When you drink beer, your liver has to go into overdrive to detoxify the alcohol. Now add a few packets of crisps to the equation, maybe some peanuts and those calories are there to stay. Beer itself is pretty calorific stuff, to the tune of around 150 calories a unit so why beer makes you fat, well that starts to seem obvious.
Now why does it give you a beer belly specifically? Well the answer is down to two things apparently - gender and age. After about the age of 35 most men's metabolisms start to slow down. While men tend to put weight on their... bellies, women's fat stores commonly go on their backside and hips.
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