Background: Small frequent meals are often recommended for weight loss, with supporting evidence often provided from studies in diabetes. Dietary meal content is also relevant, as high fat meals cause systemic inflammation via gut derived bacteria, endotoxin. As such, repeated meals may exacerbate this. In contrast, dietary induced thermogenesis, related to meal size, may reduce with small frequent meals.Aim: Therefore, the aim of this study was to compare the effect of 2 vs 5 meals on metabolic endotoxaemia and 24 h (hour) energy expenditure in lean and obese women.Methods: In a crossover study, 24 lean (age: 34 (mean±S.D.)±10 years, BMI: 22.9±2 kg/m2) and obese (age: 42±9 years, BMI: 36±8 kg/m2) women were given two or five isocaloric high (50%) fat meals, on two separate days. On both visits, 24 h energy expenditure was measured in whole body room calorimeters and blood samples taken 2 hourly (0900 to 2100 h). Serum endotoxin, glucose, insulin, lipids were measured.Results: The obese subjects had increased area under the curve (AUC) for insulin, glucose, HOMA-IR and triglyceride (TG), with decreased HDL (P<0.01), compared with lean subjects, for both meal visits. For the entire cohort, fasting endotoxin correlated with triglyceride (r=0.32, P<0.05), and AUC for endotoxin and TG correlated in the five meal visit (r=0.44, P<0.05), but not the two meal visit. In the final 2100 h blood test, the endotoxin levels were significantly higher in the five meal visit (P=0.05), but not the two meal visit. Meal frequency did not affect 24 h expenditure, in either the obese group (2124±312 vs 2142±365 Kcal/day) or lean group (1724±160 vs 1683±166 Kcal/day).Conclusion: Our findings suggest in metabolically healthy lean and obese subjects, increased meal frequency may pose an inflammatory risk posed by circulating endotoxin and TGs leading to peak levels at bedtime. As such, small frequent meals may not influence diet induced thermogenesis, but may increase metabolic disease risk.