1) It negatively affects protein synthesis.
Protein synthesis is the process where amino acids are joined together to form complete proteins. Excessive alcohol consumption slows this process down by up to 20%, and since your muscles are made up of protein, you can see how this is a problem.
2) It lowers testosterone levels and increases estrogen.
Testosterone is the most important muscle-building hormone in your body. One of the limiting factors that determines how much muscle a person can gain is their level of free-flowing testosterone.
3) It causes dehydration.
The kidneys must filter very large amounts of water in order to break down the alcohol, and this can result in severe dehydration within the body. Water plays an absolutely crucial role in the muscle-building process, and being even slightly dehydrated is a recipe for disaster. The muscles alone are comprised of 70% water.
4) It depletes the body of vitamins and minerals.
Alcohol consumption causes vitamins A, C, the B's, calcium, zinc and phosphorus to all be drained at rapid rates. Vitamins and minerals keep every little process in your body functioning properly, and many of these processes involve muscle growth and maintenance.
5) It increases fat storage.
With 7 empty calories per gram, alcohol can actually be quite fattening. Alcohol also disrupts the Kreb's Cycle, which plays an important role in fat burning.