Research into behavioral changes has indicated that individuals will make the kinds of changes you describe only when one of two conditions is met:
1. There is a powerful positive motivation for making the change and the perceived benefits outweigh the perceived losses. (External motivation)
or, more likely,
2. Maintaining one’s current status quo is untenable and the change comes from a desire within. (Internal motivation)
Your post surely hits home with a lot of Fluther users. I think your devotion to your parents is admirable and I sincerely hope that they respond to your suggestions. I’m sure you have found from experience however that people are not rational actors and that even if you manage to persuade them in the short term to change their diets and alter the foods that they eat, in the long run those choices have to be embraced and adopted by Mom and Dad themselves. To that end, there is not much you can do.
However, there are plenty of individuals who do make permanent or significant behavioral changes based on seeing results in others close to them. That is why we ask our rich relatives for investment advice and not our poor ones, why the guy in the office with a 32 inch waist is consulted about fitness and why nobody seeks counsel about what University to attend from a graduate of a community college. The best thing you can do to help your parents is to set a good example in your own life. Stay fit, eat healthfully, remain disease free. There is the chance that they may respond to your success.
Is there an uncle or aunt or some other peer of your parents who has made the kinds of changes you describe? Maybe it might be useful to casually point out how wonderful that person looks or how successful they have been in altering their diet. Not in a preachy way, but perhaps with a wistful look of admiration.
Finally, never lose sight of what radio personality Dave Ramsey calls the “Powdered butt syndrome”: Anyone who has powdered your butt as a baby has no interest in your advice on money or sex. (To that, I might add “or nutrition and health”)
Good luck!