Yes, metabolism affects weight gain and loss but not how most people think.

Remember Shawn Bradley after his mission? Philly had him on a crazy high calories per day and he couldn't gain weight. His body/metabolism somehow just dumped the calories and didn't convert them to body mass. However, if he'd lowered his calorie consumption to 8-10 calories per day, he'd have lost weight.

Yes, some people seem to eat whatever they want and not gain weight, but EVERYONE will lose weight if they cut calories enough. Metabolism doesn't matter on the bottom end. As other posters have stated, law of thermodynamics rule there.

For most people, if you eat 1500-2000 calories a day you will likely lose weight. Above 2500, you will likely gain weight.

IF diet drinks or other foods or additives or drugs can affect your metabolism, they wouldn't change it so much that the above generalization wouldn't still hold.

MAYBE something could increase your metabolism so that consuming more calories wouldn't result in weight gain or even cause weight loss (a tape worm can do this), but there is nothing that can happen to your metabolism that will cause you to gain weight if your caloric consumption is as low as above. Its simply impossible.

Food is sad. Eat what you want and feel like crap. Eat right...and food sucks. 🙁