Hepatic fatty acid synthesis and partitioning: the effect of metabolic and nutritional state.
When we consume dietary fat, a series of complex metabolic processes ensures that fatty acids are absorbed, transported around the body and used/stored appropriately. The liver is a central metabolic organ within the human body and has a major role in regulating fat and carbohydrate metabolism. Studying hepatic metabolism in human subjects is challenging; the use of stable isotope tracers and measurement of particles or molecules secreted by the liver such as VLDL-TAG and 3-hydroxybutyrate offers the best insight into postprandial hepatic fatty acid metabolism in human subjects. Diet derived fatty acids are taken up by the liver and mix with fatty acids coming from the lipolysis of adipose tissue, and those already present in the liver (cytosolic TAG) and fatty acids synthesised de novo within the liver from non-lipid precursors (known as de novo lipogenesis). Fatty acids are removed from the liver by secretion as VLDL-TAG and oxidation. Perturbations in these processes have the potential to impact on metabolic health. Whether fatty acids are partitioned towards oxidation or esterification pathways appears to be dependent on a number of metabolic factors; not least ambient insulin concentrations. Moreover, along with the phenotype and lifestyle factors (e.g. habitual diet) of an individual, it is becoming apparent that the composition of the diet (macronutrient and fatty acid composition) may play pivotal roles in determining if intra-hepatic fat accumulates, although what remains to be elucidated is the influence these nutrients have on intra-hepatic fatty acid synthesis and partitioning.