Daily exercise vs. caloric restriction for prevention of nonalcoholic fatty liver disease in the OLETF rat model

Abstract

The maintenance of normal body weight either through dietary modification or being habitually more physically active is associated with reduced incidence of nonalcoholic fatty liver disease (NAFLD). However, the means by which weight gain is prevented and potential mechanisms activated remain largely unstudied. Here, we sought to determine the effects of obesity prevention by daily exercise vs. caloric restriction on NAFLD in the hyperphagic, Otsuka Long-Evans Tokushima Fatty (OLETF) rat. At 4 wk of age, male OLETF rats (n = 7-8/group) were randomized to groups of ad libitum fed, sedentary (OLETF-SED), voluntary wheel running exercise (OLETF-EX), or caloric restriction (OLETF-CR; 70% of SED) until 40 wk of age. Nonhyperphagic, control strain Long-Evans Tokushima Otsuka (LETO) rats were kept in sedentary cage conditions for the duration of the study (LETO-SED). Both daily exercise and caloric restriction prevented obesity and the development of type 2 diabetes observed in the OLETF-SED rats, with glucose tolerance during a glucose tolerance test improved to a greater extent in the OLETF-EX animals (30-50% lower glucose and insulin areas under the curve, P < 0.05). Both daily exercise and caloric restriction also prevented excess hepatic triglyceride and diacylglycerol accumulation (P < 0.001), hepatocyte ballooning and nuclear displacement, and the increased perivenular fibrosis and collagen deposition that occurred in the obese OLETF-SED animals. However, despite similar hepatic phenotypes, OLETF-EX rats also exhibited increased hepatic mitochondrial fatty acid oxidation, enhanced oxidative enzyme function and protein content, and further suppression of hepatic de novo lipogenesis proteins compared with OLETF-CR. Prevention of obesity by either daily exercise or caloric restriction attenuates NAFLD development in OLETF rats. However, daily exercise may offer additional health benefits on glucose homeostasis and hepatic mitochondrial function compared with restricted diet alone.

Fig. 1.

Weekly food consumption (A) and body weight gain (B). Values are means ± SE (n = 6–8 animals in each group). *Otsuka Long-Evans Tokushima Fatty sedentary (OLETF-SED) significantly different from other animal groups at respective ages (P < 0.001). Inset, values with different superscripts are significantly different (P < 0.001). OLETF-EX, exercised OLETF rats; LETO-SED, sedentary Long-Evans Tokushima Otsuka rats; OLETF-CR, calorie-restricted OLETF rats.

Fig. 2.

Systemic glucose homeostasis as assessed by an intraperitoneal glucose tolerance test. Glucose responses across time (A), glucose area under curve (AUC; B), insulin responses across time (C), and insulin AUC (D). Values are means ± SE (n = 5–6). Values with different superscripts are significantly different (P < 0.05).

Fig. 3.

Representative images of hematoxylin and eosin (H&E) (×10 and 40 fields of view; A and B), Oil-Red O (C), and picrosirius red (D) staining. Note the large lipid vacuoles (A and B), macro- and microvesicular steatosis (A-C), hepatocyte ballooning and nuclear displacement (A and B), and perivenular fibrosis (C) in the 40-wk-old OLETF-SED animals compared with the other animal groups.

Fig. 4.

Quantification of hepatic triglyceride (TG) and steatosis scores is shown in A and B and hepatic diacylglycerol (DAG) content is shown in C. Values are means ± SE (n = 6–8). Values with different superscripts are significantly different (P < 0.01).

Fig. 5.

Effects of daily exercise and caloric restriction on hepatic complete palmitate oxidation to CO₂ (A), total mitochondrial palmitate oxidation [CO₂ and acid soluble metabolites (ASMs); B], total extramitochondrial palmitate oxidation (C), total lignocerate oxidation (CO₂ and ASMs; D), β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity (E), and citrate synthase activity (F). Values are means ± SE (n = 6–8). Values with different superscripts are significantly different (P < 0.05).

Fig. 6.

Effects of daily exercise and caloric restriction on hepatic carnitine palmitoyl-CoA transferease (CPT)-1 protein content (A), peroxisome proliferator-activated receptor (PPAR)-α protein content (B), and oxidative phosphorylation (OXPHOS) complex I-V protein content (D). Representative Western blots are shown in C and E. Values are means ± SE (n = 6–8). Values with different superscripts are significantly different (P < 0.05). AU, arbitrary units; ACOX1, acyl-CoA oxidase 1; CYP4A, cytochrome P-450 A2.

Fig. 7.

Effects of daily exercise and caloric restriction on the following hepatic markers of de novo lipogenesis: acetyl-coenzyme A carboxylase (ACC, A), phospho (p)-ACC (B), fatty acid synthase (FAS, C), phospho-mammalian target of rapamycin (mTOR, D), sterol regulatory element binding protein 1c (SREBP-1c, E), and stearoyl-CoA desaturase-1 (SCD-1, F). Representative Western blots are shown in G. Values are means ± SE (n = 6–8). Values with different superscripts are significantly different (P < 0.05).