Strength Training Prevents Hyperinsulinemia, Insulin Resistance, and Inflammation Independent of Weight Loss in Fructose-Fed Animals

Abstract

The aim of this study was to compare the effects of aerobic, strength, and combined training on metabolic disorders induced by a fructose-rich diet. Wistar rats (120 days old) were randomized into five groups (n = 8-14): C (control diet and sedentary), F (fed the fructose-rich diet and sedentary), FA (fed the fructose-rich diet and subject to aerobic exercise), FS (fed the fructose-rich diet and subject to strength exercise), and FAS (fed the fructose-rich diet and subject to combined aerobic and strength exercises). After the 8-week experiment, glucose homeostasis, blood biochemistry, tissue triglycerides, and inflammation were evaluated and analyzed. The strength protocol exerted greater effects on glucose homeostasis, insulin sensitivity, and liver lipid contents than other protocols (all P < 0.05). All three exercise protocols induced a remarkable reduction in inflammation, tissue triglyceride content, and inflammatory pathways, which was achieved through c-Jun NH2-terminal kinase (JNK) phosphorylation and factor nuclear kappa B (NFkB) activation in both the liver and the muscle. Our data suggest that strength training reduced the severity of most of the metabolic disorders induced by a fructose-rich diet and could be the most effective strategy to prevent or treat fructose-induced metabolic diseases.

Figure 1. Aerobic exercise seems to reduce food intake and body weight.

(A) (i) Minimum lactate (ML) test in one animal: the estimated ML was 2.67% of body weight, while the interpolated blood lactate concentration was 4.87 mM. In the second animal (ii) the estimated ML was 4.37% of body weight, while the interpolated blood lactate concentration was 5.03 mM. (B) All of the (i) Body weight change, (ii) Final Weight, and (iii) Area Under Curve of Body Weight did not change during the experiment. (iv) F animals showed higher food intake (AUC) than the FA group (p ≤ 0.01). C: Control; F: Fructose; FA: Fructose Aerobic; FAS: Fructose Combined; FS: Fructose Strength. n = 14 animals per group. Different letters mean significant difference.

Figure 2. Hyperinsulinemia, glucose intolerance and insulin resistance were attenuated by aerobic physical exercise while strength training was able to restore all these markers to control levels.

(A) (i) Serum glucose kinetics (mg/dl), (ii) Area Under Glucose Curve (mg*120 min/dl, AUC) (iii) Serum Insulin Kinetics (ng/dl), and (vi) Area Under Insulin Curve (ng*120 min/dl, AUC) during the oral glucose tolerance test (oGTT). Animals in the F and FA groups showed higher AUC of glucose than C (p ≤ 0.01) and FS (p ≤ 0.01 vs F, and p ≤ 0.05 vs FA The F group animals showed higher insulin values at 30-minute peak; p ≤ p ≤ 0.01–0.005) than all other groups. In addition, the animals of the F group showed a higher AUC of insulin (p ≤ 0.001–0.005) than all other groups. (B) (i) The fructose-rich diet was able to induce a hyperinsulinemia (F diet vs C diet; p ≤ 0.0001). On the other hand, all exercise protocols were successful in reducing the insulin levels (Sedentary vs Exercised; p ≤ 0.01–0.005), but still, these levels were higher than the control animals (p ≤ 0.05–0.01). (ii) Glucose kinetics (mg/dl), and (iii) Glucose Removal Rate (KITT in %/min⁻¹) during the insulin tolerance test (ITT). The F group showed a lower insulin sensitivity (KITT) than all other groups (p ≤ p ≤ 0.001-0.0001). Also, the FA group showed a lower insulin sensitivity than C (p ≤ 0.001) and FS p ≤ −0.005). C: Control; F: Fructose; FA: Fructose Aerobic; FAS: Fructose Combined; FS: Fructose Strength. n = 14 animals per group. Different letters mean significant difference.

Figure 3. Physical exercise results in an almost complete restoration of the liver microscopic aspect to that of control animals with a reduction in liver triglycerides and inflammation.

(A) C: Control; F: Fructose; FA: Fructose Aerobic; FAS: Fructose Combined; FS: Fructose Strength. A portion of the right lobe was extirpated to the H&E staining histology. The histology revealed that fructose-fed animals had larger triglyceride storages characterizing diffuse macro vesicular steatosis. (B) IκB-α and p-IκKα/β levels were the same for all groups. On the other hand, JNK phosphorylation and total NF-κB (p50) were higher in all Fructose-fed animals than in C (C diet vs F diet, p ≤ 0.01) and in the FA and FAS than in F and FS groups (p ≤ 0.01). Gels have been run at same conditions. For IκB-α, NF-κB (p50), phospho-JNK and JNK two different gels were blotted in the same PVDF membrane to avoid differences in the transfer and exposure. Western blotting signal was detected by special films and later scanned for density quantification. C: Control; F: Fructose; FA: Fructose Aerobic; FAS: Fructose Combined; FS: Fructose Strength. n = 4–6 animals per group. Different letters mean significant difference.

Figure 4. Physical exercise attenuated skeletal muscle inflammation.

All exercises protocols increased IKB-α and the aerobic and strength increased IL-10 level in the soleus muscle more than in F (p ≤ 0.05). Also, fructose-exercised animals (FA, FAS and FS) present reduced levels of NFκB (p50) as compared to F (p ≤ 0.05). Gels have been run at same conditions. The western blotting signal was detected by special films and later scanned for density quantification. C: Control; F: Fructose; FA: Fructose Aerobic; FAS: Fructose Combined; FS: Fructose Strength. n = 2–3 animals per group. Different letters mean significant difference.

Figure 5. Heatmap correlating IKbip genes to insulin pathway, inflammatory cytokines, and mitochondrial biogenesis markers.

The heatmap contains data from mRNA obtained from a large cohort of BXD family (mice strains generated by crossing C57BL/6J and DBA/2J); (row max is related to higher levels of each gene, in each row, being red for the positive correlation and blue negative correlation). Gene functions/pathways: IKBIP: chosen inflammatory marker; GSK3A: gluconeogenesis marker; TNFAIP, RIPK1, TRAF2: inflammatory markers; INSR, IRS2, PIK3IP1, PDK4, IGF1R: genes related to insulin pathway; ACOT2, SIRT1, PPRC1, CYP27A1, COX18: genes related to mitochondrial functions; UCP3: gene related to energy expenditure; IL6st: gene related to exercise-induced adaptation; NFRKB: gene inversely related to IKBIP levels.