FABP2 and obstructive sleep apnea syndrome: Li et al. [89] measured plasma D- lactic acid (D-LA) and intestinal fatty acid-binding protein (I-FABP) levels using colorimetry and ELISA, respectively as biomarkers of gut barrier integrity in adults with OSA (defined as AHI ≥ 5 events per hour) and those without OSA (AHI < 5 events per hour).