The real story on training and supplements — with sources.

Insulin does promote muscle protein synthesis (MPS) and suppress muscle protein breakdown (MPB). However, amino acid elevation from dietary protein alone can maximize MPS stimulation — the additional insulin spike from carbohydrates is not always necessary (Staples et al. 2011). RCTs show that when adequate protein is present, adding carbohydrates does not further increase MPS. Carbohydrates indirectly support hypertrophy (energy supply, training performance), but their direct role via insulin, compared to protein, is limited.

Compound exercises simultaneously stimulate multiple muscle groups, providing broad stimulus to agonists, antagonists, and stabilizers. However, they have limitations in providing targeted, maximal stimulus to each individual muscle. For example, bench press works chest, deltoids, and triceps simultaneously, but there are load-angle tradeoffs that prevent maximally loading each. Isolation exercises complement compound work by targeting specific muscles (e.g., biceps, triceps, lateral delts, lower hamstrings) that compounds under-stimulate.