High-fat diet negatively impacts both metabolic and behavioral health in outbred heterogeneous stock rats

Abstract

Obesity is influenced by genetics and diet and has wide ranging comorbidities, including anxiety and depressive disorders. Outbred heterogeneous stock (HS) rats are used for fine-genetic mapping of complex traits and may be useful for understanding gene by diet interactions. In this study, HS rats were fed diets containing 60% kcal from fat (high-fat diet, HFD) or 10% kcal from fat (low-fat diet, LFD) and tested for metabolic (study 1) and behavioral (study 2) outcomes. In study 1, we measured glucose tolerance, fasting glucose and insulin, fat pad weights and despair-like behavior in the forced swim test (FST). In study 2, we assessed anxiety-like (elevated plus maze, EPM; open field test, OFT) and despair-like/coping (splash test, SpT; and FST) behaviors. Body weight and food intake were measured weekly in both studies. We found negative effects of HFD on metabolic outcomes, including increased body weight and fat pad weights, decreased glucose tolerance, and increased fasting insulin. We also found negative effects of HFD on despair-like/coping and anxiety-like behaviors. These include increased immobility in the FST, decreased open arm time in the EPM, and increased movement and rest episodes and decreased rearing in the OFT. The diet-induced changes in EPM and OFT were independent of overall locomotion. Additionally, diet-induced changes in OFT behaviors were independent of adiposity, while adiposity was a confounding factor for EPM and FST behavior. This work establishes the HS as a model to study gene by diet interactions affecting metabolic and behavioral health.

Keywords: anxiety; behavior; depression model; obesity; outbred.

Fig. 1.

Study timelines. A: in study 1, heterogeneous stock (HS) rats were weaned at 3 wk of age and placed on high-fat diet (HFD) or low-fat diet (LFD) at 6 wk of age. After 8 wk of diet consumption, intraperitoneal glucose tolerance test (IPGTT) assayed glucose tolerance. Forced swim test (FST) was performed after 11 wk of diet consumption. Necropsy was performed after 12 wk of diet consumption. B: in study 2, HS rats were weaned at 3 wk of age and began HFD or LFD at 4 wk of age. Behavioral tests began after 8 wk of diet consumption and administered on a weekly basis for 4 wk. Behavioral tests, except splash test (ST), were repeated after 16 wk of diet consumption. Necropsy was performed after 21 wk of diet consumption. EPM, elevated plus maze; OFT, open field test.

Fig. 2.

Body weight and energy intake in studies 1 and 2. A, B: body weight was measured weekly in rats from diet start. Rats fed high-fat diet (HFD) showed a significantly higher body weight over time compared with rats fed low-fat diet (LFD). C, D: in study 1, but not study 2, rats on HFD had higher average daily energy intake compared with rats on LFD. Dotted line indicates beginning of testing, IPGTT in study 1 and EPM in study 2. Full statistics are reported in results.

Fig. 3.

Negative metabolic outcomes in rats fed high-fat diet (HFD) compared with low-fat diet (LFD). A, B: rats on HFD had significantly increased fat pad sizes compared with rats on LFD in both studies, except study 2 omental fat. C–E: in study 1, rats fed HFD had significantly increased fasting insulin and decreased glucose tolerance compared with rats fed LFD. Fasting glucose levels were not different between diet groups. Means ± SE are shown. ▲ and ○ represent individual rats on LFD or HFD, respectively; *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 4.

High-fat diet (HFD) induces despair-like phenotypes in the forced swim test (FST). A, B: in both study 1 and study 2, FST immobility increased in rats fed HFD compared with low-fat diet (LFD). C, D: a strong positive correlation was found between FST immobility and retroperitoneal fat pad size in study 1 and study 2 (r = 0.5399, P < 0.05 and r = 0.4161, P < 0.05, respectively). Means ± SE are shown. ▲ and ○ represent individual rats on LFD or HFD, respectively; *P < 0.05, **P < 0.01.

Fig. 5.

High-fat diet (HFD) induces anxiety-like phenotypes in the elevated plus maze (EPM). A–C: a significant increase in EPM closed arm time was seen after 8 wk of HFD compared with low-fat diet (LFD) consumption, as well as a significant decrease in EPM open arm time. No significant difference in closed arm explorations, a measure of general locomotion, was found between diet groups. Means ± SE are shown. ▲ and ○ represent individual rats on LFD or HFD, respectively; *P < 0.05.

Fig. 6.

High-fat diet (HFD) induces anxiety-like phenotypes in the OFT. A–C: after 9 wk of diet consumption, rats fed HFD showed a significant increase in rest episodes, movement episodes, and a significant decrease in rearing episodes after 5 min in the open field test (OFT) compared with rats fed low-fat diet (LFD). D: total distance traveled in the OFT was not significantly different between diet groups. E, F: after 30 min, there was no difference between rats fed HFD vs. LFD in rest episodes or movement episodes, but rearing episode count was significantly decreased in rats fed HFD compared with LFD. Means ± SE are shown. ▲ and ○ represent individual rats on LFD or HFD, respectively; *P < 0.05, **P < 0.01.

Fig. 7.

High-fat diet (HFD) increases grooming time in splash test. A: no difference was found between rats fed HFD vs. low-fat diet (LFD) in latency to begin grooming. B: rats fed HFD spent significantly more time grooming in the SpT after 10 wk of diet consumption compared with rats fed LFD. Means ± SE are shown. ▲ and ○ represent individual rats on LFD or HFD, respectively; *P < 0.05.

Fig. 8.

Diet-specific behavioral correlations between anxiety-like measures. A: rats fed high-fat diet (HFD) show a significant positive correlation between elevated plus maze (EPM) closed arm time and open field test (OFT) movement episode count (r = 0.6785, P < 0.05), while rats fed low-fat diet (LFD) did not correlate between these measures (r = 0.1995, P > 0.05). B: a negative correlation was found between EPM closed arm time and OFT rearing episode count in rats fed HFD (r = −0.6611, P < 0.05) but not LFD (r = 0.0788, P > 0.05).