This exploratory study investigated the association between genetic variants and physiological outcomes during the preprandial and postprandial digestive responses in thirty healthy young men. We report a strong association between the IRS1 gene variant rs2943641 and BMI with elevated fasting insulin levels, the PPARγ2 gene polymorphism rs1801282 and the UCP1 gene variant rs1800592 with participants’ BMI. Therefore, our finding confirmed three associations between genetic variants and physiological outcomes. This was remarkable, despite the relatively small sample size of men and the complex genetic assessment. Our finding regarding the lack of association between many of the SNPs tested and the physiological markers may be false negatives, given the small sample size.
Studies have shown that a person’s metabolic efficiency and subtle differences in genetic variability influence postprandial digestive response as an independent risk factor for health and disease (Berry et al., 2020; Lopez-Miranda & Marin, 2010; Vincent et al., 2002). Several factors, including genetics, may influence interpersonal differences in preprandial and postprandial meal responses and physiological outcomes (Berry et al., 2020). Thus, genetic variation and the digestive response are important areas of study (Ellis et al., 2021).
GWAS have identified regions of the genome where gene–nutrient interactions have been associated with disease-causing effects, but the strength of these associations may be weak (Dib et al., 2019). For instance, studies have associated LDL levels and lipoprotein metabolism with the ATP-binding cassette subfamily G member 8 (ABCG8) gene (Kathiresan et al., 2009). Excretion of the ABCG8 gene is via the intestines, and during this process, cholesterol absorption from the intestines reduces (Feingold & Grunfeld, 2015). The ABCG8 gene variant rs6544713, T allele, is associated with high LDL levels and can elevate cholesterol uptake and lower secretion from the intestines (Acalovschi et al., 2006; Schroor et al., 2021). Diet and lifestyle choices can increase the risk of diseases, such as high LDL intake, which can contribute to atherosclerosis and eventually, coronary artery disease or cardiovascular disease (Röhrl & Stangl, 2013). Another study combined 21 GWAS, using 46,186 nondiabetic European subjects, that included loci associated with fasting glucose near the adenylate cyclase 5 (ADCY5) gene (Dupuis et al., 2010). Meta-analysis after adjustments for BMI demonstrated that the ADCY5 gene was associated with elevated fasting glucose levels of 0.027 mmol/L in A allele carriers (P = 0.0001). Therefore, A allele carriers had an increased risk of type 2 diabetes compared to G allele carriers. It should be noted, however, that studies differ regarding allele frequency, effect size, and the population they studied, and associations contributed only to increasing odds of disease occurrence (Tam et al., 2017; Wray et al., 2007). Currently, the challenge is confirming gene–nutrient associations with disease risk and providing dietary advice to individuals with risk variants (Reddy et al., 2018). The use of genetic information to guide dietary decisions is in its infancy. Generally, advancement has been slow due to insufficient evidence and a low replication of studies (Corella et al., 2009).
Investigations on the IRS1 gene have reported its possible association with insulin levels and type 2 diabetes (Alharbi et al., 2014; Almgren et al., 2017; Kovacs et al., 2003). This is consistent with a finding from a population-based cohort study where they recruited 3,344 Swedish participants born between 1923 and 1950 and 4,905 Finnish participants (Almgren et al., 2017). The study searched for a link between nondiabetic participants and fasting insulin levels and found that a location near the IRS1 gene variant rs2943641 showed a significant association (P = 2.4 x 10− 7). Almgren et al. (2017) concluded that participants who carried the CT or CC genotypes had greater fasting insulin levels than the TT genotype. This association has been reported in subjects with type 2 diabetes from a randomised control trial performed on 376 type 2 diabetes and 380 healthy participants from Saudi Arabia (Alharbi et al., 2014). Our study provides new evidence of a positive relationship between the IRS1 gene variant rs2943641 and elevated fasting insulin levels in thirty healthy young men. This evidence suggests that carriers of the C allele are more likely to have an increased risk of elevated fasting insulin levels, which could increase their risk of other noncommunicable diseases such as obesity, type 2 diabetes and cardiovascular disease. Besides diet and lifestyle, insulin levels can also be influenced by obesity, which is a causal factor affecting insulin resistance (Phillips et al., 2012). This agrees with our finding that BMI (P = < 0.0001) significantly predicted elevated fasting insulin levels.
