To the authors’ knowledge, this was the first study investigating the interaction between CETP Taq1B polymorphism and DII and DIL on CVD risk factors in T2DM patients. The key findings of the current study were the significant interaction effect of CETP rs708272 polymorphism with DII and DIL on obesity indices (WC and BMI), lipid profiles (TG, HDL, LDL/HDL), inflammatory markers (IL-18, CRP, and PGF2α), and antioxidant markers (TAC and SOD) in T2DM patients.
T2DM patients have an elevated risk of CVD. Additionally, CETP is a key gene related to CVD pathogenesis (18). The relation between the CETP Taq1B polymorphism and the risk factors of CVD in patients with T2DM has been evaluated in previous studies. A significant interaction between CETP Taq1B polymorphism and DII, DIL was demonstrated in relation to obesity indices, including WC and BMI. CETP Taq1B polymorphism was found to be able to increase the effects of DII and DIL on obesity. In the present study, B2B2 genotype carriers in the last tertile of DII and DIL were more obese. Although several studies have reported an association between DIL and DII scores and obesity (19, 20), the interaction between DII and DIL with CETP polymorphism on obesity was not evaluated in previous reports.
DII and DIL may represent insulin concentrations in response to consuming certain foods, and are perhaps more appropriate than dietary GI and GL in assessing the relation between insulin exposure and metabolic disorders (21). It is proposed that DIL and DII have a direct and indirect effect on obesity. A high score of DIL and DII may lead to greater body fat formation by promoting pre-adipocytes differentiation and proliferation to adipocytes (19, 20). Besides, a high score may cause reductions in insulin sensitivity and lipolysis, while increasing insulin growth factor-1 (IGF-1) and insulin secretion levels (22, 23). Experimental studies have shown that a high stimulation of IGF-1 on pre-adipocytes proliferation could be associated with body fat accumulation (24). IGF-1 increased glucose uptake and glucose oxidation in adipocytes, elevated lipogenesis, and inhibited lipolysis in cells (25). According to some studies, insulinogenic foods result in a higher risk of insulin resistance and DII/DIL, independent from potential risk factors (26). Dietary pattern interventions, such as restricting insulinogenic foods, have a key role in the management of IR and its metabolic disorders (27). CETP is a protein synthesized by various tissues, especially adipose tissue in humans, which leads to the storing of CEs in adipocytes, resulting in the formation of fatty deposits (28). Results have suggested a potential pathway relating CETP polymorphisms with an elevated risk of obesity and obesity-related diseases (29). Additionally, there is a linear association between CETP activity and insulin resistance in obese T2DM patients; high CETP activity also contributes to high insulin secretion (30). Therefore, high DII/DIL score and CETP polymorphism may act synergistically to elevate a patient’s susceptibility to insulinemic spikes, which relate to obesity.
The present findings are consistent with previous studies that have reported elevated plasma CETP activity in obese participants. Dullaart et al. demonstrated that plasma CETP activity was increased in obese subjects. Also, CETP activity was related to BMI and plasma C-peptide (31). Arai et al. revealed that CETP activity and protein levels were elevated in obese participants (32). In contrast to the present study, however, Heilbronn et al. reported that BMI was higher in obese B1B2 participants, compared to obese subjects with B1B1 and B2B2 (33). Maroufi et al. reported that Taq1B polymorphisms had no effect on related metabolic syndrome parameters, including WC (34). Also, some studies have shown that CETP polymorphism is not associated with anthropometric parameters (35, 36). These inconsistencies seem due to an important role of gene-diet interactions. It has been shown that relations between CETP Taq1B and anthropometric indices can be population-specific, and consequently are regulated by environmental factors, especially dietary factors (37).
