- Reduction of serum retinol-binding protein-4 levels in nondiabetic obstructive sleep apnea patients under continuous positive airway pressure treatment. Nena, E., Steiropoulos, P., Tzouvelekis, A., Tsara, V., Hatzizisi, O., Kyriazis, G., Froudarakis, M., Trakada, G., Papanas, N., Bouros, D. Respiration (2010)