In the current investigation, the PPARγ2 gene variant rs1801282 was correlated with BMI as the physiological measure. A previous study reported that the PPARγ2 gene polymorphism of the ALa allele in Pro12Ala is associated with fat cell formation in adipose tissue, linked to insulin resistance, obesity and BMI (Garaulet et al., 2011). They analysed the PPARγ2 polymorphism Pro12Ala (rs1801282) association with participants’ BMI and fat loss by studying 1,465 overweight and obese Spanish subjects (89% completed the study). The study found a positive gene–diet interaction between the PPARγ2 polymorphism Pro12Ala (rs1801282) and participants’ BMI. After the fat loss intervention, carriers of the minor G allele were significantly less obese with lower BMI (P = < 0.001) than homozygous major CC genotypes (P = 0.037) when their dietary intake of monounsaturated fats was high. Participants in this study with the high-risk variant could have a more significant weight loss if they consumed a diet high in monounsaturated fats compared to the ten participants with a BMI > 25 kg/m2 and were genotype CC. Therefore, the finding in this study suggests that the positive association between BMI and the high-risk variant transcends diet and fatty acid consumption.
BMI has been identified as the physiological measure associated with the UCP1 gene (Cannon & Nedergaard, 2004). The UCP1 gene is located on chromosome 4 of the human genome and encodes the UCP1 protein (Dalgaard & Pedersen, 2001). This protein is found chiefly in brown adipose tissue (BAT) and facilitates proton transport across the mitochondrial inner membrane (Flouris et al., 2017). During UCP1 activation, there is an increase in fatty acid oxidation to compensate for the decrease in ATP synthesis, and this allows energy expenditure to be maintained (Brondani et al., 2012; Dalgaard & Pedersen, 2001). Research suggests that the UCP1 gene increases energy expenditure and decreases the mitochondrial membrane potential due to the UCP1 gene polymorphism of rs1800592 (-3826 GA, -1766GA, and − 112AC) in the intraperitoneal adipose tissue (Brondani et al., 2012; Nagai et al., 2011; Vimaleswaran & Loos, 2010). Several studies suggest that carriers of the UCP1 gene polymorphism rs1800592 are at greater risk of multifactorial diseases, including obesity and type 2 diabetes; however, findings are still ambiguous (Brondani et al., 2012; Vimaleswaran & Loos, 2010). A cohort study used 82 Japanese females aged 20–22 from the same university campus in Japan and genotyped for the UCP1 gene polymorphism rs1800592 (Nagai et al., 2011). The study explored the gene variant’s association with BMI and body weight. The conclusion was that G allele carriers had lower energy expenditure; therefore, their energy needs were lower than the AA genotypes. According to our findings, there was a significant association between the UCP1 gene polymorphism rs1800592 and participants’ BMI. Therefore, participants with the high-risk variant may have an increased risk of obesity, especially if they have a high BMI (Hill et al., 2012). These participants may need to reduce their intake of calories whilst increasing their energy expenditure because their energy needs are lower than AA genotypes (Brondani et al., 2012; Nagai et al., 2011; Pfannenberg et al., 2010).
Based on published articles, the best-matched genetic variant and physiological measure were chosen for this study. However, a significant limitation of the current investigation is that some physiological measures may be mismatched to the genetic variant as newly published research is being produced. For example, hunger was associated with the NMB gene; another association has been linked to the FTO gene variant rs9939609, which is associated with the postprandial sensation of hunger (Magno et al., 2018).
The association between physiological measures, genetic variation and health benefits can be complex since other factors may also play a role. SNPs, for instance, are linked with other genetic risk variants causing causal variants (linkage disequilibrium) and are affected by causal effects (lifestyle and environment) (Dandine-Roulland & Perdry, 2015).
According to PREDICT1, the largest nutritional study in the world, researchers found that a person’s gut microbiome has a greater influence on postprandial lipemia than meal composition. Genetic variants were found to have only a small impact on individual postprandial metabolism, indicating that modifiable environmental factors (e.g., lifestyle, exercise and sleep) are important factors to consider (Berry et al., 2020). Even so, more research and evidence will need to confirm any causal inference. Eventually, gene–nutrient studies could help clinicians, dietitians, and nutritionists tailor nutrition advice based on a genetic test (Braakhuis et al., 2021; Horne et al., 2021). However, all registered practitioners should consider the strength of the relationship between SNPs and the physiological parameter before incorporating nutrigenomics into their practice (Collins et al., 2013).