Within versus between group designs, and not timing of onset of puberty, influence sex and age differences in intake of palatable food in rats

•Developmental changes in intake of a palatable food from early adolescence to adulthood depended on design.•Sex differences were observed in a within-group design, not in a between-group design.•A within-group design may promote bingeing (excessive consumption in a discrete period of time).•The practice of normalizing intake by body mass is questioned for intake of palatable foods. Understanding normative development of sensitivity to palatable food in adolescence is key to developing animal models for preclinical research of disorders of reward systems (e.g., eating disorders). Nevertheless, few studies have investigated changes in consumption of palatable food in both sexes and the role of the timing of onset of puberty as a factor. Here, we tested multiple ages of adolescence in both female and male Long-Evans rats and used both a within-group and between-group testing paradigm to compare effects of repeated testing on consumption of diluted sweetened condensed milk. In the within-group, female rats consumed more per body mass at postnatal day (P) 27 and declined in intake thereafter. Male rats also had the highest intake per body mass at P27 and declined thereafter, although a second peak was evident at P48, and the intake of females was greater than that of males from P41 on except at P48. In those tested at one age only (between-group design), there were no sex differences across ages, and the decline reached a plateau at P48 in both sexes. Further, intake per body weight was less in the between-group rats than in the within-group rats, suggesting that the within-group design used here as in previous studies may induce bingeing (excessive consumption in a discrete period of time). Thus, such a design may not capture normative development but rather sensitized intake akin to behavioural sensitization to drugs of abuse and/or binge-eating. There was no evidence of an effect of timing of puberty onset on intake in either design. The results show how methodological factors may compromise the interpretation of the development of, and sex differences in, the intake of palatable foods.