2.2. Measurement of AGEs in the Fingertip Skin

To estimate AGEs, the participants underwent measurements of the sAF levels using the AGEs Sensor (Air Water Biodesign Inc., Kobe, Japan). The sAF values obtained with the excitation and emission wavelengths of 365 nm and 440 nm, respectively, were used to estimate the levels of AGE accumulation. The obtained sAF levels are correlated positively with the level of the hyperglycemia-associated AGE, Nδ-(5-hydro-5-methyl-4-imidazolone-2-yl)-ornithine (MG-H1) [41]. The sAF is correlated with collagen-linked fluorescence (CLF) (excitation at 370 nm, emission at 440 nm) that degrades in skin biopsy specimen [19,42,43]. The levels of skin CLF reflect the actual levels of fluorescent and non-fluorescent AGEs such as pentosidine and MG-H1, and these levels of AGEs are correlated with well-known abundant AGEs such as Nε-(carboxymethyl)-lysine (CML) and Nε-(carboxyethyl)lysine [19,42,43]; therefore, the sAF is a good proxy for AGEs accumulation in tissues.

The finger clip feature of the device enables measurement of the intensity of the fluorescence from the middle finger of the non-dominant hand where the least skin melanin is present. Although the intensity of autofluorescence from veins is approximately 1.5-fold higher than that of skin, the influence of the veins is negligible in the measurement of the sAF using the AGEs Sensor, since the fingertips have only capillaries and no veins. Therefore, the fingertip is suitable to avoid non-specific skin fluorescence [41]. Trained examiners performed all measurements.

The measured AGEs were expressed in arbitrary units. According to our pilot study, the coefficient of variation and intraclass correlation coefficient (Cronbach’s α) of three repeated AGE measurements were 6.7 ± 7.3% and 0.938, respectively.