Serum proline concentration measured in early life demonstrated inverse associations with several later-life biomarkers that are directly associated with MetS—notably, sCD40L (–0.80; 95% CI, –1.54 to –0.06), TNF-α (–0.29; 95% CI, –0.50 to –0.07), ApoA-II (–0.18; 95% CI, –0.36 to –0.01]), insulin, FGF21, and Vaspin (with the latter three not significant at the α = 0.05 level), and a direct association with PON1 (0.56; 95% CI, 0.10—0.83), which is itself directly associated with MetS. Here, FGF21 is linked to metabolic syndrome.