Background:Energy-yielding liquids elicit weak suppressive appetite responses and weak compensatory responses, suggesting that liquid calories might lead to a positive energy balance. However, data is often derived from foods differing in many characteristics other than viscosity.Objective:To investigate the effect of viscosity on ad libitum food intake in real-life setting and to investigate whether a difference in ad libitum intake is related to eating rate and/or eating effort.Design:In real-life setting 108 nonrestrained subjects (26+/-7 years, BMI 22.7+/-2.4 kg m(-2)) received a chocolate flavored liquid, semi-liquid and semi-solid milk-based product, similar in palatability, macronutrient composition and energy density. In laboratory setting 49 nonrestrained subjects (24+/-6 years, BMI 22.2+/-2.3 kg m(-2)) received the liquid or semi-solid product. Effort and eating rate were controlled by means of a peristaltic pump.Results:In real-life setting the intake of the liquid (809+/-396 g) was respectively 14 and 30% higher compared to the semi-liquid (699+/-391 g) and semi-solid product (566+/-311 g; P<0.0001). In laboratory setting, removing eating effort, resulted in a 29% (P<0.0001) intake difference between liquid (319+/-176 g) and semi-solid (226+/-122 g). Standardizing eating rate resulted in 12% difference between liquid (200+/-106 g) and semi-solid (176+/-88 g; P=0.24). If not controlled, the difference in intake between liquid (419+/-216 g) and semi-solid (277+/-130 g) was comparable to the real-life setting (34%; P<0.0001).Conclusions:Products different in viscosity but similar in palatability, macronutrient composition and energy density lead to significant differences in intake. This difference is partially explained by the higher eating rate of liquids.International Journal of Obesity advance online publication, 11 December 2007; doi:10.1038/sj.ijo.0803776.