Systems that permit cells to capture and transport the available nutrient molecules to their interior ensure that nutrients are used effectively.
But if several nutrients are available, cells can select those that are of most interest and discard undesired molecules. Inside cells, nutrients are conducted to the mitochondria, the specialized cell organelles in which nutrients are combusted to release the energy held in their chemical bonds. Both sugars (glucose) and fats (fatty acids) are 'burned' in mitochondria, but these organelles need to adjust their molecular apparatus in different ways depending on whether their main fuel supply comes from sugars or fats.
According to Dr. Enríquez, "This adjustment can be likened to refitting your boiler to burn natural gas or butane." The proportion of different food fuels available to cells can be affected by diet, exercise, or a period of fasting, and cells need to be able to adapt to these changes. In some specific situations, for example during the activation of immune cells to defend the body against infection, cells change their activity even if the fuel source is unaltered, and this can be accompanied by changes in the preferential use of glucose instead of fatty acids or vice versa. In all these cases, mitochondria need to adapt their 'fuel burners', technically known as the electron transport chain (ETC). "The ETC was known to adapt, but the signals that promote this change and the molecules responsible for it were not known."