Low aerobic capacity (AC) is a powerful predictor of early mortality and risk for metabolic disease including excessive hepatic fat storage (steatosis). Conversely, high AC is clinically associated with protection against hepatic steatosis and a healthier, longer lifespan even in the face of obesity. We will utilize a rat model selectively bred for divergent intrinsic AC (high or low running capacity [HCR/LCR]) to unravel mechanisms by which AC impacts hepatic steatosis and metabolic pathologies. In a sedentary condition, HCR rats have a 40% higher intrinsic AC and are protected against high fat/sucrose (HFD)-induced hepatic steatosis and insulin resistance while LCR are highly susceptible. We have shown that differences in hepatic mitochondrial function (MitoFX: defined here as fat oxidation, and respiratory capacity) between the HCR and LCR play an important role in their protection or susceptibility for hepatic steatosis, respectively. New data suggests that hepatic metabolic flux through TCA cycle and gluconeogenesis are also elevated in the HCR over the LCR rat, but these pathways have yet to be examined in the context of protection against steatosis. In addition, novel preliminary data suggests HCR rats have elevated bile acid (BA) synthesis paired with increased fecal sterol and BA excretion compared to LCR. Exercise trained mice which also have elevated MitoFX and are protected from steatosis show evidence of a similar upregulation of BA synthesis and excretion. We will test the hypothesis that increases in hepatic BA synthesis and fecal excretion is critical to the high AC and chronic exercise phenotype(s) and contributes to hepatic MitoFx, metabolic flux, and protection of steatosis by: 1) Pulling acetyl-CoA out of the mitochondria (minimizing feedback inhibition and mitochondrial protein acetylation) and 2) diverting acetyl-CoA away from accumulation and de-novo-lipogenesis (DNL) and towards BA synthesis and subsequent fecal loss via a “siphoning mechanism”. We will test these mechanisms utilizing pharmacological and molecular tools to modulate CYP7a1 activity and BA synthesis combined with in-vivo metabolic tracers in HCR/LCR rats and exercise vs. sedentary mice. Additionally, HCR livers display greater metabolic and transcriptional adaptability in response to high-fat diet (HFD) feeding than LCR. Our preliminary data suggests that enhanced transcriptional adaptability in the HCR livers is caused by increases in the acetylation of histones (H3K9ac and H3K27ac) that coordinate expression of genes involved in mitochondrial metabolism and specifically for BA synthesis. Thus, we posit that high AC and exercise induced increases in metabolic flux and enhanced BA synthesis likely increase acetyl CoA flux out of the mitochondria and into the cytosol where it can serve as a substrate for histone acetylation. This proposal will also test the hypothesis that livers from HCR rats and from exercised mice can transcriptionally adapt to high fat di...