What are the effects of beer on your stomach?

Because of the calories and carbohydrates.

Beers and most other alcohol products are very high in calories.

Beer is also brewed from wheat and barley, which contains high carbohydrates.

This combination of high calories and high carbohydrates is what causes the feared "beer belly".

Beer it typically consumed while relaxing, stereotypically while watching television. So there is no exercise involved after consuming it, which allows the unburnt calories and unburnt carbohydrates to be "stored" in your body, increasing weight. You don't need to consume fatty foods to gain weight.