Randomized trial and rodent experiments have shown that SFA rich diet induce body fat, serum lipids, and liver lipids accumulation 22–25. However, discordant results also be reported 26, 27. Our present study also found that SFA rich diet (lard) induce body fat (p>0.05) and serum lipids (p<0.05) accumulation than PUFA rich diet (sunflower oil or soybean oil), but liver lipids accumulation higher in PUFA rich diet (sunflower oil (p>0.05) or soybean oil) than SFA rich diet (lard), especially in soybean oil diet. Deol et al. showed that soybean oil diet induces greater weight gain, adiposity, and fatty liver than coconut oil diet that rich in SFA 26. Liver damage caused by dietary cholesterol in mice was strongly enhanced by a high fat diet containing soybean oil, but not by a lard-based high fat diet, soybean oil-based diet enhanced cholesterol-induced mitochondrial damage and amplified the ensuing oxidative stress 27. Di Rienzi proved that toxicity of soybean oil fatty acid inhibits growth of Lactobacilli, beneficial members of the small intestinal microbiota 28. Jurgoński also found that cecal butyrate level higher in lard diet rats than soybean oil diet rats, benefit gut metabolism 22. Studies showed that the abundance of short chain fatty acid bacteria such as Bifidobacterium, Enterococcus and Allobaculum increased with high lard diet 29–31. Thus, it is speculated that different fat/oil might exert different effects in different tissues, mix lard with vegetable oil (a balance fatty acid diet) benefit for lipids metabolism from multiple metabolic pathways. In the next step, we will explore effect of lard and soybean oil diet on gut microbiota and metabolite change in gut and liver.
Our previous study (under 25% fat energy) found that a mixture of lard and soybean oil could reduce body fat accumulation compared to lard or soybean oil 20, it was also proved in present study (under 30% fat energy), and can be extended to sunflower oil. However, under 35% fat energy, the function cannot be observed 21. Catta-Preta M et al. found that a lard diet can induce adipose mass generation compared with the sunflower oil diet and other vegetable oils 25. In fact, most studies evaluated the nutritional benefits of dietary oils based on a high-fat diet model 32, 33. In these models, the energy obtained from fat is typically 40–60%. Part of studies with lower fat energy present inconsistent results with Catta-Preta, Bourgeois F et al. showed that mice fed with various kinds of dietary fat/oil (43% energy from lipids), specifically lard, beef tallow, sunflower oil, and soybean oil, can induce obesity 34. A high-fat diet with 42% energy for 12 weeks also showed that rats fed with lard and olive oil were the most obese, having no significant differences between these diets 35. The contradictory results may be attributed to the different level of fat energy, but the mechanism worth further exploration.
In the present study, mixture of lard and vegetable oil (sunflower oil or soybean oil) activated AMPK compared to single oil diet, and mixture of lard and soybean oil increased serum GLP-1 and glucagon levels. AMPK plays a key role in regulating energy metabolism 36. Liver AMPK can decrease the rate of hepatic lipogenesis. Its phosphorylation leads to the inhibition of fatty acid biosynthesis via phosphorylation of acetyl Co-A carboxylase (ACC), thus affecting malonyl-CoA content that synthesis catalyzed by ACC 37. The activity of CPT-1α can be regulated by malonyl-CoA 38, besides, activation of AMPK also can induce HSL and inhibit FAS expression 39. HSL is a key enzyme that catalyzes the rate-limiting step of adipose tissue lipolysis from TG to FFA 40. A previous study showed that administration of a specific HSL inhibitor can reduce the serum FFA levels in mice, rats, and dogs, demonstrating its role in vivo 41. Thus, the expression of HSL is closely associated with FFA content. HSL protein expressions were higher in lard, L-SFO, and L-SBO groups, benefit serum TG lipolysis to FFA, however, serum FFA cannot be oxidized due to inactivation of the AMPK and lower expression of CPT-1α, it also explains why serum FFA levels in the mice fed with lard were significantly higher than those fed with other oils. Interestingly, the mixture of lard and vegetable oil not only induced the expression of HSL protein but also activated AMPK pathway, thereby increasing the expression of CPT-1α proteins. This, subsequently, reduced fatty acid synthesis, induced higher levels of CPT-1 proteins, and, ultimately, strengthened fatty acid oxidation, indicating that the energy expenditure was higher. Palmitoleic acid has been reported to improve metabolic functions by increasing the AMPK phosphorylation in the fatty liver induced by high-fat-diet 42.
Fasted GLU levels among groups were no significant different, even though GLU of mice fed with lard and mixture of lard and soybean oil were lower than other three groups. Wang et al showed that mice fed with high lard diet and high SBO diet (15% weight of oil) for 18 weeks showed no significant difference in terms of glucose level, although those fed with lard showed a decreasing glucose level 43. Other studies yielded similar results 44. PAI-1 is an important regulator of the fibrinolytic process. Correlation heatmap showed that PAI-1 had no significant correlated with phenotypes of ectopic fat accumulation and GLU, actually, there still have debate that whether PAI-1 could be a predictor of atherosclerotic 45. It was put forwarded that the contradiction may be due to neglecting the distinction between physiological and pathological conditions, PAI-1 level still within physiological conditions 46. The main source of glucagon is the pancreatic α-cell, while intestinal L-cells and neurons in the nucleus of the solitary tract are the principal producers of GLP-1, and it was reported therapeutic approaches to treating obesity based on glucagon/GLP-1 synergistic interaction 47. Glucagon’s main action is related to glucose homeostasis, where glucagon stimulation of hepatic glucose production by increasing glycogenolysis and gluconeogenesis, and simultaneously inhibiting glycogen synthesis, serves to restore glucose levels in hypoglycemic states. Metabolic actions of glucagon are exerted through a unique receptor which is mainly expressed in liver. GLP-1 secreted following food intake, and its action including inhibits glucagon secretion, stimulates insulin release, and decreases hepatic gluconeogenesis 48. In recent years, it was point out that glucagon and GLP-1 play a role in brown fat thermogenesis without decrement of food intake, glucagon and GLP-1 induced oxygen consumption and body temperature 49. Dual agonists glucagon/GLP-1 induced a massive decrease in body weight in diet-induced obese mice 50–52. The co-upregulated of glucagon and GLP-1 in mice fed with mixture of lard and soybean oil may contribute to improved lipids metabolism.