Alcohol calories and the deficit
Fat loss occurs when total calorie expenditure exceeds total calorie intake. A deficit of 7,700 kcal leads to approximately 1 kg of fat loss. If you maintain a daily deficit of 500 kcal (3,500 kcal per week) but also consume 600 kcal of alcohol per week, your effective weekly deficit is 2,900 kcal — 17% smaller. Over 12 weeks, this equates to a meaningful difference in total fat lost.
The impact is proportional to how large alcohol intake is relative to the overall deficit. Someone maintaining an 800 kcal daily deficit is much less affected by 300 kcal of weekly alcohol than someone maintaining a 200 kcal deficit — in the latter case, alcohol calories could eliminate more than half the weekly deficit.