Yeasty reasons from the brewery:
In ancient times beer was named liquid bread. Today we possess the evidence that this is the truth. Depending on the extract, one litre of beer has 400-500 kcal. Of this, 250-390 kcal are the calories which come from the alcohol that is in beer. The rest of the calories come from the part of the extract the human body can use for its own maintenance.
With a tip to El Reg.