When Fiber Does Not Protect: Evaluating Statistical Interaction Between Dietary Fiber, Sodium-to-Potassium Ratio, and Metabolic Syndrome in South Korean Adults

Objectives: Metabolic syndrome (MetS) represents a major public health concern strongly associated with dietary factors. High sodium and low potassium intakes are established cardiometabolic risk factors, and dietary fiber has been hypothesized to exert a buffering influence. In this cross-sectional study, whether dietary fiber intake moderates the association between the dietary sodium-to-potassium (Na/K) ratio and MetS outcomes was evaluated in South Korean adults. Methods: A final unweighted sample of 8354 adults aged 20–59 years from the pooled 2022–2024 Korea National Health and Nutrition Examination Survey (KNHANES) was analyzed, representing a weighted population of 27,293,126 adults. The dietary Na/K ratio and fiber intake were estimated using a single 24 h dietary recall. MetS was defined using modified National Cholesterol Education Program Adult Treatment Panel III criteria alongside a validated continuous MetS severity score. Multivariate survey-weighted regression models were estimated to evaluate associations with metabolic outcomes, with adjustment for age, sex, body mass index, smoking status, alcohol consumption, and total energy intake. Moderation by dietary fiber was assessed using product-interaction terms, simple-slope analyses, and Johnson–Neyman techniques. Results: Overall MetS prevalence was 23.0%. A higher Na/K ratio was independently associated with increased odds of hypertension (OR = 1.144, 95% CI: 1.023–1.279, p = 0.018) and metabolic syndrome (OR = 1.204, 95% CI: 1.076–1.347, p = 0.001), as well as a higher continuous MetS severity score (β = 0.088, 95% CI: 0.034–0.141, p = 0.001). Contrary to the hypothesized buffering role, dietary fiber intake demonstrated a statistically significant interaction with the Na/K ratio for metabolic syndrome outcomes (binary MetS interaction: OR = 1.010, 95% CI: 1.002–1.018, p = 0.013; severity score interaction: β = 0.0060, 95% CI: 0.0023–0.0096, p = 0.001). This interaction was not statistically significant for hypertension (p = 0.581). Conclusions: Dietary fiber intake demonstrated a positive statistical interaction with the dietary Na/K ratio, contrary to the hypothesized attenuating influence. This observed statistical interaction likely reflects contextual dietary factors, such as overlapping plant sources of sodium and fiber, residual confounding, or reverse causality rather than an inherently adverse biological mechanism. These interaction patterns warrant longitudinal confirmation before informing public health dietary guidance.