In this cross-sectional study, we found a positive association between total BCAAs, leucine, isoleucine, and valine intake and odds of general obesity. Such association was found in men, but not in women. There was no significant association between total and individual BCAAs intake and abdominal obesity.
Increase consumption of dietary BCAAs result in increased plasma levels of BCAAs. Earlier studies have found a positive association between plasma levels of BCAAs and visceral adipose tissue (VAT) and insulin resistance (8, 9). A cross-sectional study in China reported that children of mothers with GDM who had higher intake of BCAAs had higher odds of overweight and insulin resistance (11). In addition, in an experimental study, restriction of dietary BCAAs resulted in weight loss and improved glucose tolerance and insulin sensitivity in diet-induced obese mice (21). On the other hand, some publications reported the beneficial effects of decreased consumption of BCAAs on metabolic health in humans (22, 23). In contrast to our findings, a cross-sectional study revealed a significant inverse association between dietary BCAAs and prevalence of overweight or obesity (13). Another cross-sectional study demonstrated that obese subjects, compared with non-obese individuals, had lower intakes of dietary BCAAs (24). In a recent meta-analysis, a contra-positive correlation between BCAAs intake and risk of obesity was found (10). Different findings might be explained by methodological discrepancies across studies including difference in target population, dietary assessment tools, and controlling for confounding factors in the analyses. In addition, different sample size along with nature of study design can also provide some other explanations.
We found no significant association between dietary total and individual BCAAs intake and odds of abdominal obesity. Contrary to our results, Li et al. showed that higher BCAAs intake was associated with a decreased odds of abdominal obesity (24). In addition, a clinical trial revealed that subjects who consume a hypocaloric diet high in BCAAs had greater weight loss and lower percentage of body fat and abdominal obesity than those in hypocaloric control, hypocaloric high-protein, hypocaloric low-protein, and control diet groups (25). Some studies have shown higher circulating levels of leucine, isoleucine, and valine among subjects with central obesity compared with peripheral obesity (26). One possible explanation for lack of a significant relationship in the current study might be the definition of central obesity. We used the criteria suggested for Western countries (27, 28), while this definition and the cut-off points used might be different in Middle Eastern countries like Iran. In addition, we assessed abdominal obesity based on WC only. Others have used waist-to-hip and waist-to-height ratios for this definition (29-31). Additionally, we evaluated all anthropometric measures, including WC, through self-reported data. Although a significant association was seen between self-reported and actually measured data in our validation study, the correlation was not as strong as for weight and height. Therefore, some sort of misclassification of participants in terms of abdominal obesity might have occurred.
There are several potential mechanisms through which BCAAs might influence body weight. BCAAs might stimulate secretion of both insulin and glucagon and, when administered orally, result in increased prolonged insulin and glucagon secretion (32). The role of insulin in the pathogenesis of obesity and excess adiposity has been previously proposed (33, 34). On the other hand, insulin can increase the uptake of amino acids such as BCAAs and facilitate the protein synthesis in the body (35, 36). As a result, this leads to increased skeletal muscle weight and eventually body weight (37). Furthermore, some studies indicated that insulin activity is impaired by BCAAs supplementation, which can in turn increase the activity of mammalian target of rapamycin (mTOR) (38, 39). Moreover, 3-hydroxy-isobutyrate (3-HIB), a metabolite of valine, activates uptake of fatty acids to the muscle, which can result in accumulated body fat in animals (7). It is demonstrated that the changes in this metabolite were correlated with metabolic improvements with weight loss in humans (40). In our study, we found a significant positive association between BCAAs intake and general obesity in men, but not in women. The reasons of this gender discrepancy are unknown, but it has been suggested that BCAAs can boost testosterone secretion (41) and, therefore, increase lean body mass (42), which might have been resulted in a greater weight in men.
The present study has some strengths including large sample size and controlling for a wide range of potential confounders in the analyses. However, some limitations must be considered. First, this study has cross-sectional design that would not allow to infer causal relationship between dietary BCAAs and obesity. Therefore, further studies are needed in this regard to confirm our findings. Second, despite for adjusting for several variables in the study, the role of residual confounding cannot be excluded. Third, although we used a validated FFQ for assessment of dietary intakes, some degree of measurement error and misclassification is unavoidable. Finally, we used self-reported data of anthropometric measures in the current study, which can further bring some sort of bias into our findings.
In conclusion, we found a significant positive association between dietary BCAAs and general obesity in the whole population. Such association was found in men, but not in women. Dietary BCAAs was not associated with abdominal obesity.
What is already known on this subject?
Several studies found that higher circulating BCAAs were associated with increased risk of obesity-related conditions such as metabolic disorders and type 2 diabetes. BCAAs and their metabolites can result in insulin resistance and impaired glucose tolerance and eventually increased visceral fat accumulation and obesity. Some studies have revealed that dietary intake of BCAAs was associated with increased circulating levels of these amino acids, but other studies had the opposite findings.
What does this study add?
Given the controversial findings with regard to the association between dietary BCAA intake and obesity, and increasing prevalence of obesity in Middle-Eastern countries, we decided to conduct the present study. We found a positive association between dietary BCAA intake and obesity among adults. By gender, such association was found in men, but not in women. Dietary BCAAs was not associated with abdominal obesity.