Alcohol has many detrimental effects in the body, and you cannot replace carbs with it.
- Lowers Growth Hormone and Testosterone
- Inhibits recovery
- Causes dehydration
- Lowers muscle glycogen
- Decreases aerobic capacity
- It is caloric, but non-nutrient
Alcohol has a catabolic effect on protein synthesis; the result is lower muscle mass/smaller gains, but it also inhibits other proteins in the body. One effect is that neurogenesis (the creation of new neurons) in the hippocampus is shut off for 4-5 days after alcohol consumption.
One huge effect this has is that it decreases the synthesis and release of the peptide (protein based) growth hormone. This is reason enough to abstain from alcohol if you are exercising.
Besides his fact, it also decreases testosterone release, so that less is available to increase muscle mass. This eventually leads to a decreased lean muscle mass as well. (http://www.ncbi.nlm.nih.gov/pubmed/24658221) - Some research shoes that testosterone increases following low doses of alcohol, and decreases with moderate and high doses. However, the synthesis is inhibited, leading to long-term effects. Alcohol inhibits gonadotropin-releasing hormone which otherwise stimulates the production of sex hormones.
Alcohol also slows down recovery. This happens because energy expenditure is increased for the detoxification from alcohol, and as such less energy is used for muscle regeneration. As far as research goes, this effect depends on the type of training performed.
Another effect is that it functions as a diuretic. This leads to a state of dehydration which induces fatigue, lowers performance and decreases ATP levels. Also, dehydration in itself causes an increase in the heart rate. This makes aerobic training less effective and harder to perform.
Alcohol decreases glycogen levels in the muscles. When doing medium to high intensity training, muscle glycogen is the main energy source; depletion results in premature ending of training, with subsequent lower gains than otherwise.
Finally, alcohol is pretty caloric, but is rather low on nutritive value. There are better options to increase calorie intake both during gains and cutting.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4056249/ - This review sums up many of the effects I mentioned above concerning hormonal function. I will try to find studies on the other aspects as well.