Being one of the richest sources of ALA, perilla oil consists of 69.11% ALA. After a 16-week dietary treatment with a diet rich in perilla oil, the level of lipids in the serum lowered significantly and hepatic fatty oxidation improved. In contrast, insulin sensitivity, as measured by a euglycemic-hyperinsulinemic clamp, was decreased by the perilla oil-rich diet.
Excessive ALA intake did not cause significant increases in body weight or total body fat in the HP group , which is similar to the role of EPA/DHA used to prevent obesity and overweightness . Contrarily, after high-fat diet derived from lard or soybean oil were fed to rats or mice, the results showed that the animals had severe intra-abdominal fat and fat deposition, and their body weights were much higher than those of the control group [18–20]. These results suggest that perilla may affect triglyceride deposition in adipose tissue and fat cell activities (such as cell differentiation and cytokine secretion).
In the present study, the serum levels of triglycerides and total cholesterol of rats in the HP group decreased by 86.5 and 40.2%, respectively, compared with those in the control group. In accord with our data, when the mice were respectively fed a high-ALA diet (12% ALA and 4% LA) and a high-LA diet (12% LA and 3% ALA), the serum levels of total cholesterol of the mice fed with the high-dose ALA decreased significantly after 35 days of feeding . Clinical trials demonstrated that patients with metabolic syndrome had a daily ALA intake of 3.5 g/d on their body weight, and their serum levels of triglycerides and total cholesterol decreased significantly after six months of dietary intervention . Therefore, the data suggest taking high doses of ALA was effective in their accepted role of lowering TG and TCH, and this method may be used to treat patients to whom statins are clinically not applicable. However, the n-3 PUFAs currently used mainly originate from marine products (EPA/DHA) [23–25], while few studies have focused on ALA. In addition, the serum levels of high-density lipoprotein cholesterol in the HP group were significantly lower than those in the control group. The reason for this result may be that peripheral cholesterol does not require being transported to the liver, due to cholesterol reduction.
Liver plays a central role in maintaining energy balance and contributing to energy storage in the fed state. The results of the present study show that, compared with the control group, the hepatic mRNA levels of PPAR-α and CPT1A in the HP group increased by 171 and 85%, respectively, with a significant increase in hepatic CPT1A protein expression. However, no difference in hepatic PPAR-α protein expression was shown, suggesting that the high-perilla intake enhanced β-oxidation. 3 T3-L1 cells were treated with ALA, and by DNA microarray analysis the results show that a 1.7-fold increase occurred in the CPT-1a gene expression . In animal experiments, it has been proven that ALA may enhance fatty acid oxidation. When rats were fed with flaxseed oil and perilla oil, the mRNA expression levels of hepatic CPT-I and CPT-II significantly increased, but no effect was shown in terms of fatty acid synthesis . Gonzalez-Manan D. fed rats with chia (Salvia hispanica) and rosa mosqueta (Rosa rubiginosa) rich in ALA, and found significant increases in the PPAR-α transcript level and CPT1A protein expression level after 21 days of feeding . In the present study, the mRNA expressions of hepatic SREBP-1, FASN and ACC of rats in the HP group were significantly higher, while the protein expressions of FASN and ACC were significantly lower, compared with those of the control group. In addition, the SREBP-1 protein expression showed no significant difference between the two groups. These results indicate that the high-dose ALA inhibited fat synthesis by regulating the translation pathway. SREBP-1 has been found to be a positive transcriptional regulator of the cytosolic lipogenic enzymes, which work in sequence to citrate carrier (CIC). Direct evidence was provided that PUFA decrease CIC gene promoter activity, the CIC transcriptional suppression by PUFA was clearly mediated by the SRE/SREBP-1 regulatory system as its effects on CIC promoter-driven luciferase activity was abolished by mutations in the SRE site of the CIC gene [29–31]. However, the rats in the HP group were given a long-term diet with high energy, and excessive fat deposition did not occur in the liver, due to an increase in fatty acid oxidation and a decrease protein expression of key enzymes in fat synthesis. This finding is in agreement with the observation, obese Zucker rats were fed a diet containing 4% ALA, and hepatic fat accumulation was also significantly inhibited after one month of feeding .
In the present study, the mass proportion of ALA in the high-fat diet was 15.74%, and the data from the 16-week clamp experiment show a significant reduction in insulin sensitivity in the HP group. In our previous studies, rats were fed a high-fat diet with the same proportion of perilla, and their insulin sensitivity indexes also decreased significantly after eight weeks of feeding . There have been very few studies in this area to directly compare our data to. However, in terms of application in humans, one study found that an intake of high-dose long-chain n-3 PUFAs (5–8 g/d) increased the blood glucose level in patients with type II diabetes . The reason for this insulin resistance caused by long-term excessive ALA may be that the activity of fatty acid β-oxidation leads to acetyl-CoA accumulation, then pyruvate dehydrogenase activity inhibited by excessive intracellular acetyl-CoA causes citric acid accumulation. As a potential inhibitor of phosphofructokinase, citric acid blocks glucose oxidation in the initial stage, resulting in a decrease in the glucose transportation rate of GLUT4 . Many studies have shown that an excessive saturated fat intake caused an increase of FFA, and elevated FFA had a “toxic” effect on the body, causing damage to the pancreatic beta cell function, promoting cell apoptosis, and resulting in impaired glucose-stimulated insulin secretion . Male Sprague–Dawley rats were fed a chow-diet or a diet high in saturated fat or PUFA (derived from lard and coconut oil or pufa derived from safflower oil). The results show that after 8 weeks, both high-fat diets increased the plasma FFA by 30% . However, excessive ALA intake did not cause elevated serum FFA levels of the rats in the HP group.
After long-term excessive perilla intake, no significant changes were shown in the body weight, hepatic fat deposition or intra-abdominal fat of the rats. In addition, a significant reduction was seen in the serum lipid level, as well as significant increases in the expressions of key enzymes in fatty acid oxidation, along with significant decreases in the protein expressions of key enzymes in fatty acid synthesis. However, the experimental results of the insulin sensitivity reduction will incite interest in researchers studying ALA dosage. More studies regarding the functions and mechanisms of ALA in the bodies of animals and their verification in the human body must be conducted in order to determine the most effective doses of ALA for prevention and treatment.