Daylength and body mass affect diet self-selection by Siberian hamsters.

Siberian hamsters exhibit a decrease in peak body mass/fat and caloric intake when they are exposed to short winter-like days (SDs) compared with hamsters exposed to long summer-like days (LDs). We hypothesize that the photoperiod may directly or indirectly mediate alterations in gustatory processing that may account for shifts in the preferences for specific food types and/or macronutrients. Two experiments lasting 14 days each were conducted to: 1) identify photoperiod-related differences in diet selection, and; 2) determine whether these differences were a primary effect of SD exposure or a secondary effect due to SD-induced decreases in body fat/mass. In Experiment 1, two groups of hamsters were exposed to either LDs or SDs and given access to three complex food types [sunflower seeds (SF); rabbit chow (RC); millet seeds (M)]. In addition to their photoperiod-induced decreased body and fat pad mass, and caloric intake, SD-housed hamsters exhibited significant differences in their patterns of diet selection and relative amounts of fat, carbohydrate (COH), and protein eaten compared with LD-housed hamsters. By the end of the experiment, SD-housed hamsters selected a diet higher in COH and protein and lower in fat than that selected by LD-housed hamsters. Because SD-housed hamsters decrease their body mass/fat, this may reflect an adaptive strategy to maintain a leaner body mass. A second experiment was conducted to test whether the diet selection pattern of SD-housed hamsters was due to the SD-induced decrease in body mass/fat. LD-housed hamsters were allowed to feed ad lib (AL group), or were food restricted (FR group) to simulate a SD-induced decrease in body mass. Both groups were then released into the diet self-selection paradigm. FR hamsters reversed their food restriction-induced decrease in body mass and showed concurrent alterations in diet self-selection patterns that were distinct from the SD-induced patterns seen in Experiment 1. Specifically, they showed a persistent hyperphagia and selected a larger proportion of their total calories as M (high COH) and less as SF (high fat) than did AL hamsters during their return to LD body mass levels. However, once LD body mass levels were achieved, they increased their SF intake, and thus fat consumption. Despite these dynamic daily changes, the average selection patterns of both the diet types and macronutrients were not different by the end of the experiment. Therefore, it appears that the SD-induced changes in the selection of food sources (and ultimately, the macronutrient composition of the diet) cannot be solely attributable to the SD-induced decreases in body mass.