Chemical contamination and nutritional degradation of freshwater fish marketed in West Kazakhstan

Background and Aim: Freshwater fish are an important source of high-quality protein, essential amino acids, omega-3 polyunsaturated fatty acids, vitamins, and minerals. However, contamination of aquatic ecosystems with toxic elements and radionuclides may compromise both food safety and the nutritional and biochemical integrity of fish. This study aimed to evaluate the association between toxic elements, radionuclides, and biochemical degradation of freshwater fish marketed for human consumption in West Kazakhstan. Materials and Methods: A cross-sectional observational study was conducted from April to September 2025. A total of 800 samples were analyzed, including 500 fresh freshwater fish specimens representing five species and 300 processed fish products collected from retail outlets supplied by 15 water bodies and one aquaculture facility. Concentrations of lead (Pb), cadmium (Cd), arsenic (As), and mercury (Hg) were determined using validated standard methods. Radionuclide activity of cesium-137 (Cs-137) and strontium-90 (Sr-90) was also measured. For biochemical analysis, fish were divided into a control group (no detectable contamination; n = 50) and a contaminated group (low-dose Pb and/or Cd detected; n = 50). Nutritional composition, mineral profile, vitamin content, amino acid composition, and fatty acid profiles were analyzed using standardized laboratory techniques. Statistical analysis included effect size estimation, false discovery rate correction, and multivariate modeling. Results: Pb and Cd were detected in fish from 7 of 15 water bodies, with localized exceedances reaching 1.15 mg/kg and 0.245 mg/kg, respectively. As and Hg were not detected, while radionuclide activity remained within permissible limits. Compared with the control group, contaminated fish showed significant reductions in lipid content (−26.3%) and energy value (−12.7%), along with decreases in essential minerals, vitamins, and total amino acid content (−15.8%) (p ≤ 0.05). Omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid and docosahexaenoic acid, were markedly reduced. Multivariate analysis confirmed that contamination status was an independent predictor of biochemical deterioration irrespective of species. Conclusion: The findings demonstrate that compliance with regulatory contaminant limits does not guarantee preservation of nutritional quality. Chronic low-dose exposure to Pb and Cd is associated with substantial biochemical degradation of fish muscle tissue, even when organoleptic quality remains acceptable. These results highlight an overlooked dimension of food safety and support the integration of biochemical and nutritional indicators into routine monitoring frameworks to improve consumer protection. Keywords: biochemical degradation, cadmium contamination, food safety, freshwater fish, heavy metal toxicity, nutritional quality, public health risk, West Kazakhstan.