Photoperiod, but not a high-fat diet, alters body fat in Shaw's jird.

Many animal species living in temperate zones show annual body fat cycles. In an apparent regulation of total body fat, species showing naturally occurring decreases in adiposity in the fall (e.g., meadow voles and Siberian hamsters) are resistant to high-fat diet (HFD)-induced obesity in short, "winter-like" days (SDs), and in long, "summer-like" days (LDs) at their peak adiposity. SD-exposed Shaw's jirds (Meriones shawi) show SD-induced decreases in body fat; therefore, we predicted they also would be resistant to HFD-induced obesity. Male jirds were fed a standard chow diet or a HFD, and half of each group was exposed to LDs or SDs. SD-exposed jirds significantly decreased their carcass lipid content and testes mass compared with LD controls, but not body or WAT pad masses. HFD feeding in either photoperiod did not affect any of these measures, nor did it trigger overeating. Thus, it appears disadvantageous for this, and other species, that exhibit body fat peaks in the LDs of summer and nadirs in the SDs of winter, to fatten further or dampen their body fat losses, respectively, whereas species that exhibit body fat nadirs in the LDs of summer and peaks in the SDs of winter do become fat in LDs, and even fatter in SDs, when fed a HFD (e.g., Syrian hamsters). This dichotomous separation of HFD-induced body fat responses to HFD feeding among species showing opposite seasonal lipid mass peaks and nadirs may prove useful in understanding resistance or susceptibility to HFD-induced obesity, especially because they are naturally occurring.