Dietary protein restriction in rats leads to a rapid within-session preference for protein

Evolution has provided species with adaptive behavioural mechanisms that guarantee tight dietary protein regulation. However, an unsolved question is whether the well-established preference for protein-containing food manifested during states of protein restriction is innate or learned. Here, we tackled this problem by maintaining male rats on either a low-protein diet (4 % protein, protein-restricted) or a control diet (22 % protein, non-restricted) for 9-12 days, and then offered them two novel foods, a protein-containing solution (4 % casein) and a carbohydrate-containing solution (4 % maltodextrin) during a daily 60-minute free-choice test, repeated for 5 consecutive days. We assessed both the total and cumulative intake of each solution throughout each test, as well as the microstructure of licking behaviour as an index of the solutions' palatability. In a second experiment, we exposed a different cohort of rats, before any behavioural test, to the same protein source (i.e., casein) that they would encounter during the free-choice tests, to assess whether familiarity with casein would drive subsequent casein intake even in non-restricted rats. We found that dietary protein restriction leads to a rapid preference (within 5 min of first exposure) for a casein-rich solution, and this preference is persistent over subsequent exposures. Increased palatability of protein during initial exposure correlated with protein preference in the restricted rats. Moreover, familiarity with casein did not lead to protein preference in non-restricted rats. This study demonstrates that, when in need of protein, protein preference is a rapid adaptation that requires minimal experience of protein.