I kind of wondered the same thing as above...
While I admit that I do drink some Coca Cola, I fully believe that soda, and all of the High Fructose Corn Syrup, and chemicals and crap, is basically just toxic.
We never got my son hooked on soda, at all.
My DH never, ever, drinks soda...
And we are from the time when nobody drank actual water, and people thought you could live on the stuff. Just sucking it down all day long!
And, it is pretty well known now that all of the sugar and HFCS, etc, is a big factor on why such a high percentage of people might develope Type II diabetes.
If your Dad's comments are limited to the soda and that type of thing.. he might just be well-meaning and not good at keeping his mouth quiet.
Now, of course, if he is truly one of those people who make negative comments that are personal and damaging.. that's a different story.
I am just not so sure that this is really the category that your dad falls in???
Just try to take him as he comes, and not get upset over small things and over-react.