Provided are methods for identifying or monitoring a subject having, or at risk of developing, impaired glucose homeostasis. Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) is shown to be a biomarker for impaired glucose homeostasis and/or conditions characterized by β-cell dysfunction. Comparing a test level of CMPF in a subject to a control level identifies subjects having, or at risk of developing, impaired glucose homeostasis. Also provided are methods of causing impaired glucose homeostasis or β-cell dysfunction and methods of screening for compounds that affect the activity of β-cells. Also provided are methods for the treatment of β-cell dysfunction by reducing the physiological levels of CMPF in a subject as well as the use of a OAT modulator for the treatment of β-cell dysfunction.