text: During physical exertion or moderate intensity exercise lactate released from working muscle and other tissue beds is the primary fuel source for the heart, exiting the muscles through monocarboxylate transport protein (MCT).   This evidence is supported by an increased amount of MCT shuttle proteins in the heart and muscle in direct proportion to exertion as measured through muscular contraction.
  Furthermore, both neurons and astrocytes have been shown to express MCT proteins, suggesting that the lactate shuttle may be involved in brain metabolism. Astrocytes express MCT4, a low affinity transporter for lactate (Km = 35mM), suggesting its function is to export lactate produced by glycolysis. Conversely, neurons express MCT2, a high affinity transporter for lactate (Km = 0.7mM). Thus, it is hypothesized that the astrocytes produce lactate which is then taken up by the adjacent neurons and oxidized for fuel.