It's true that the best way to convince people PB is real is to show them by changing. But humans are skillful at not seeing what they don't want to see.
"Sure, your test results are good, this time. But test results can be wrong."
"All the vegetables you are eating are somehow fighting off all the damage you are doing by eating all that fat. It will catch up with you sooner or later."
What examples do you have?