It has been shown that digestibility is closely associated with contents of oil. In general, there are two factors that have an influence on lipid digestibility, and the first element is the amount of oil intake. It has been suggested that the level of oil in diet is related to lipid digestibility17,18. The second element is the chain length of fatty acid and the proportion of SFAs, MUFAs and PUFAs. It has been reported that soybean oil, which has high polyunsaturated fatty acids, enhances nutrient digestibility19. This finding has been verified in this study showing that the weight and body fat rate of mice fed with diet 6.5% soybean oil increased, which may be associated with the increased percentage of UFAs of FAs, particularly the amount of PUFAs (nearly 59%), and its proportion was significantly higher than lard and soybean oil.

Cholesterol, an important lipid, comprises the structure of cell membrane. Low-density lipoprotein (LDL) is responsible of transferring endogenous cholesterol from liver to peripheral tissues. The accumulation of cholesterol in serum is strongly related to the increase of LDL-C in serum20. High-density lipoprotein (HDL) is a lipoprotein which plays a role in anti-atherogenic property by reversing cholesterol transport from the peripheral tissues to liver. The content of HDL-C, LDL-C and TG was increased in mice fed with 6.5% blended oil compared to 6.5% lard and soybean oil. This is likely to be attributed to the effect of the increased metabolism of lipid in liver and adipose. Additionally, the ratio of serum HDL-C and LDL-C is one of the most intuitive evaluation indicators for serum lipids. In mice fed with 3.8% blended oil, the ratio of HDL-C/LDL-C is lower than the other groups. It is obvious that those levels of blood lipids can be adjusted with diet oil blended with lard and soybean at 25 g per day for one person.

When the lipid transfer or metabolism in the liver exceeds the range of its own capacity, it causes too much ectopic fat deposition which results in nonalcoholic fatty liver disease21. We found that there was obvious decrement of TG in liver in mice fed with diet 6.5% oil. In liver sections, lipid deposition was not observed. In contrary to TG in liver, TG in fat was significantly increased in mice fed with diet 6.5% oil. It was suggested that TG was mainly transported to adipose tissue for storage while taking 6.5% oil. The mechanism underlying the distinction of lipid transport between mice fed with diets of different proportion of oils needs to be explored further.

Adipose tissue plays a crucial role in lipid storage, energy homeostasis, and insulin sensitivity in the body. The redundant energy uptake from food is stored in adipose tissues in the form of triglyceride, and released to plasma in the form of fatty acid. The redundancy of subcutaneous and visceral adipose are the main factors of obesity. Adipogenesis is the process of cell differentiation by which pre-adipocytes become mature adipocytes in adipose tissues, and the morphology and function of cells were changed from pre-adipocytes to mature adipocytes.

It has been demonstrated that the level of PPARγ expression is increased during the development of readipocytes into mature adipocytes22, and that the differentiation process could not complete with the deficiency of PPARγ23. C/EBPα is also important for adipogenesis, and C/EBPα knockout mice fail to accumulate lipids in adipose tissues, and die of hypoglycemia and liver defect shortly after birth. In this study, the mRNA and protein expression levels of PPARγ were higher in the adipose of mice fed with 6.5% lard and 6.5% soybean oil or blended oil, and their transcripts were higher in mice fed with 6.5% lard than fed 3.8% lard. The C/EBPα mRNA expression level also higher in the adipose of mice fed with 6.5% lard than the mice fed with the same volumes of the other two types of oils. Previous studies have reported that the natural ligands of PPARs mainly include natural or modified PUFAs and eicosanoid24,25, and the inhibiting adipogenesis effect of the SFAs-derived feeds are stronger than the UFAs-derived diets in swine26. Our results revealed that the mRNA expression levels of PPARγ and C/EBPα were parallel to the volume of lard and SFAs, which is inconsistent with the studies showed above. Duan et al. reported that a diet with a lower n-6: n-3 PUFA ratio could reduce the expression levels of PPARγ in pig models27, we speculated that the lower expression levels of PPARγ and C/EBPα on account of a lower n-6: n-3 PUFA ratio in blended oil, which need to be further examined. Collectively, the higher doses of lard, improves the differentiation of adipocytes in adipose tissues of mice.

