The ratio of measurement values of glucose, citric acid, and cis-aconitic acid in a biological sample of a subject to the measurement values of healthy subjects is calculated, and the glucose ratio, citric acid ratio, and cis-aconitic acid ratio are used to assess and/or determine fatigue. In the same way, the isocitric acid ratio, succinic acid ratio, malic acid ratio, and lactic acid ratio are calculated, combined with the three abovementioned ratios, and used to assess and/or determine fatigue. Thus enabled is the objective and simple diagnosis and assessment of fatigue.