We found a significant interaction between saturated fatty acid intake and the −256T>C APOA2 genotypes for BMI. Our results are consistent with the findings of previous studies conducted in the three American populations (Corella et al. 2009), Mediterranean population, Chinese and Asian Indian subjects (Corella et al. 2011) who participated in a Singapore National Health Survey. These studies showed the association of high SFAs intake with higher BMI and obesity in CC genotype.
In our population, the frequency of homozygous minor allele carriers (CC) was 12.5 % while the distribution of CC genotype differed in various populations (1–16 %; Corella et al. 2009, 2011; Smith et al. 2011). In FOS (16 %; Corella et al. 2009), GOLDEN study (15 %; Corella et al. 2009) and Mediterranean population (15 %; Corella et al. 2011), the prevalence of CC subjects was higher than our and other population, whereas the frequency of CC genotype was the same of our study (12 %) in Spanish population (Smith et al. 2011).
In the current study, we divided the SFAs intake into two groups (low and high) according to previous studies (Corella et al. 2009). In subjects with lower SFAs intake, after adjusting for age, gender, physical activity and total energy intake, there was no significant association between the mean weight or BMI and APOA2 genotypes. However, these relations were significantly different in patients with high SFAs intake. After adjusting for age, gender, physical activity, total energy intake and BMI, in both high and low SFAs intake groups, a significant interaction between the waist circumference and two groups of APOA2 genotypes was not found.
The mechanism by which −256T>C APOA2 SNP affects fat metabolism is still unknown. So far, researchers have been described that APOA2 transcription is controlled by an array of A to N regulatory elements in the promoter region of the gene. The −265T>C polymorphism is located in the middle of the element D (Chambaz et al. 1991). This element binds several different nuclear factors (Cardot et al. 1994). It is suggested that the nuclear-binding factors in the presence of C allele were associated with lower APOA2 expression in liver cells that results in reduced APOA2 plasma concentration (Van’t Hooft et al. 2001), while the results of this study indicate that the probable difference in APOA2 plasma concentration had no effect on anthropometric variables, but lifestyle changes, such as high intake of SFAs, may change the unknown metabolic pathways which lead to increase susceptibility to weight gain and obesity.
This is the reason that individuals with CC genotypes, who consume SFAs <22 g/day, have a lower prevalence of overweight. We could conclude that the CC group with high SFAs intake is more prone to obesity and overweight, compared to CC individuals with low SFAs intake.
According to our data, there was no significant difference between the serum leptin levels of TT + TC and CC groups, but the latter group showed significantly higher ghrelin concentration. Based on SFAs intake of these groups, no significant difference was found in the serum levels of these hormones. So far, Smith et al. study has been the only published report about the relation of these hormones and TT + TC or CC groups. Based on their research, plasma ghrelin of the CC group with low SFA intake was lower than the CC group with high SFA intake. It was also lower in TT + TC subjects with low SFA intake than TT + TC subjects with high SFA intake. However, regarding SFA intake, they found no significant difference between the serum leptin of CC group and T allele carriers (Smith et al. 2011). Since ghrelin is an orexigenic and appetite hormone, it seems natural that the serum level of ghrelin in C allele carriers to be higher than T allele carriers. This suggests a new role for APOA2 as a satiety signal. Due to the higher level of serum ghrelin in CC group, it was expected that these subjects had more energy intake, compared to T allele carriers. However, since all the subjects of our study were diabetic patients, who followed dietary advises for controlling their disease, then we did not find a significant difference of energy intake between CC and TT + TC groups.
Regarding the limitation of our research, there could be some measurement errors, like underestimation or overestimation of some food items in a food frequency questionnaire. We also did not measure the plasma concentration of APOA2. Despite all these limitations, this current study was the singular attempt to investigate the relation between anthropometric variables and SFA intake with APOA2 polymorphism in type 2 diabetic patients. Our data could help to detect the high-risk genotypes, to design the screening programs and to provide dietary suggestions for the prevention or treatment of overweight and obesity.
In conclusion, we have investigated a gene–diet interaction between the APOA2 SNP and saturated fatty acids intake for BMI in Iranian patients with type 2 diabetes mellitus. In fact, individuals with the CC genotype consuming a high SFA diet appear to be more prone to obesity and overweight than T allele carriers. The APOA2 −256T>C polymorphism was associated with elevated levels of serum ghrelin, but there were no significant APOA2-saturated fat interactions with regard to hormonal variables. Based on these findings, genotype-based personalized dietary recommendations for SFAs intake might be useful in the prevention of obesity.