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 occu rring decreases in adiposity in the fall (e.g., meadow voles and Siberian h amsters) are resistant to high-fat diet (HFD)induced obesity in short, "win ter-like" days (SDs), and in long, "summer-like" days (LDs) at their peak a diposity. SD-exposed Shaw's jirds (Meriones shawi) show SD-induced decrease s 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 hal f of each group was exposed to LDs or SDs. SD-exposed jirds significantly d ecreased their carcass lipid content and testes mass compared with LD contr ols, 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 a ppears disadvantageous for this, and other species, that exhibit body fat p eaks in the LDs of summer and nadirs in the SDs of winter, to fatten furthe r or dampen their body fat losses, respectively, whereas species that exhib it body fat nadirs in the LDs of summer and peaks in the SDs of winter do b ecome fat in LDs, and even fatter in SDs, when fed a HFD (e.g., Syrian hams ters). This dichotomous separation of HFD-induced body fat responses to HFD feeding among species showing opposite seasonal lipid mass peaks and nadir s may prove useful in understanding resistance or susceptibility to HFD-ind uced obesity, especially because they are naturally occurring. (C) 1999 Els evier Science Inc. All rights reserved.