J Endocrinol Metab

Journal of Endocrinology and Metabolism, ISSN 1923-2861 print, 1923-287X online, Open Access

Article copyright, the authors; Journal compilation copyright, J Endocrinol Metab and Elmer Press Inc

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Original Article

Volume 16, Number 4, August 2026, pages 167-171

Nonlinear Relationship Between Serum 25-Hydroxyvitamin D and Glycemic Control Among US Adults: A Survey-Weighted Analysis of NHANES 2017–2018

Figure

↓ Figure 1. Restricted cubic spline regression demonstrating the nonlinear association between serum 25(OH)D concentrations and predicted HbA1c levels. Shaded regions represent 95% confidence intervals. HbA1c: glycated hemoglobin; 25(OH)D: serum 25-hydroxyvitamin D.

Tables

↓ **Table 1.** Baseline Characteristics of Study Participants (NHANES 2017–2018)
Characteristic	N = 5,136
Data are presented as mean ± standard deviation (SD) or number (percentage). BMI: body mass index; HbA1c: glycated hemoglobin; 25(OH)D: serum 25-hydroxyvitamin D.
Age, years	50 ± 19
Male sex	2,474 (48%)
Female sex	2,662 (52%)
Mexican American	718 (14%)
Other Hispanic	486 (9.5%)
Non-Hispanic White	1,792 (35%)
Non-Hispanic Black	1,153 (22%)
Other race	987 (19%)
BMI, kg/m²	30 ± 7
HbA1c, %	5.84 ± 1.09
25(OH)D, nmol/L	68 ± 32
Vitamin D deficient	1,518 (30%)
Vitamin D insufficient	1,790 (35%)
Vitamin D sufficient	1,828 (36%)
Physically inactive	2,691 (52%)
Physically active	2,445 (48%)

↓ **Table 2.** Multivariable Association Between Serum 25(OH)D and HbA1c
Variable	β	SE	95% CI	P-value
BMI: body mass index; CI: confidence interval; SE: standard error.
Vitamin D insufficient	−0.051	0.036	−0.120 to 0.019	0.156
Vitamin D sufficient	−0.189	0.038	−0.263 to −0.115	< 0.001
Age (per year)	0.020	0.001	0.018 to 0.021	< 0.001
Sex (female vs. male)	−0.076	0.029	−0.132 to −0.020	0.008
BMI (kg/m²)	0.024	0.002	0.020 to 0.028	< 0.001
Physical activity (active vs. inactive)	−0.063	0.029	−0.121 to −0.006	0.030