Discussion
Several advantages are associated with the use of the pig as a model for human nutrition research. These include its similar organ sizes compared to humans, similar digestive tract architecture, and similar lipid and carbohydrate metabolism. Additionally, the Ossabaw pig is an excellent model for the study of metabolic syndrome as this animal model easily develops dyslipidemia when fed a diet high in fatty acids and cholesterol. Consumption of a high fat, high calorie diet is also associated with increased risk of obesity, diabetes and cardiovascular disease. Soy oil is a major vegetable oil consumed in the US. It is rich in PUFAs which are highly susceptible to oxidative damage. Therefore, soy oil is usually hydrogenated to increase the shelf life of products made from it, a process that results in generation of trans fatty acids.
Consumption of high amounts of trans fat is linked to increased risk of cardiovascular diseases. Lowering the content of α-linolenic acid to reduce soy oil unsaturation is a strategy for eliminating the need for soy oil hydrogenation and improving the shelf life of food products containing soy oil. Although this approach lowers the degree of unsaturation of soy oil, it increases its (n-6)/(n-3) ratio and the implications of long-term consumption of this oil on metabolic status are unknown. There has been no systematic study to characterize the metabolic response to consumption of the low α-linolenic compared to the standard soy oil. We report herein a comparison of several metabolic responses that may be associated with human consumption of the two types of soy oil using the pig model.
The estimated daily intakes of linoleic and α-linolenic acid in all treatments groups far exceeded the estimated daily intake for humans which ranged from 10.4-14.7 g/d for linoleic acid and 1.1-1.6 g/d for α-linolenic acid. A major reason for this difference is the much larger feed intake in pigs relative to humans and the difference in feed composition between pigs and humans in general. The pig diet is also different from typical human diet due the absence of long chain PUFA such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The n6:n3 ratios of 12.36 and 7.54 in the control and SBO diets are not too far off from the 10.6:1 ratio in human diets. However the ratio of 29.2 consumed in the LLO diet is quite different from human diet and far from the recommended n6:n3 of 4.0 in human diets. The higher dietary n6:n3 ratio in the LLO diets is further reflected in the elevation of this ratio in the serum and subcutaneous adipose tissue as well. This underscores the disruption that may occur to this ratio in serum and adipose tissue if low α-linolenic acid represents a major source of n3 fatty acids in human diets. Because insulin resistant state if often marked by elevated serum insulin and glucose concentrations, the elevated glucose and insulin concentrations in the SBO diet relative to the CON and LLO diets could suggest development of insulin resistance in pigs on this treatment, and indicate that the LLO oil prevents pigs from developing insulin resistance, despite consuming similar level of dietary fat as in the SBO diet. Likewise, the higher levels of triglycerides, LDL-cholesterol and the lower level of HDL-cholesterol in pigs on the SBO diet may point to development of dyslipidemia, a condition that is marked by elevated levels of blood lipids. Thus consumption of LLO oil may offer some protection against development of both insulin resistance and dyslipidemia. Therefore, the disruption of the n6:n3 ratio in the LLO did not result in a significant adverse metabolic response. Consumption of both SBO and LLO diets resulted in a lower level of CRP than in pigs on the CON diet. CRP is a cardiovascular disease marker. The reduction in CRP concentration in the SBO and LLO diets is consistent with the reported effect of PUFA in reducing CRP concentration and indicates that despite the reduced content of α-linolenic acid, the low α- linolenic oil was effective in lowering the concentration of this cardiovascular disease marker and this might indicate that α-linolenic acid may not be the major factor in soy oil regulating the concentration of CRP. Soy is rich in linoleic acid, and due to its anti-inflammatory property, this fatty acid could be the major player in lowering the concentration of CRP.
Several distinct differences in the serum fatty acid profiles between pigs on the SBO and LLO diets could explain the different serum metabolic profile in the pigs. The LLO diet resulted in higher cis 9, trans11 CLA, reduced total SFA, increased total PUFA content and higher PUFA: SFA ratio in the serum than the SBO diet. It has been reported previously that dietary inclusion of cis 9, trans11 CLA in mice led to improved glucose tolerance, insulin sensitivity and reduction in triacylglycerol content compared to control fed mice. Choi et al. showed a higher response to insulin in rats whose diets were supplemented with CLA. Furthermore, inclusion of cis 9, trans11 CLA in ob/ob mice improved insulin signaling. SFAs have been shown to impair insulin signaling whereas PUFAs enhance it. Therefore, the overall changes in the fatty acid profile in the LLO treatment may support enhanced insulin sensitivity in vivo. Increased insulin sensitivity will also result in reduced serum cholesterol concentration. This is in agreement with the reduced serum cholesterol content in the serum of LLO pigs observed in this study. As found earlier, consumption of cis 9, trans11 CLA by ApoE knockout mice resulted in reduced triglycerides, improved glucose tolerance and insulin sensitivity compared to control fed mice. Thus cis 9, trans11 CLA may have played a big role in the apparent enhanced insulin sensitivity in the LLO diet. Finally, the lower SFA, higher PUFA content and higher PUFA: SFA ratio in the LLO fed pigs could have contributed to their improved metabolic profile as well. Consumption of lower amounts of SFA and consumption of higher amounts of PUFA are both associated with reduction in risks for development of inflammation and metabolic syndrome. Although associations between consumption of individual fatty acids and serum metabolites are well established, it is probable that the overall fatty acid profile and the complex interactions it engenders between individual components may play important roles in determining the dietary effects observed. A comprehensive metabolomics analysis may be needed in the future for such a determination. Obesity is currently regarded as low-grade chronic inflammatory disease and consumption of n3 fatty acid enriched diets may offer protection against obesity-induced inflammation. In this study consumption of both SBO and LLO diets resulted in elevated adipose tissue expression of IL-6. However, the higher expression of TNFα in the adipose tissue of pigs on the LLO diet than those on the CON diet may suggest a potential loss of some anti-inflammatory effects of ALA in the LLO diet and this could be a disadvantage of consuming a diet low in ALA. Nevertheless, the overall limited effect of reducing ALA level in the diet of pigs on the LLO diet may suggest a weak link between ALA and the regulation of inflammation. Indeed, in healthy human subjects with large waist circumferences, increased ALA consumption from flaxseed oil consumption failed to reduce inflammatory makers. Therefore, the importance of ALA consumption in regulating adipose tissue inflammation could be complex and additional studies are required to further elucidate the importance of ALA in the regulation of inflammation.
Adiponectin is an adipocyte-derived hormone that can improve insulin sensitivity by regulating glucose utilization and fatty acid metabolism. It is possible that the elevated serum and subcutaneous adipose tissue adiponectin abundance in the SBO group is related to the elevated oleic acid abundance in the serum and subcutaneous adipose tissue of pigs in this group. In support of this hypothesis, a recent study by Granados et al. showed that incubation of 3T3-L1 adipocytes with oleic acid increased adiponectin mRNA levels. Positive association between adipose tissue oleic acid content and serum adiponectin concentration has also been reported. Therefore, elevated oleic acid level in the serum and adipose tissue of SBO pigs may partly contribute to the higher expression of adiponectin in this treatment.
In summary, we report herein that feeding the low α-linolenic soy oil resulted in alteration of serum metabolite profile marked by reduced serum glucose, insulin, triglycerides and total and LDL cholesterol concentrations compared to regular soy oil. However, there were no significant changes in the expression of inflammatory markers. We speculate that these changes may be driven by the reduction in SFA content, elevation of PUFA and cis 9, trans11 CLA content and the increase in PUFA: SFA ratio following consumption of the LLO diet.