Associations of Classical and Novel Anthropometric Indices with Vitamin D Status in Thai Adults

Background: Vitamin D deficiency remains prevalent in tropical regions despite abundant sunlight and is linked to adverse skeletal and cardiometabolic outcomes. The relationship between vitamin D status and adiposity is inconsistent, partly due to the use of classical anthropometric indices that do not account for visceral fat. This study explores whether both classical and novel anthropometric indices reflecting visceral adiposity provide better insight into vitamin D variation in Thai adults. Methods: A cross-sectional study of 403 adults categorized participants into deficient (<12 ng/mL), insufficient (12–20 ng/mL), and normal (>20 ng/mL) serum 25-hydroxyvitamin D [25(OH)D] groups. Classical anthropometry indices (waist circumference [WC], waist–hip ratio [WHR], body fat percentage [BF%]) and novel indices (body roundness index [BRI], conicity index [CI], weight-adjusted waist index [WWI], abdominal volume index [AVI]) were assessed. Multivariable linear and logistic regression models evaluated independent associations between adiposity markers and serum 25(OH)D after adjustment for age, sex, lifestyle, supplement use, and sun exposure time. Results: Although the crude comparisons mainly reflected differences in age and sex distribution across groups, adjusted analyses showed a different pattern of associations. Participants with normal vitamin D levels showed higher central adiposity indices (WHR, BRI, CI, and WWI) than those with deficiency. After adjustment, WC demonstrated a positive association with 25(OH)D (β = 3.53, p = 0.01), whereas AVI showed an inverse association (β = −3.90, p = 0.01), suggesting that indices capturing visceral fat volume rather than simple girth better reflect the adiposity–vitamin D relationship. BF% consistently predicted vitamin D deficiency across all logistic regression models (OR range = 2.18–2.28, all p < 0.05). Conclusions: AVI appeared to be the most informative indicator of visceral adiposity associated with lower vitamin D levels. Measures that more accurately capture fat distribution, rather than overall body size, may better identify individuals at risk of vitamin D deficiency, especially in tropical settings where sun-exposure behavior may exceed adiposity alone.