Associations of Serum Niacin and Nicotinamide with Child Behavior: A Longitudinal Study in China

Background/Objectives: Nicotinic acid (NA; niacin) and nicotinamide (NAM) participate in nicotinamide adenine dinucleotide metabolism, but pediatric evidence relating serum concentrations of these vitamers to behavioral-problem scores is limited. Methods: We conducted an exploratory analysis of children assessed in Yiwu, Zhejiang Province, China, with serum NA and NAM concentrations recorded in 2021 and 2025 and caregiver-reported Child Behavior Checklist (CBCL) outcomes in 2025. Vitamin B3 was measured by high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS). Internalizing, externalizing, and total-problem raw scores were the broadband outcomes; seven common raw subscales were exploratory outcomes. Multivariable linear models and three-knot restricted cubic splines (RCSs) were conducted to assess the associations between vitamin B3 and CBCL scores. Benjamini–Hochberg correction and exposure-by-sex interaction tests were used. Results: Pooled associations were compatible with the null after correction. Per interquartile-range (IQR) increase in 2021 NA, boys’ adjusted contrasts were −2.18 (95% CI, −4.22 to −0.13) for internalizing, −3.04 (−5.05 to −1.04) for externalizing, and −5.33 (−9.18 to −1.47) for total problems. In sex-stratified RCS analyses, 2021 NA was associated with externalizing (q-overall = 0.040) and total problems (q-overall = 0.023) in boys, whereas 2025 NA was associated with these outcomes in girls (both q-overall = 0.042). Sex-stratified two-degree-of-freedom RCS models had limited sensitivity for small-to-modest associations. Conclusions: The sex-interaction and sex-stratified RCS patterns for NA are hypothesis-generating and require independent, prespecified replication.