Associations of Plasma Homocysteine Reflecting Vitamin B12 and Folate Status with Fatigue-Related Outcomes in Healthy Adults

<b>Background/Objectives</b>: Fatigue and reduced motivation impair daily functioning and quality of life. Homocysteine (Hcy) has been implicated in oxidative stress and one-carbon metabolism, but its relationship with fatigue-related outcomes remains unclear. We explored associations between plasma Hcy and fatigue-related measures in healthy adults. <b>Methods</b>: We analyzed cross-sectional data from 602 community-dwelling adults. Plasma Hcy concentrations were categorized into sex-specific tertiles. Fatigue and motivation were assessed using the Chalder Fatigue Scale and a visual analog scale (VAS). Sex-stratified multivariable linear models adjusted for lifestyle and biochemical covariates were used to examine associations. Sensitivity analyses additionally modeled Hcy as a continuous variable. <b>Results</b>: Higher Hcy tertiles were associated with lower serum folate and vitamin B₁₂ concentrations in both sexes (<i>p</i> < 0.001). In men, the lowest Hcy tertile was associated with lower Chalder physical fatigue scores, whereas in women the highest Hcy tertile was associated with lower VAS motivation scores in multivariable analyses. Pairwise contrasts indicated higher physical fatigue in men in the highest tertile compared with the lowest (T3-T1: 1.55; 95% CI 0.24-2.86; <i>p</i> = 0.022) and lower motivation in women (T3-T1: -5.62; 95% CI -10.65 to -0.59; <i>p</i> = 0.029). However, no significant associations were observed when Hcy was modeled as a continuous variable. <b>Conclusions</b>: In this exploratory cross-sectional analysis, plasma Hcy showed associations with fatigue-related outcomes in sex-stratified analyses. These findings should be interpreted cautiously and considered hypothesis-generating. Longitudinal and mechanistic studies are needed to clarify potential causal relationships.