The regulatory, informational, and immunomodulatory roles of fat fuels.

Fat oxidation provides a fuel for many tissues and it provides an important signal to decrease glucose utilization and oxidation in muscle and so conserve glucose for essential organs such as the brain. The control of fatty acid oxidation is achieved in part through its plasma concentrations, which may be precisely controlled by the triacylglycerol-fatty acid substrate cycle, which can also, if oxidation is taken into account, be viewed as a branch point in this important pathway. Branch points may provide precision in regulation if one of the fluxes at the branch is low compared with the other flux. Both branch points and substrate cycles are energetically expensive and may account for some of the increases in energy expenditure in conditions of injury, burns, and sepsis and in the postexercise condition. Fatty acids, through effects on plasma free tryptophan concentrations and hence 5-hydroxytryptamine concentrations in the brain, may play a role in central fatigue. Polyunsaturated fatty acids are claimed to have immunosuppressive properties. Work has been done to provide a biochemical analysis of how they might influence some functions of cells of the immune system.