Protein elicits a stronger early (30 min) glucose-dependent insulinotropic polypeptide (GIP) response than fat ingestion in lean individuals, with no difference in glucagon-like peptide-1 (GLP-1). We assessed the incretin hormone response to protein versus fat ingestion in obesity. Equicaloric (8 kcal/kg) fat (olive oil) or protein (whey protein) was ingested by non-diabetic obese male volunteers [body mass index (BMI) >30 kg/m(2) ; n = 12] and plasma GIP and GLP-1 were determined. We found no difference in the early GIP or GLP-1 responses to fat versus protein. However, the total 300-min GIP response was greater after fat than after protein ingestion (20.3 ± 3.9 vs. 10.0 ± 2.8 nmol/l × min; p = 0.026), whereas the 300-min GLP-1 responses were the same. Thus, in obesity, protein and fat ingestion elicit similar early (30 min) incretin hormone responses, whereas 300-min GIP secretion is more pronounced after fat than protein ingestion.