Insulin resistance and reduced metabolic flexibility: cause or consequence of NAFLD?

Abstract

Whether non-alcoholic fatty liver disease (NAFLD) precedes insulin resistance (IR) or IR preludes/causes NAFLD has been long debated. Recent studies have shown that there are two phenotypes of NAFLD, ‘genetic’ vs ‘metabolic’ NAFLD. The former patients are more at risk of hepatocellular carcinoma and chronic liver disease the latter are more IR and at increased risk of type 2 diabetes (T2D). Even if they are not yet diabetics, from a metabolic point of view having NAFLD is equivalent to T2D with reduced peripheral glucose disposal and impaired suppression of hepatic glucose production, but without fasting hyperglycaemia. T2D develops only when hepatic autoregulation is lost and glucose production exceeds the capacity of muscle glucose disposal. In NAFLD adipocytes are resistant to the effect of insulin, lipolysis is increased and excess plasma free fatty acids (FFA) are taken up by other organs (mainly liver) where they are stored as lipid droplets or oxidized. Increased adiposity is associated with worsen severity of both ‘genetic’ and ‘metabolic’ NAFLD. FFA oxidative metabolism is increased in NAFLD and not shifted towards glucose during insulin infusion. Although this reduced metabolic flexibility is an early predictor of T2D, it can be seen also as a protective mechanism against excess FFA. In conclusion, IR precedes and causes ‘metabolic’ NAFLD, but not ‘genetic’ NAFLD. Reduced metabolic flexibility in NAFLD might be seen as a protective mechanism against FFA overflow, but together with IR remains a strong risk factor for T2D that develops with the worsening of hepatic regulation of glucose production.