Concomitantly, the chicken fed mice had deteriorated plasma lipid profile and enlarged liver mass with elevated hepatic TG levels. Thus, we demonstrate that different protein sources affect diet-induced obesity and associated co-morbidities in C57BL/6J mice when given at normal levels in a HF background diet. Body fat accretion was reduced, evident as lower body mass gain, lower adipose tissue masses and reduced liver TG, in the casein and cod/scallop fed compared to the chicken fed mice. Interestingly, the apparent fat digestibility was reduced from an average of about 98% in LF, chicken and cod/scallop fed mice, to an average of about 95% in casein fed mice. Assuming that the apparent fat digestibility was constant for the entire seven week period, the casein fed mice absorbed approximately five g less fat than LF, chicken and cod/scallop fed mice. In mice, intake of a HF casein diet has previously been reported to cause higher fecal fat excretion and a leaner phenotype as compared to intake of a HF salmon diet. Hence, it is likely that the reduced apparent fat absorption was a contributing factor to the reduced fat accretion in casein fed mice in the present study. The cod/scallop fed mice maintained a lean phenotype, relative to chicken fed mice, without a reduction in fat absorption. To elucidate whether the protein sources modulated energy metabolism, we subjected the mice to indirect calorimetric measurements before onset of obesity at the transition from LF to HF feeding. HF diets have previously been shown to disturb feeding pattern and behavioral circadian rhythm in mice, such that the LF diet-induced fluctuations in RER PI-103 PI3K inhibitor between light and dark phases, reflecting different feed intake and substrate oxidations, are completely abolished after a switch to a casein based HF diet. Accordingly, the RER was promptly reduced, and the differences in RER between light and dark phases disappeared after the switch to HF diets in the present study. There was no protein source effect on RER in the present study. However, following the transition from LF diet to HF diets EE decreased less in the cod/ scallop fed than in casein fed mice, but we observed no significant difference in EE between chicken fed and cod/scallop fed mice that could explain the difference in adiposity. Our indirect calorimetry setup monitored gas exchange of each mouse for 1.9 minutes every 30 minutes, and it has been argued that the monitoring frequency has to be considerably higher in order to detect the 2–5% changes in diet-induced EE sufficient to elicit long term alterations on energy balance. Decreased spontaneous locomotor activity has previously been demonstrated at the transition from LF to HF diets. Accordingly, a decrease in spontaneous locomotor activity was observed in the light phases concurrent with a tendency towards decreased total activity after the shift from LF diet to HF diets in the present study. Importantly, cod/scallop feeding tended to avert this decrease in activity at the transition from LF to HF feeding. In line with this notion, we have previously observed an inverse correlation between locomotor activity and development of dietinduced obesity, without being able to detect differences in EE.