Background/Aims: The 825T allele of the G protein β3-subunit gene (GNB3) is a thrifty genotype associated with an increased risk for obesity. We aimed to determine whether the 825T allele is modifying the subjective response to extended fasting. Methods: We genotyped 108 subjects who underwent an 8-day modified medical fasting treatment [total energy intake <350 kcal (1465 kJ)/day] for the GNB3 C825T polymorphism. Perceived hunger and mood were recorded daily by self-rating visual analogue scales. Results: Whereas weight loss was not dependent on genotype, both mood and hunger were significantly associated with genotype, with homozygous CC genotype carriers having best mood (p = 0.004) and least hunger (p = 0.036) during fasting compared to TT genotype carriers. Conclusions: Pronounced mental discomfort during fasting in 825T allele carriers might partly explain their increased risk for obesity. The strong association between the subjective response to fasting with GNB3 genotypes indicates a role of the gene in the behavioural regulation of food intake, which should be further considered in nutritional intervention studies.