Previously, using the breath hydrogen test, we investigated seasonal variations in the amount of unabsorbed dietary carbohydrate from the intestine after breakfast in female Japanese university students and young Polish subjects. In the study we found that there were significant seasonal variations in both countries with the smallest unabsorbed dietary carbohydrate in autumn and the biggest in winter. Considering the theory of human adaptation to living environments, we suggested a hypothesis that this seasonal variation in the efficiency of carbohydrate absorption in the intestine may reflect human adaptation and/or a response to seasonal change in the living environment. In order to prove this hypothesis, we carried out the same examination in Chiang Mai, Thailand, where there is different seasonal change in the living environment from that of Japan and of Poland. In this examination, we measured the amount of unabsorbed dietary carbohydrate (UDC) from the intestine after breakfast and the oro-cecal transit time (OCTT) of the breakfast in female Thai university students using the same method and experimental protocol as previously carried out with Japanese subjects. We conducted the examination in April (the hot season), in August–September (the rainy season), and in November–December (the dry season) of 2008, at the Medical School of Chiang Mai University, Chiang Mai. The results are summarized as follows: (1) there were no significant seasonal variations in the amounts of unabsorbed dietary carbohydrate from breakfast in Thai subjects; (2) there were no significant seasonal variations in the oro-cecal transit times of breakfast or a soluble indigestible trisaccharide (lactosucrose) solution in Thai subjects; (3) there were no significant differences in the oro-cecal transit times of breakfast between the two countries in any season; (4) the UDC of Thai subjects was significantly less than that of the Japanese subjects in the three seasons. These results suggest that differences in seasonal change in a living environment have different effects on seasonal variations in the efficiency of carbohydrate absorption in the intestine after breakfast.