Results from our case-control study in Hanoi preschool children found that the ADRB3-rs4994 polymorphism was strongly associated with obesity as an independent risk factor.
As a crucial component for regulating energy balance in mammals, the essential role of ADRB3 gene is due to its effects on lipid degradation, transport of fatty acid, thermogenesis [18, 19]… Binding to β3-adrenergic agonists leads to the activation of ADRB3, which then activate adenylyl cyclase. Activated adenylyl cyclase initiates a signal pathway through intracellular signaling cascades including cyclic AMP, kinase proteins... that finally results in the production of heat in brown and white adipocytes [20, 21]. According to the research by Krief et al. (1993), ADRB3 also participates in the regulation of lipid metabolism including the absorption of fat during digestion, storage, and mobilization of lipids from adipocytes [18]. Thus, dysfunction of ADRB3 directly affects the process of lipolysis and energy expenditure, which consequently causes obesity as well as metabolic disorders through excessive accumulation of fat in adipose tissue.
ADRB3-rs4994 polymorphism was demonstrated that associated with some health problems such as being overweight, abdominal obesity, HDL-C levels, harder to lose weight, a reduced basal metabolic rate, type 2 diabetes in previous studies [16],[22],[23],[24]. Adipocytes with TC and CC genotypes exhibited a weaker response to β3-adrenergic agonists as their cytosolic cAMP and glycerol were nearly 70% lower compared with that of TT genotypes [25, 26]. In addition, the existence of C allele in genotypes also leads to a lower rate of enzyme activity for the lipolysis induced by ADRB3 [27]. Thus, the replacement of T with C in codon 64 of ADRB3 gene significantly decreased lipolysis rate in brown adipocytes.
Much research has been carried out to clarify the relationship between ADRB3-rs4994 and obesity in humans, but the outcomes are still controversial. ADRB3-rs4994 were demonstrated to associate with obesity in many populations such as Chinese [28], France [29], Japan [29]... According to research by Daghestani et al. in 329 unrelated persons (31% male and 66.9% female) with the age range from 18 to 36, there was a substantial correlation between the development of overweight and obesity in the Saudi population and the ADRB3-rs4994 variant. All anthropometric parameters in the overweight and obese group were higher than in the control group (p < 0.0001). In the obese group, the frequency of TC and CC genotypes was 15.8% and 4.8%, while in the control, the frequency of these genotypes was only 2.6% and 0%. Besides, the research also found that homozygotes for the C allele had a greater value for all anthropometric indices and almost biochemical parameters compared with two other genotypes (p < 0.05) [30]. Research by Sipilä et al. (1997) showed that the ADRB3-rs4994 polymorphism affects basal metabolic rate in obese Finns [31]. After adjustment for body weight and age, basal metabolic rate was lower in individuals with TC genotype than in TT genotypes (1,569 vs. 1,635 kcal/day, p < 0.05). Thus, it can be speculated that rs4994 polymorphism is associated with obesity through partly reducing basal metabolic rate, which results in an increase in fat accumulation and difficulty with losing weight [31].
Conversely, some research indicated that there was no association of ADRB3-rs4994 polymorphism with obesity. According to research by Kurokawa et al. on 87 Japanese children, BMI index and body fat percentage were not statistically different among three genotypes TC CC, and TT [32]. Research by Porto et al. (2004) on 934 high school students (121 normotensive and 54 hypertensive students) showed that there was no association between increasing BMI and rs4994 polymorphism of ADRB3 gene. The frequency of TT, TC, and CC genotypes were 85%, 14.5%, and 0.5%, respectively [33]. Research by Chou et al., in 559 adolescent volunteers in Taiwan, was not found any association between obesity and rs4994 polymorphism of ADRB3 gene in Taiwanese adolescents. The frequencies of TT, TC, and CC genotypes were 72.3%, 26.1%, and 1.6%, respectively. And these genotypes were found to be Hardy-Weinberg equilibrium [34].
A study by Yilmaz et al. (2019) conducted on 441 children and adolescents aged 6–18 years in Turkish also resulted in no association of ADRB3-rs4994 with obesity. However, the frequency of rs4994 variant was higher in obese girls, which can lead to weight gain. The frequencies of TT, TC and CC genotypes in the obese group were 84.8%, 14.4%, and 0.8%, while in the control group, the frequencies of these genotypes were 89.4%, 10.6%, and 0.0%, respectively (p = 0.247). There was no significant difference in terms of allele and genotype frequencies between two study groups. The frequency of T and C alleles were 92.0% and 8.0% in the obese group, whereas the frequencies of these alleles were 94.7% and 5.3% (p = 0.127) in the control group [35].
Generally, the inconsistencies among the previous studies’ results may be due to differences in study populations (sex, age, socio-economic status, etc.), environmental or lifestyle factors (levels of energy intake and level of physical activity). In addition, the allele frequency at locus rs4994 varies in different ethnic populations. With a frequency of 0.38 in Eskimos and 0.31 in Indians, the C allele was very common in both populations [36]. Japanese population had a high C allele frequency of 0.21, which is twice as common as in white populations [37]. In our study, the C allele accounted for 0.12 in the controls and 0.16 in the cases.
Along with the development of social life, Vietnam has experienced a sharp rise in the prevalence of overweight and obesity in recent years, particularly in children. This phenomenon may be caused by the gradual replacement of healthy foods with various high-calorie but low-nutrient foods and beverages, which leads to a substantial change in eating behaviors of Vietnamese children, especially those living in big cities. When emotions change, students often have no control over the amount of food they eat. According to Derks et al. (2018), EOE has a two-way relationship with BMI, both as a predictor and as a result of high BMI [38]. Ashcroft et al. (2008) also showed that emotional overeating increased over time from 4 years of age to 10 years of age [39]. In addition, a study of twins in the UK has shown that overeating in response to negative emotions is a learned behavior rather than a genetic factor [40]. In addition, food response (such as demanding food, if allowed, students can eat a lot, when they are full, they can still eat…) is also one of the risk factors for increasing BFP. A longitudinal study of 3,331 children in the Netherlands showed that, by the age of 4, food response index (FR) and food preference (EF) were higher in children with high BMI. When children were 10 years old, there was a positive relationship between food response index (FR), food preference (EF) and mood swings (EOE) with BMI and body fat mass. Meanwhile, satiety response (SR) was negatively associated with BMI and body fat mass.[38] Therefore, the association of ADRB3-rs4994 polymorphism with childhood obesity was adjusted for children's eating behaviors.
Nutrigenomics is a new field of study whose objective is to clarify the relationship between the genetic factors of an individual and the corresponding dietary intake, and also the effect of nutrition on gene expression. Based on the PubMed database, Pavlidis et al. (2015) conducted a meta-analysis to examine the association of 38 candidate genes with dietary intake and/or pathologies of nutrient-related diseases. Although his results showed that there was no significant association in any of these 38 genes, it is clear that additional research on nutrigenomics needs to be carried out because this is a potential tool that provides many benefits for medicine [41].
Since our results indicated that there was a significant association between ADRB3-rs4994 variant with obesity regardless of children's eating behaviors, it may be useful for doctors to categorize children into groups with different risks of obesity, thus providing suggestions for their parents about specific diets or exercise routines to prevent obesity. Despite this, our cross-sectional study also had certain disadvantages due to not considering the contribution of lifestyle factors and their complicated interaction with other genetic factors to the development of childhood obesity. Therefore, further research needs to be conducted in the future to examine the roles of other factors in the epidemiology of obesity.