The objective is to present the case of a 21-yr-old athlete observed with non-physiological immediate postprandial insulin response (1162 pmol/l; normal glucose excursion: 6.6 mmol/l), in a warm environment. No suspicion or evidence of any underlying pathology was found in this well-trained Afro-Caribbean male runner. He never reported any hypoglycemic episode. When performing the same protocol performed in a cooler environment (21.0°C vs 30.3°C), only physiological responses were observed. We conclude that 1) youth, leanness and regular exercise training are not absolutely protective against glucose metabolism impairment in apparently healthy subjects; 2) ambient temperature should be regarded as a potential source of glucose metabolism impairment.