Bascially you gain weight if you intake more calorie than what you actually burn.
So, if I take cola zero, which has 1 kcal per can, does that make me fat or gain weight?
What are the disadvantages?
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Getting something clear: you can gain weight either by the accumulation of fat or the accumulation of muscle mass. So, if you accumulate fat, you'll gain weight.
However, it seems the actual question is if one can gain weight by drinking cola zero. The answer is this: It depends on how many cans you drink and how often. While you won't gain weight from drinking a single can, you might begin to feel pudgy after consuming it on a regular basis.
This is because while the advertised calorie is 1 kcal per can, we know that it might actually contain more than that, based on past experiences.
Also, you are consuming other chemicals that might allow your other food intake to increase your fat mass.
Personally, I see no reason to drink such drinks. If you want to indulge, go for the regular soda. That way, at least, you know what you are getting and you know what you need to do to burn it off.
The jury is still out on the actual negative effects of these diet drinks. But no, you cannot gain fat weight from drinking a single can.
Unless you drink enough cans to put you in a ~3500 calorie surplus over the span of a few days you won't gain fat weight. It's really the poor dieting habits that people who drink diet sodas still undertake that causes them to continue to gain or not lose weight, rather than the drink itself. This is referred to as the "big mac and diet coke" phenomenon.
With that said, it doesn't mean that diet soda is any better off as a drink that you would want to consume or should feel safe consuming. It's still just as highly processed and packed with chemicals the body doesn't need, doesn't want, and chemicals that can cause health issues.