Current study showed that consumption of 30 g/day of both canola or soybean oil in comparison to control significantly decreased fasting blood glucose and increased proinsulin to insulin ratio in overweight and obese patients with T2D. Consumption of these two oils also significantly reduced serum levels of TC and LDL and increased HDL concentrations, which was more considerable among those who consumed soybean oil. Body weight and waist circumference significantly decreased only in canola oil group. No significant changes were seen in serum levels of TG and HbA1C, and serum levels of inflammatory factors (ESR and CRP), urea and Cr, and in white blood cell (WBC), red blood cell (RBC), and platelet (PLT) levels.
Consumption of either soybean or canola oil resulted in significant reduction of FBS. Soybean and canola oil consumption also was associated to increased proinsulin to insulin ratio. Increased proinsulin to insulin ratio is a common manifest of T2D associated to insulin resistance or impaired conversion of proinsulin to insulin . Significant improvement in blood glucose without changes in HbA1C and significant elevation of proinsulin to insulin ratio shows that consumption of these two oils had no significant effect on insulin resistance, as the major complication of T2D mellitus. Consumption of non-fried soya, but not its oil, resulted in significant reduction of insulin resistance in Japanese adults in a cross-sectional study . Consumption of canola oil in patients with T2D reduced insulin resistance and serum levels of insulin in a randomized clinical trial . It should be noted that most studies in this area have been done on animal models and further human studies are needed to reach a clear conclusion. Our study also found significant reduction of anthropometric measures in T2D patients following consumption of canola oil. Due to significant beneficial changes in anthropometric measures, it might be suggested that consumption of soybean or canola oil for a longer time influences more considerably insulin resistance in patients with T2D.
This study also showed that consumption of 30 g/day of soybean and canola oil has beneficial effects on lipid profiles, including TC, LDL, and HDL in patients with type 2 diabetes mellitus (T2D). These effects were more pronounced in the patients who consumed soybean oil that those at CO group. Earlier studies reported inconsistence findings for the association of soybean and canola oil with serum lipid profiles. Gulesserian et al. examined seventeen 4-19-year-old children and adolescents with baseline TC of 233±35 mg/dl who received canola oil for 5 months (15 gr/day in the first two months and 22 gr/day in the next three months). Serum levels of TG, TC and LDL-c decreased following canola oil consumption in that study, with no significant changes in serum HDL-c concentrations . Negele et al. used lipid-lowering diets with canola and sunflower oils for 3 weeks in 95 hyperlipidemic subjects. In both regimes LDL, TC, HDL and TG decreased . In another study by Bierenbaum et al., 30 ml/day of canola oil rather than other edible oils in daily diet of 46 patients with hyperlipidemia (baseline LDL level was 173±9 mg/dl) for 4 months decreased LDL-c, but had no significant influence on TC and HDL levels . It seems that baseline level of lipid profiles is a crucial confounder for the effect of consumed oils on serum levels of these variables.
Canola and soybean oil are dietary sources of phytosterols . Phytosterols (plant sterols) are structural analogs of cholesterol . Several studies have shown that consumption of phytosterols reduces blood cholesterol, through which they reduce risk of cardiovascular diseases in diabetic and non-diabetic patients [29, 30]. In addition, soybean oil is a major source of linoleic acid and linolenic acid . Consumption of linoleic acid has been associated to reduced risk of cardiovascular diseases by modulation of serum lipids . Moreover, canola oil (11%) and soybean oil (7%) are two major sources of dietary alpha-Linolenic acid (ALA) [15, 17]. Several studies have indicated that consumption of ALA can improve function of the adipose tissue and inhibit fat accumulation through the activated protein kinase-related activities . It has been generally accepted that the effects of n-3PUFA can be due to peroxisome proliferator-activated receptors (PPARs), especially PPARα, such that n-3PUFA acts as a ligand for PPARs . Some other studies have shown that long-chain fatty acids can regulate carnitine palmitoyltransferase I (CPT1), by which they restrict rate of mitochondrial fatty acid oxidation [35, 36]. In addition, n-3PUFA stimulates 5'AMP-activated protein kinase in the adipose tissue . AMP-activated protein kinase (AMPK) is an enzyme that plays an important role in the energy homeostasis in adipose tissues . These mentioned mechanisms might help to explain effects of CO and SO on the anthropometric measures.
Small sample size and short duration of intervention were among the main limitations of the current study. In addition, because participants were at an age range of 45-49, the results cannot be generalized to young and elderly populations. It is necessary to done similar studies on other individuals and groups in order to investigate effects of canola and soybean oils on biochemical and anthropometric indices in patients with diabetes mellitus.