Elevated plasma branched-chain amino acid (BCAA) levels are a characteristic feature of type 2 diabetes (T2DM) and are associated with insulin resistance and mitochondrial dysfunction. The exact mechanisms driving these elevated levels remain unclear, with hypotheses including increased protein intake, blunted insulin inhibition of proteolysis, and compromised mitochondrial BCAA metabolism. A positive association between plasma BCAA and intrahepatic lipid (IHL) content has been reported, suggesting a possible link between BCAA and hepatic fat accumulation. Animal studies have shown BCAA can stimulate hepatic lipogenesis. Given the established benefits of physical activity and exercise training in improving insulin sensitivity and reducing IHL, this study investigates whether these interventions affect the BCAA-IHL relationship. Previous studies have shown conflicting results regarding the impact of physical activity and exercise on plasma BCAA levels, with some showing an association between high physical activity and low BCAA levels, while others found no effect of exercise training on BCAA despite improved insulin sensitivity. This study hypothesized that moderate-to-high physical activity and exercise training would result in lower plasma BCAA levels and decreased IHL content.

The literature review summarizes existing research on the relationship between BCAA, IHL, and physical activity. Studies have shown a positive correlation between plasma BCAA and IHL content, particularly in individuals with overweight/obesity. The impact of glucose-lowering therapy on both BCAA and IHL was mentioned, but the specific correlation was not investigated in detail in previous studies. Longitudinal studies highlight the predictive role of BCAA changes for the development of NAFL. The mechanisms underlying BCAA's contribution to hepatic fat accumulation remain to be fully elucidated, but animal studies suggest BCAA stimulation of hepatic lipogenesis via activation of lipogenic genes. The influence of physical activity and exercise on BCAA and IHL, separately and in conjunction, had yielded mixed results and warrant further investigation. Existing literature points to a complex interplay of metabolic factors and physical activity impacting BCAA levels and liver fat content, motivating this study to investigate these relationships further.

This study employed a two-pronged approach: a cross-sectional analysis and an intervention study. The cross-sectional analysis used data from 1983 participants (45–65 years) in the Netherlands Epidemiology of Obesity (NEO) study. Physical activity was assessed using the SQUASH questionnaire, blood samples were collected for BCAA analysis, and IHL content was measured using proton-magnetic resonance spectroscopy ('H-MRS). Linear regression analyses were performed to examine the association between BCAA and IHL, stratified by physical activity frequency and adjusted for relevant covariates. The intervention study involved a 12-week supervised combined aerobic resistance-exercise program in 21 male participants (7 with NAFL, 7 with T2DM, and 7 BMI-matched controls). IHL, insulin sensitivity (hyperinsulinemic-euglycemic clamp), and plasma BCAA levels were assessed before and after the intervention. Linear mixed model analyses were used to compare changes among the groups. Detailed methodology is outlined in the paper, including the exercise training protocol, 'H-MRS procedures, hyperinsulinemic-euglycemic clamp details, laboratory analysis of amino acids, and statistical methods used for both the cross-sectional and intervention analyses.

Cross-sectional analysis revealed a positive association between plasma valine, isoleucine, and leucine levels and IHL content (1.29, 1.52, and 1.54 times IHL per SD of amino acid levels, respectively). These associations were similar in both less and more active individuals, suggesting that physical activity level does not influence this relationship. Interestingly, no association was found between dietary protein intake and plasma BCAA levels. The intervention study demonstrated a significant reduction in IHL content after exercise training in both the NAFL and control groups, but not in the T2DM group. Importantly, plasma BCAA levels did not change significantly after the exercise program across all groups. Positive correlations were observed between fasting plasma BCAA levels and IHL content, and negative correlations were observed between BCAA levels and peripheral insulin sensitivity. In contrast, a strong positive correlation was found between BCAA levels and hepatic insulin sensitivity. The insulin-suppressive effect on BCAA was significantly greater in the control group compared to the NAFL and T2DM groups.

This study's findings do not support the hypothesis that changes in plasma BCAA levels mediate the reduction in IHL content following exercise training. While a positive association between plasma BCAA and IHL was consistently observed, this relationship was not modulated by physical activity level. The significant reduction in IHL content after exercise training, independent of changes in BCAA, suggests that other mechanisms, such as improved insulin sensitivity, are more likely responsible for the observed reduction in liver fat. The greater insulin-suppressive effect on BCAA in the control group compared to those with NAFL and T2DM highlights the potential impairment in insulin signaling pathways in these groups. The lack of association between dietary protein and plasma BCAA concentration could be attributed to several factors that warrant further exploration. Further studies could investigate the role of other metabolic pathways and factors contributing to the observed IHL reduction. The study's strength lies in its use of a robust methodology involving both cross-sectional and interventional study designs, supplemented by detailed metabolic assessments.

The study concludes that the association between plasma BCAA levels and IHL content is not influenced by physical activity level. Exercise training effectively reduces IHL content without significantly altering plasma BCAA levels, suggesting that other metabolic mechanisms drive this beneficial effect. Future research should focus on identifying these mechanisms and exploring potential personalized approaches to improve hepatic health.

The cross-sectional design limits the ability to establish causality. The relatively small sample size of the intervention study may limit the generalizability of the findings. The study was limited to male participants, hence gender differences in the response to exercise might not be fully captured. The use of self-reported physical activity data may be prone to recall bias. The results might not be generalizable to other populations due to the specific participant characteristics. Furthermore, the impact of other lifestyle factors such as diet was only partially accounted for. The study focused on a specific exercise protocol; different exercise modalities might have varying impacts on BCAA and IHL.