Serum and Skin Carotenoid Levels in Older Adults with and Without Metabolic Syndrome: A Cross-Sectional Study

Introduction: Metabolic syndrome (MetS), a clustering of cardiovascular disease (CVD) risk factors, is associated with increased mortality. Fruit and vegetable (FV) intake is inversely associated with CVD risk, and carotenoids, bioactive compounds found in brightly colored FVs, can be measured in serum and skin as biomarkers of intake. While serum and skin carotenoids are correlated in healthy populations, this relationship is not well understood in older adults with MetS, who may have altered carotenoid absorption or metabolism. Methods: In this cross-sectional study, adults aged 55+ were assessed for serum carotenoid concentrations, pressure-mediated reflection spectroscopy (RS) skin carotenoid scores, self-reported FV intake, sociodemographic characteristics, and comorbidities. MetS status was determined using the National Cholesterol Education Program Adult Treatment Panel III criteria (77 with MetS, 63 without). Linear regression models evaluated group differences in carotenoid levels. Associations between serum and skin carotenoids were examined using Spearman correlation and multivariable regression. Results: Participants with MetS had significantly lower serum alpha-carotene (52%), beta-carotene (39%), and total carotenoids (22%) than those without MetS (all p < 0.002). Differences remained after adjustment for sociodemographic and health-related factors. No significant group differences were found for lycopene, lutein, cryptoxanthin, or skin carotenoid scores. Total serum carotenoids were positively correlated with skin scores (r = 0.58, p < 0.001), and this association persisted in adjusted models. Conclusions: Older adults with MetS had lower serum carotenoid levels, primarily due to alpha- and beta-carotene. This serum–skin correlation supports RS-based skin measurement as a practical, non-invasive assessment of carotenoid status.