Objectives: This study aimed to investigate both the linear and non-linear associations between serum 25-hydroxyvitamin D [25(OH)D] levels and serum uric acid concentrations in Korean adults, with a particular focus on the vitamin D-insufficient range (<30 ng/mL), and to explore the potential metabolic implications of this relationship. Methods: Using data from the Korea National Health and Nutrition Examination Survey (KNHANES), we analyzed 10,864 adults aged 19 years and older. Serum vitamin D levels were categorized into quartiles (Q1–Q4), and their relationships with uric acid concentrations were examined using Pearson correlation, analysis of variance (ANOVA), and restricted cubic spline regression. Multivariate models were adjusted for potential confounders including age, sex, body mass index (BMI), kidney function, chronic disease status, and macronutrient intake. Results: In unadjusted analysis, a statistically significant but weak negative correlation was observed between serum 25(OH)D and uric acid levels (Pearson’s r = −0.092, p < 0.001). However, in multivariate regression adjusting for confounders, a weak positive association emerged. Restricted cubic spline analysis revealed significant positive associations in the lower quartiles (Q1–Q3), with the strongest association in Q3 (β = 0.769, 95% CI: 0.34–1.19, p < 0.001). No significant association was observed in the highest quartile (Q4). Conclusions: Serum vitamin D and uric acid concentrations show a non-linear relationship, with a significant positive association within the vitamin D-insufficient range (<30 ng/mL). These findings provide new insights into the potential metabolic role of vitamin D and highlight the need for longitudinal and interventional studies to clarify causality and clinical significance.

Background/Objectives: Resistant starch intake has been shown to influence gut microbiota composition and affect metabolic markers. These effects may be partially attributed to enhanced short-chain fatty acid (SCFA)-mediated energy harvesting and hepatic lipogenesis induced by resistant starch fermentation. However, there is a lack of prospective research addressing these associations. To address this gap, we performed a double-blind, randomized dietary intervention study to investigate the impact of high versus low resistant starch consumption on metabolic markers and gut microbiota among adult women presenting with risk factors for metabolic syndrome. Methods: A total of 30 participants were randomly assigned to either the low-resistant starch (LRS) or high-resistant starch (HRS) diet groups. Each group, comprising 15 participants, consumed one food product per day enriched with either high or low resistant starch for 8 weeks. Changes in metabolic indices and gut microbiota were assessed and compared with baseline values, as assessed before diet (Week 0). Results: After 8 weeks of intervention, the HRS diet significantly increased body weight, body fat, and triglyceride (TG) level (mean change ≈ +40 mg/dL), while reducing blood pressure. Analysis of intestinal microbiota in the HRS group revealed a statistically significant increase in the genus Veillonella following the intervention. Conversely, the genus Marvinbryantia increased significantly in the LRS group. Conclusions: In women with metabolic risk factors, resistant starch supplementation elicited mixed metabolic responses—showing a modest reduction in blood pressure but concurrent increases in adiposity and TG concentrations. As the TG elevation reached a clinically meaningful magnitude, dietary interventions involving high-resistant starch should incorporate regular lipid monitoring to ensure cardiometabolic safety. Collectively, these findings highlight the complex interplay between SCFA-producing gut microbiota and host energy metabolism, suggesting that individualized dietary strategies may be required to optimize metabolic outcomes.

Background: Sarcopenia and metabolic dysfunction share common physiological mechanisms, and insulin resistance has been recognized as a major contributor to muscle loss. However, the independent association between circulating fasting insulin and muscle strength remains unclear. Methods: We analyzed data from 8343 Korean adults aged ≥ 20 years who participated in the 2015 and 2019 Korea National Health and Nutrition Examination Surveys. Multivariate outliers were removed using the Mahalanobis distance, and sampling weights were applied to account for the complex survey design. Multivariable linear regression models were constructed with progressive adjustments for demographic and metabolic covariates, and stratified analyses were conducted by age, BMI category, and diabetes status. Results: Crude models showed a weak positive association between fasting insulin and handgrip strength in both sexes. However, after adjustment for age and BMI, the association became significantly inverse and remained consistent in fully adjusted models. The inverse association was most pronounced in individuals aged ≤ 65 years, with BMI < 23 kg/m², and without diabetes. Conclusions: Elevated fasting insulin levels were independently associated with lower handgrip strength in Korean adults. These findings suggest that hyperinsulinemia may reflect early metabolic changes linked to subclinical muscle weakness, warranting further longitudinal investigation.

Objective: Resistant starch is associated with metabolic benefits, but whether autonomic regulation links resistant starch intake to cardiovascular outcomes remains unclear. This study examined the effects of resistant starch on heart rate variability and blood pressure in women with features of metabolic syndrome. Methods: In this 8-week randomized, double-blind pilot trial, 30 women (aged ≥ 40 years) with ≥2 metabolic syndrome components were assigned to a high-resistant starch or low-resistant starch group. The primary outcome was the change in systolic blood pressure from baseline to week 8. Secondary outcomes included heart rate variability-related autonomic parameters, whereas other blood pressure and metabolic syndrome-related variables were evaluated as exploratory outcomes. Results: Systolic blood pressure decreased significantly within the high-resistant starch group (p = 0.037), although the baseline-adjusted between-group difference was not statistically significant. This change occurred without corresponding improvements in body mass index or triglyceride levels. Group-level heart rate variability differences did not reach statistical significance; however, exploratory analyses suggested a favorable directional pattern in global heart rate variability composite scores among participants with lower baseline autonomic activity (p = 0.06). Conclusions: High-resistant starch intake demonstrated exploratory signals of reduced systolic blood pressure and heart rate variability-related autonomic adaptation, particularly among participants with lower baseline autonomic activity. These findings warrant confirmation in adequately powered randomized controlled trials.

Objectives: To investigate a non-linear relationship between serum vitamin D level and uric acid concentration and to evaluate the potential of vitamin D as a biomarker for inflammatory diseases such as hyperuricemia. Methods: Using data from the Korea National Health and Nutrition Examination Survey (KNHANES), an analysis was conducted on 10,864 adults aged 19 years or more. Serum vitamin D levels were categorized into quartiles (Q1–Q4) for analysis. Their linear and non-linear relationships with uric acid concentrations were assessed using Pearson correlation analysis, analysis of variance (ANOVA), and restricted cubic spline regression. Confounding variables including age, sex, BMI, kidney function, chronic diseases, and macronutrient intake were adjusted for in the analysis. Results: In univariate analysis, a statistically significant but weak negative correlation was observed between serum vitamin D and uric acid levels (Pearson’s correlation coefficient: -0.089, < 0.001). However, after adjusting for confounders, multivariate regression revealed a weak positive association. Restricted cubic spline regression identified significant positive associations in lower quartiles (Q1–Q3), with the strongest effect observed in Q3 (beta-coefficient: 0.643, 95% CI: 0.09–1.20, p = 0.023). No significant association was found in the highest quartile (Q4). Conclusions: Vitamin D and uric acid metabolism have a non-linear relationship, particularly showing a positive association in vitamin D insufficiency (< 30 ng/mL). These findings support the potential of vitamin D as a biomarker for hyperuricemia and related inflammatory-metabolic diseases.