How leucine helps insulin make muscle store more sugar

Leucine, an amino acid, triggers a cellular pathway called mTOR that starts the production of muscle proteins. When carbohydrates are consumed at the same time, the resulting increase in insulin levels boosts this process.

In human muscle cells, leucine by itself does not change the activity of AKT, GSK3β, or glycogen synthesis, but when insulin is present, it temporarily increases ERK1/2 phosphorylation, with the highest effect at 10 minutes and no effect remaining after 60 minutes.

At a concentration of 100 micromolar, leucine produces the maximum possible increase in insulin-driven GSK3β phosphorylation and glycogen synthesis in human muscle cells; higher concentrations up to 400 micromolar do not produce a greater effect.

In human muscle cells deprived of nutrients, adding 100 micromolar leucine before insulin increases the activation of key signaling proteins and boosts glycogen production by 50% compared to insulin alone.

When human muscle cells are exposed to leucine before insulin stimulation, there is an increase in the phosphorylation of a specific protein site (4EBP1 at Thr37/46), but this increase is not consistently observed across all experiments and cannot be detected when only a single time point is measured.