Alcohol has a two-fold negative effect on our ability to lose fat:
1. Alcohol is highly calorific and easily over-consumed. Compare its calorific value with the other components of our diet: Alcohol is 7kcal/g, fat is 9 kcal/g, both protein and carbohydrate are roughly 4kcal/g).
2. The simple presence of alcohol in your system has a hugely negative impact on your ability tometabolise fat. Period!
This was illustrated by a study where 8 men were given two drinks of vodka and lemonade separated by 30 minutes. Each drink contained just under 90 calories. Fat metabolism was measured before and after consumption of the drink.
The reason why alcohol has this dramatic effect on fat metabolism has to do with the way alcohol is handled in the body. Rather than getting stored as fat, the main fate of alcohol is conversion into acetate and the presence of acetate in the system puts the brakes on fat loss. The greater the quantity of alcohol, the greater the quantity of acetate created, the less likely fat is metabolised.
In other words, your body tends to use whatever you feed it, and after a time becomes adapted to the macro nutrient intake. Unfortunately when acetate levels rise, your body burns the acetate preferentially. So the body simply burns the acetate first, this basically pushes fat oxidation out of the metabolic equation.