The star – who is making history on Strictly as part of the show’s first all-male duo with Johannes Radebe – said working out is his way of keeping things “under control” as he is bulimic and has struggled with body dysmorphia.
In a long post on Instagram, he suggested people commenting on him should focus on themselves.
“Since I started working out I’ve had so many messages from members of the gay community telling me how disappointed they are with me for becoming ‘yet another muscle Mary’,” said the star, who found fame when he won Bake Off in 2012.
“I don’t work out so I can post pictures of myself on Instagram.
"I don’t work out because I feel having muscular body shape makes me a better person.
"I work out because I am bulimic and because I have struggled with body dysmorphia since I was a little boy."
"Going to the gym is my way of keeping things under control," he said.
"It allows me to feel like I have a small grip on the rudder of my life."
Whaite, 32, said he also listens to classical music while he exercises so it gives him "a little time to zone out of this very hectic world we all share".
He went on: "But please, if you’re going to post platitudinal screenshots about being kind and caring, please put that into practice. You don’t know what any single one of us is going through.
"If you feel the need to comment on someone else’s life, direct that energy inwards and focus on yourself. It’ll make you a better person."
The star's comments were welcomed by his fans and celebrity friends.
"Hear hear," Loose Women star Nadia Sawalha posted in the comments section, while screenwriter Russell T Davies wrote: "Well said."
Actor Charlie Condou wrote: "And more importantly, it’s none of anyone’s business! You look great."
Watch: Who is Strictly's John Whaite?