. National Health and Nutrition Examination Survey (2007–2012) documented a median BCL of 0.31 μg/L. In the Yangtze Plain, 9.7% of men and 8.5% of women had BCLs ≥5 μg/L, indicating significant environmental exposure.
3. Inverse Association Between BCL and 25(OH)D in Women
Multivariable linear regression revealed a significant inverse relationship between BCL and 25(OH)D in women. Each doubling of BCL corresponded to a 0.401 nmol/L decrease in 25(OH)D (95% confidence interval [CI]: −0.697 to −0.105; P = 0.008) after adjusting for age, education, smoking, BMI, diabetes, and seasonal variation. Women in the highest BCL tertile (≥2.45 μg/L) had mean 25(OH)D levels of 38.64 ± 9.79 nmol/L, compared to 40.52 ± 10.62 nmol/L in the lowest tertile (≤0.69 μg/L; P for trend = 0.001).
In contrast, no significant association was observed in men, even after similar adjustments. This sex-specific effect may stem from hormonal differences, as estrogen has been shown to interact with cadmium metabolism and vitamin D synthesis.
4. Factors Influencing BCLs
BCLs varied significantly across demographic groups:
- Gender: Men had higher median BCLs (1.80 μg/L vs. 1.40 μg/L; P < 0.05), likely due to higher smoking rates (51.3–58.5% in men vs. 3.1–3.4% in women).
- Season: Winter sampling (February–March) correlated with higher BCLs (median 1.60 μg/L) than spring (April–June; 1.10 μg/L), possibly reflecting seasonal dietary or behavioral changes.
- BMI: Overweight individuals (BMI 25.0–29.9 kg/m²) had lower BCLs (1.34 μg/L) than those with normal BMI (1.62 μg/L; P < 0.05), suggesting weight-related differences in cadmium distribution or excretion.
Mechanisms and Implications
The study hypothesizes that cadmium disrupts vitamin D metabolism through hepatic and renal pathways. The liver converts vitamin D to 25(OH)D, and cadmium’s hepatotoxicity may impair this process. Additionally, cadmium may interfere with cytochrome P450 enzymes involved in vitamin D hydroxylation. Animal studies support this, showing cadmium-induced reductions in 1,25-dihydroxyvitamin D (1,25(OH)₂D), the active form of vitamin D.
The sex-specific association in women could also involve estrogen’s role in modulating cadmium toxicity and vitamin D synthesis. Lower estrogen levels in postmenopausal women, who comprised a large subset of the cohort, may exacerbate susceptibility to cadmium’s effects.
Public Health Considerations
The Yangtze Plain’s rapid urbanization has intensified cadmium exposure through contaminated crops, industrial emissions, and inadequate waste management. Rice, a dietary staple, is a primary cadmium source due to its high bioaccumulation in polluted soils. Mitigation strategies should prioritize soil remediation, stricter industrial regulations, and public health campaigns promoting dietary diversification and sunlight exposure.
Limitations and Future Directions
As a cross-sectional study, this research cannot establish causality or temporal relationships. Longitudinal cohorts and animal experiments are needed to confirm cadmium’s role in vitamin D deficiency and elucidate mechanisms. Future studies should also explore interactions between cadmium, other heavy metals, and endocrine-disrupting chemicals.
Conclusion
This study highlights the dual burden of environmental cadmium exposure and vitamin D deficiency in China’s rapidly urbanizing Yangtze Plain. The inverse association between BCL and 25(OH)D in women underscores the need for gender-specific interventions. Addressing these challenges requires integrated efforts in environmental management, healthcare, and policy to safeguard population health amid urbanization.
doi.org/10.1097/CM9.0000000000001068
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