The best way I know to get someone off one's back is to "agree" with them. They don't have anywhere to go if you do that :-)

"Yes, you're right, I shouldn't be eating this candybar right now, but I am and I aim to enjoy it too!" and then "laugh." What can they say? If you have a definate "opinion" and act a certain way but do it pleasantly without putting the other person down, they "give up."

Another thing I might try is to, if I like any of it, eat things around him that he would approve of? I love fruit and veggies so use to bring in veggie trays w/dip for the whole group at work. Think of things you like that he can't complain about and do more eating of them around him and he shouldn't have much to say then either?

I'd stop his boundary crossing though if he says anything about "your need" to do/eat/whatever something. He doesn't get to decide that and you have to let him know that although you appreciate his concern, you will not tolerate his telling you how to run your life, that is your concern. There's a difference between his encouraging you to come to the gym with you and his telling you what you need. You don't have to be unpleasant in any way, just look him in the eye and be
"firm" Maybe soften it by reminding him of the joke, "Don't tell me what kind of day to have!" :-)