Have any you watched as your adult child made decisions or lived their lives in such a way that you know they will have long term health problems because of their choices? If you say too much to them then you're nagging and they get offended or shut down. It's like watching your child in a very slow car wreck and you can do nothing to stop it. All I can do is be supportive and offer help and advice when she asks for it. How do you deal with it?
Once my kids were 18 years old, I left them alone to smoke, take drugs, or eat junk food as they liked. I always told them they could do anything they wanted once they were paying their own health insurance.
My daughter eventually reverted back to her organic, healthy diet, but dunno about my son.