In mammals, fatty acids are necessary for lipid accumulation, and the principle source of fatty acids is de novo synthesis in liver and adipose tissues. FAS is the key enzyme of de novo synthesis for endogenous fatty acids, which is mainly expressed in liver and adipose tissues and catalyze carbohydrates to fatty acids. SREBP-1 belongs to the family member of transcription factors that regulate cellular lipogenesis and lipid homeostasis and control, it regulates the expression of several enzymes that are involved in the synthesis of cholesterol, fatty acid, triacylglycerol and phospholipid28, which increases the expression of FAS and PPARγ mRNA and participates in the formation of endogenous ligands for PPARγ29. In this study, both of the SREBP1 and FAS mRNA expression levels in adipose of mice fed with 3.8% oils were lower than the mice fed with 6.5% oils. Among the mice fed with 6.5% oils, the mRNA and protein expression levels of SREBP1 and FAS in adipose of mice fed with soybean oil were higher than the mice fed with lard and blended oil. Previous researchers have showed that PUFAs have remarkable effects on reducing expression of FAS mRNA, but increasing the level of SFAs30,31. From the perspective of fatty acids of oils, we found the contrary results, showing that high dose of soybean oil, which is rich in PUFAs, has the ability to stimulate the expression levels of SREBP-1 and FAS. Considered together, digestibility for lipids and other nutrients is stronger in mice fed with 6.5% soybean oil than mice fed with lard and blended oil. Higher dose of soybean oil has the potential to improve lipogenesis in adipose of mice. However, higher concentration of blended oil may have the reverse effect.

ATGL plays an essential role in lipolysis—a series of orderly controlled process of TG hydrolysis, which leads to the formation of DG and FA32. In contrast to wild type mice, there is a reduction of FA release from WAT by more than 75% in ATGL-deficient mice33. We observed that higher lard enhanced the expression level of ATGL mRNA, which was in accordance with the expression of PPARγ mRNA and protein. This was consistent with previous studies showing that the expression of ATGL mRNA is positively regulated by PPARγ34,35. APN is a proteohormone that is secreted by adipose, and the circulating APN is involved in appetite, glycometabolism and lipid metabolism regulation. Leptin has been regarded as a signal in the brain to inhibit food intake and decrease weight36. The secretion concentrations of both leptin and APN showed positive correlation with mRNA expression levels. Obesity is accompanied by high leptin levels and leptin resistance37, and consequently the circulating leptin level is proportional to adipose tissue mass38. Among the mice fed with 6.5%t oils, lard had a distinct ability to increase the expression level of APN mRNA, but soybean oil assumed a reverse effect. The expression levels of APN mRNA were also in accord with PPARγ mRNA, which is confirms with the previous findings reported by Li et al.39. In the present study, a lower expression level of leptin mRNA in mice fed with blended oil at 6.5% level, suggesting that a lower n-6: n-3 PUFA ratio in blended oil, which is agreement with previous findings40. The expression levels of leptin mRNA approximately identify with body fat rate in mice, indicating that leptin resistance occurs in the obese mice fed with higher soybean oil. Therefore, 6.5% lard or blended oil has a much stronger capacity of stimulating lipolysis than soybean oil in mice.

In summary, we have demonstrated that the blended oil has a significant effect on anti-obesity. The blended oil also has the ability of decreasing blood lipids, preventing the damage of lipid peroxidation in liver and reducing the abnormity of renal function. The mechanisms of anti-obesity effect of blended oil may be attributed to the regulation of PPARγ, C/EBPα, SREBP1, FAS and ATGL. In adipose tissues, the processes of adipogenesis and lipogenesis are suppressed, and the hydrolysis of stored triglycerides is promoted. As a result, body weight and body fat rate of the subjects are decreased. Soybean oil at the intake of current Chinese residents (41.4 g/d), results in the increase in the body weight and body fat mass. Alternatively, lard at 41.4 g/d leads to the reduction in anti-oxidant in liver, an increase in the burden of kidney, an elevation of adipogenesis and hydrolysis of triglycerides in adipose tissues. We have revealed that the traditional Chinese eating habit, taking in oils blended with lard and soybean oil, is one of the factors of lower percentage of overweight and obesity in China. This study provides a novel insight into reasons for increasing rates of obesity over the past 20 years in China, which might be attributed to the increase of dietary oil intake and the change of the oil components.