The 26-year-old singer is known for speaking out against body shaming, and sarcastically branded a troll as "super sweet" earlier this week after they requested she send them a private message on social media in order to set up a diet plan for her.
The fan - who has since deleted their Instagram account - posted a paparazzi shot of the 'Sorry Not Sorry' hitmaker leaving the gym and tagged her alongside the caption: "hey demi dm me so we can discuss a diet plan for you (sic)"
And Demi found the post, and commented: "I actually am choosing not to diet and fall back into unhealthy behaviors at the risk of losing my sanity or mental health but thank you for the offer! Super sweet. (sic)"
The singer has long been open about her struggles with her weight, and once battled an eating disorder after being left unhappy with her body image.
In March, Demi hit out at a headline which commented on her "fuller" body size, and said she was "more" than her weight.
She wrote at the time: "I am more than my weight. Unlike the past, I'm not triggered, I'm not upset that someone wrote a headline about my 'fuller figure.' I'm angry that people think it's OK to write headlines about people's body shapes."
The 'Confident' singer also leapt to the defence of fellow singer Bebe Rexha last month, after trolls branded her as "tubby".
Bebe wrote on social media: "To all the people tweeting mean things to me about my weight you just want a response and you will not get it.
"Unless you are completely perfect you have no f***ing right to talk about anybody else's body. The hate you breed that stems from Insecurity don't look pretty on you.
"I don't give a two s**ts about what people think about my weight. I care about what I think about my weight. (sic)"
And Demi replied: "You are GORGEOUS, strong, authentic and an inspiration to me and so many. Thank you for being you you BEAUTIFUL, beautiful woman. I love you. (sic)"