The present results also demonstrated a significant interaction among CETP Taq1B polymorphism and DII/DIL in relation to inflammatory factors. The highest IL-18, CRP, and PGF2α were observed in the B2B2 genotype carriers with the highest adherence to DIL and DII. As indicated in the findings, there was a significant interaction effect between Taq1B CETP polymorphism and DII/DIL in association with anthropometric indices. Numerous findings have reported obesity as causing chronic low-grade inflammatory disorder, contributing to the progression of T2DM and CVD (38). In these conditions, human adipose tissue secretes a high level of inflammatory markers, including IL-18, CRP, and PGF2α (39).
A further novel finding is the significant interaction between CETP Taq1B polymorphism and DII/DIL towards antioxidant markers, including TAC and SOD. CETP Taq1B polymorphism was able to inverse the effect of DII and DIL on oxidant status, so that the highest TAC and SOD was observed in the B2B2 genotype with the highest adherence to DIL and DII. However, while there is no available study about the relation between DIL/DII with antioxidant status, several studies have reported that insulin concentrations and insulin resistance lead to an imbalance between oxidant and antioxidant systems, a condition known as oxidative stress (40). In recent years, oxidative stress has been implicated in T2DM pathogenesis by producing excessive free radicals, decreasing glutathione, vitamin E, vitamin C, and via reduced antioxidant enzyme activity, such as SOD and TAC (41). SOD and TAC lead to the conversion of superoxide radicals into hydrogen peroxide, and decrease oxygen toxicity (41). It seems that CETP Taq1B polymorphism can invert the effect produced by insulin. However, the reason for this difference is not apparent, and further studies are warranted to evaluate the mechanism of action.
Finally, another promising finding was the significant interaction between CETP Taq1B polymorphism and DII/DIL with lipid profile markers, including TG, HDL, and LDL/HDL. The lowest TG and LDL/HDL, and the highest HDL, were observed in the B1B1 genotype carriers with the highest adherence to DIL and DII. Although several studies have shown a relationship between DII/DIL scores and lipid profiles, the interaction between DII/DIL and CETP polymorphism on lipid profiles had not been evaluated in previous studies.
Additionally, prior research has shown that the B1B1 genotype is associated with a better response to nutritional interventions, compared with carriers of B2 alleles. Previous research showed a significant interaction, where B1/B1homozygotes had a lower TG/HDL ratio after a kiwifruit intervention, compared to a control diet, while B2 carriers were not affected (42). Nahid Ramezani-Jolfaie et al. revealed that, for diabetic patients, dietary oil treatments would be more helpful (lower LDL; HDL, TG; HDL, TC; HDL, Insulin, and HOMA-IR) among subjects with B1B1 alleles than among B2 allele carriers (43).
Some authors have also suggested that subjects with B1B1 allele of CETP polymorphism showed a better response in regards to high carbohydrate dietary interventions. Overall these findings are in accordance with findings reported by Juan Dua et al., demonstrating that males with CETP Taq1B B1B1 allele have higher apo A-I and HDL concentrations after following a high carbohydrate and low fat (HC/LF) diet for 6 days (44). Perez et al. reported that carriers of B2 alleles who consumed sucrose as more than 5% of total kcal/day had higher TC and LDL serum levels, compared to B1B1 homozygotes (45).
A series of recent studies have indicated that plasma CETP activity decreased by hyperinsulinemia condition in healthy subjects, but not in diabetic patients (46). Siewert et al. suggested that insulin has a direct influence on CETP, but in IR conditions, this particular insulin action may be diminished (47). Therefore, it is suggested that diabetic patients with B1B1 allele may be counteracted with CETP activity and HDL plasma reduction.
Several studies have been shown that CETP is able to transfer HDL cholesterol esters to Apolipoprotein B (ApoB), including VLDL, remnants of VLDL, and LDL. Due to an important role of CETP polymorphisms in lipids metabolism, it is considered that genetic polymorphism in the CETP gene is related to CVD risk factors by changing serum lipid profiles (48, 49). The inconsistency may be due to distinct populations; the studies were conducted among different gender, ethnic, and geographic populations, with a variety of habitual dietary patterns and various underlying diseases.