To the best of our knowledge, the current research is the first attempt to study the interaction between CARTPT rs2239670 polymorphism and dietary NEAC on cardio-metabolic risk factors and hypothalamic hormones. Our finding suggest that the rs2239670 on chromosome 5q13-14 interacts with dietary ORAC, FRAP and TEAC to influence obesity and obesity-related metabolic phenotypes. Specifically, when compliance with ORAC was high, minor-allele-carriers exhibited obesity-related traits such as BMI and FM%. On the other hand, improving the adherence to dietary FRAP might reduce the genetic association with insulin resistance indices (HOMA-IR and QUICKI) only among G allele carriers (AG and GG). However, positive interactions of CARTPT rs2239670 on serum glucose level were found even in high compliance with FRAP and TEAC. Thus, the results of modification effect of diet on the associations of CARTPT with obesity and metabolic factors were not homogenous. According to our results, frequency of the rare allele (20.79%) was nearly similar to the range of reported values in other population or ethnic groups such as Korean (17%)  and Malaysian (30%) . This discrepancy in the minor allele frequency reported might be explained by differences in study sample size, demographic characteristics of population like age, ethnicity, and gender and also various lifestyles. Previous evidence has revealed that polymorphisms in the CARTPT gene are linked to human obesity. In this regard, leu34Phe missense mutation in CART gene was detected in Italian subjects with early-onset obesity . Likewise, it was reported that the A-156G polymorphism in the promoter region of CARTPT was related to obesity among Japanese subjects . Moreover, the studies have shown that genetic variation in the CART locus might influence susceptibility to MetS and its components such as dyslipidemia, high blood pressure and hyperglycemia . It should be taken into account that all variants in CARTPT gene are not associated with obesity phenotypes and finding in this regard is conflicting . For example, in accordance with the results of Walder study, C1442G variant of CARTPT gene was not associated with obesity in Pima Indians . The previous studies which have specifically examined the association between the CARTPT rs2239670 polymorphism and addictive behaviors such as alcohol dependence have reported a positive association . However, the studies which have evaluated this variant in relation to obesity are scarce. To the best of our knowledge, only one study have investigated the CARTPT rs2239670-obesity relation and showed no association between this variant and obesity among the Malaysian subjects . All of these heterogeneities in finding warrants further research efforts among different populations. Since the rs2239670 variant located in the intron 1 of CARTPT gene and the strong effects on obesity and related metabolic factors may be removed during mRNA splicing encoding CART proteins, effects of this variant on CART function is still unknown . However, genetic variation in the CART locus may effect on the expression of the CART peptide, which is related to hypothalamic anorectic and orexigenic neuropeptides . Totally, it seems that CART peptides have an anorexigenic effect, although the mechanism of this function is not completely known .
As far as we are aware, no previous research has investigated the gene-diet interactions of CARTPT with adherence to the dietary NEAC on metabolic profile in obesity to compare our finding. However, there have been numerous researches investigating the interaction between genetic variation and diet or dietary ingredients on obesity and its-related complications . For example, Mirzababaei and et al. examined the interaction of the rs1333048 variant on 9p21 genetic region with TAC on the risk of MetS and they revealed that high ORAC intake may modify the increased risk of MetS in homozygous subjects for the minor allele (AA genotype) . Accordingly, in other our recent study, we documented the significant interactions between dietary quality indices and CARTPT rs2239670 genotypes affecting metabolic parameters ; higher adherence to diet quality index-international (DQI-I) reduced the metabolic risk in obese individuals with the homozygous minor allele genotype (AA).
Noticeably, the main finding of our study was the association between the CARTPT rs2239670 polymorphism and cardio-metabolic factors depended on the dietary antioxidant intakes; a good compliance with NEAC blunted the association of the CARTPT gene with metabolic factors. While the underlying mechanisms behind this interactions is not still clarified, these favorable effects of the total antioxidant capacity may be mediated by vitamin C, vitamin E and its isomers, selenium, carotenoids, isoflavones, flavonoids, and proanthocyanidins [41, 42]. In this regard, there are numerous studies which have indicated the beneficial effects of antioxidant-rich foods (e.g. fruits and vegetables and tea) on the obesity, insulin resistance, glucose homeostasis and lipid profiles [43, 44]. These favorable effects of these antioxidants on the improvement of metabolic profile may be partly attributed to other activities of antioxidants such as regulation of brown adipose tissue metabolism and enhancement of thermogenesis, suppression of adipogenesis and induction of catabolism in adipose tissue .
The strengths of the present study is that according to our current knowledge, it is the first study to examine the interaction between CARTPT rs2239670 genotypes and dietary NEAC on the metabolic factors in obese subjects and identifying these gene-diet interactions may provide the best personalized dietary advice for high-risk participants according to their genetic makeup to reduce the burden of obesity and its-related chronic diseases.
However, the present study has several potential limitations that should be considered. Firstly, since the present study is a cross-sectional study, ascertained causality cannot be argued but it helps to generate hypotheses that can be examined by prospective cohort or other studies. Secondly, a rather small sample size of our study may not have sufficient statistical power to detect the interaction effect. So, the results of our study should be interpreted with caution and require replication and confirmation in larger and different populations. Thirdly, the present study was limited to the assessment of only one variant from a single gene, while multiple genes are known to be involved in the pathogenesis of obesity and its-related consequences. Fourthly, because dietary intakes and other lifestyle factors in Tabriz may differ from those in other parts of the country with different cultures, our results may not necessarily be extrapolated to the general population. Fifthly, despite adjustment for several confounders in the analyses, residual confounding by other unmeasured factors was inevitable. Lastly, under-reporting of dietary intakes particularly in obese individuals is a source of measurements error which may cause underestimation of the true effect . Thus, we excluded the extreme-energy reporters from analysis.