Sugar. Oh honey, honey
As my pudgy physique will attest to, I'm not even close to what you might consider a health nut. I have, however, pretty much cut out high fructose corn syrup from my diet (not exactly pursposefully, mostly just from buying less processed and other junk foods.) So, no soda, no fruit drinks, no ho-hos. This weekend, though, I had some lemonade with the old HFCS and man, that stuff is nasty.
Unlike some people, I don't think sugar is poison, and I don't think it (directly) makes you fat (my favorite saccharide? maltose.) But I can't believe how much my reaction to things I used to consume regularly has changed. If you like your Mountain Dew, I suggest never getting off it, you might not be able to go back.
HFCS fun fact: the sugar syrup in most sodas (not this, though!) is actually only about half fructose, with the other half being glucose. And fructose has a much lower glycemic index than glucose (and table sugar). What does this all mean? Who the heck knows.