**Background:** Functional disability in older adults leads to decreased quality of life, increased hospitalization, and higher mortality. Folate, a water-soluble vitamin with antioxidant properties, may protect against functional decline through oxidative stress reduction, anti-inflammatory effects, and homocysteine metabolism. Previous studies on serum folate and physical function yielded inconsistent results, and no study had examined 5-methyltetrahydrofolate (5-MTHF), the main bioactive form, in relation to functional disability across multiple domains.
**Methods:** Data were extracted from four NHANES cycles (2011–2018), yielding 5,850 participants aged ≥60 years after exclusions. Serum folate (5-MTHF and total folate) was measured by LC-MS/MS. Functional disability was self-reported across five domains: LEM, IADL, ADL, LSA, and GPA, using 19 validated questions. Disability was defined as any difficulty in performing one or more tasks within a domain. Serum folate was divided into sex-specific quartiles. Multivariable-adjusted logistic regression models adjusted for age, race/ethnicity, education, marital status, poverty-income ratio, physical activity, alcohol consumption, smoking, BMI, total energy intake, and multiple chronic conditions. Restricted cubic splines with 3 knots assessed dose-response relationships. Sensitivity analyses included excluding participants using medications affecting folate metabolism, those with malnutrition, and examining combined total folate plus Mefox.
**Key Results:** In fully adjusted models, higher 5-MTHF was inversely associated with IADL disability (OR Q4 vs Q1 = 0.65, 95% CI: 0.46–0.91) and GPA disability (OR Q4 vs Q1 = 0.70, 95% CI: 0.50–0.96). Total folate showed similar inverse associations with IADL (OR Q4 vs Q1 = 0.65, 95% CI: 0.46–0.90) and GPA disability (OR Q3 vs Q1 = 0.66, 95% CI: 0.44–0.99). In sex-stratified analyses, associations were significant only in females: for IADL disability, fully adjusted ORs for highest vs lowest quartile were 0.52 (95% CI: 0.35–0.79) for 5-MTHF and 0.53 (95% CI: 0.35–0.78) for total folate. No significant associations were found in males. Age-stratified analyses showed negative associations between 5-MTHF and IADL disability in those aged 70–79 years, and with GPA disability in those aged 60–69 years. BMI-stratified analyses found associations only in normal-weight participants. Alcohol consumption-stratified analyses showed associations only in non-drinkers. Restricted cubic spline analyses demonstrated a gradual decrease in IADL and GPA disability risk with increasing serum folate levels. Sensitivity analyses excluding participants using antibiotics, estrogens, and anticonvulsants (n=798) and those with malnutrition (n=192) did not substantially change results. Dietary folate intake was also inversely associated with IADL disability (OR Q4 vs Q1 = 0.62, 95% CI: 0.45–0.86), and folate supplement users had lower ADL disability prevalence (OR = 0.78, 95% CI: 0.64–0.96).
**Clinical Implications:** This study suggests that higher serum folate concentrations are associated with lower odds of IADL and GPA disability in older adults, particularly in females, those under 80 years, normal-weight individuals, and non-drinkers. The findings support recommending adequate folate intake through diet or supplementation as a potentially modifiable factor to prevent or delay functional decline in aging populations. However, as a cross-sectional study, causality cannot be established, and residual confounding may exist. The self-reported nature of disability assessment and focus on US adults aged ≥60 limit generalizability. Further longitudinal and mechanistic studies are needed to confirm these associations and elucidate underlying biological pathways.