Lactate is Only Good for Muscle Pain, Right?

We commonly think of lactate as being a waste product that accumulates during intense exercise causing the all too familiar pain. As we have learned, lactate gets transported to the liver to be used in the cori cycle for gluconeogenesis. However, that is not the only fate for lactate. Lactate is largely used for oxidation and is incorporated into glycogen in skeletal muscle (Takahashi et al., 2020). Recent studies have even shown that lactate can act as a signaling molecule and induce metabolic adaptation (Takahashi et al., 2020). If lactate gets incorporated into glycogen, could increased lactate levels during exercise help to replenish our glycogen stores post-exercise? 

Takahashi et al. (2020) studied just that. They used a lactate injection in the post-exercise period to determine if lactate can help replenish glycogen stores. In an attempt to take a deeper look what muscles lactate can be of benefit to, the researchers looked at the soleus, plantaris and gastrocnemius muscles. The combination of lactate administration and glucose intake collectively enhanced glycogen repletion during the post-exercise period in the plantaris and gastrocnemius muscles, but not in the soleus muscle (Takahashi et al., 2020). Concentrations of FBP2 and MTC1 were higher in the plantaris and gastrocnemius muscles than in the soleus muscle (Takahashi et al., 2020). FBP2 is the rate-limiting enzyme in the reaction that converts lactate to glycogen (Takahashi et al., 2020). MTC1 is a protein that is responsible for facilitating the uptake of lactate in the skeletal muscles (Takahashi et al., 2020). Even though lactate administration did not increase glycogen repletion in the soleus muscle, it still impacted this muscle (Takahashi et al., 2020). An enhanced phosphorylation of TBC1D1 was observed in the soleus muscle and this protein is important for regulating GLUT4 translocation (Takahashi et al., 2020). Overall, it appears as though lactate is important for glycogen recovery within glycolytic phenotype muscles, so is the pain of increased lactate production during intense exercise really worth it? 

Reference: Takahashi, K., Kitaoka, Y., Matsunaga, Y., & Hatta, H. (2020). Effect of post-lactate administration on glycogen repletion and signaling activation in different types of mouse skeletal muscle. Current Research in Physiology(3), 34-43. http://doi.org/10.1016/j.crphys.2020.07.002