What would you say to a well intentioned person who lectures you about something you are eating or drinking?

I have a cousin who, at family gatherings, will lecture others about their choice of drink (“Diet soda is so bad for you, blah blah blah”). The person receiving the lecture will not want to tell her to shut the f up, as the gathering is a happy occasion, but yet it’s tough to sit there and received a lecture (I know because I have had her lecture me, too).

What’s a good way to politely tell someone to end the lecture, even a well intentioned lecture?