Doc...I am a physician myself but this question perplexes me. Can we drink alcohol while dieting. The beer companies say that the alcohol does not have glycemic index

During my consultation with  weight loss patient, they (especially men) ask about if it is alright to drink beer or alcohol.

First let see what is in alcohol. Why it gives you 7 kcal for each gram rather than 4 kcal as other carb food group. 

Alcohol is made from carbohydrate commonly "Barley grain",  before it become alcohol, the grain was

1. malted to become "maltose",

2. maltose was brew to ethyl alcohol and Carbon dioxide.

Malting process concentrates the calories (that is the reason alcohol gives you higher calories than other carb).

Brewing process make the alcohol chemical different than other carb. That is the reason, alcohol does not have measurable glycemic index (because it does not break down into glucose, see below). 

So alcohol companies claims that alcohol does not have much of carb which is true. BUT then why do we see a lot of central obesity, fatty liver and wasting of muscle with abnormal fat deposition with regular alcohol consumption.

When you drink alcohol, it quickly absorb into the blood stream. That is the reason, we immediately feel the tipsy after a few minutes.

Then alcohol quickly travel into the entire body. Largely it breakdowns in liver into acetaldehyde, then to acetyl CoA and CO2. There is an enzyme which involved in degradation of the alcohol. Depends on the enzyme activity, the degree of feeling drunk differs.    

During this breakdown process, it used up the energy residue and clogged up the body essential energy transport system(the TCA cycle). The break down of the glucose, fat and the rest of the energy yielding process are blocked till alcohol breakdown is complete.No other nutrients get degraded till alcohol complete its degradation. (I think it is natural way of the body defense the bad guy to kill first. :) That is the reason, when a person drinks alcohol regularly and heavily ,it exhausted the body energy transfer which is needed in degradation of fat. It causes fat deposition especially in the liver, called the fatty liver.

During the weight loss program, one would desire the fat burning/degradation rather than the depositing/storing the fat.

So when you are trying to lose weight, you should not drink for at least two major reasons.

1. Alcohol is not a nutrient, and it gives you a lot of non nutrient calories.

Non nutrient calories means it does not satisfies you nutritionally, does not make you full, and satisfied.

2. Alcohol make fat accumulation especially central obesity that is opposite of what we want during the weight loss.

I do not support to drink alcohol during the weight loss plan. 

Do I not drink alcohol at all while I maintain my weight?

I may have two sips of wine at the most in rare occasions because I want to eat something else to give me energy and nutrition.