The 28-year-old actor spoke out in an Instagram post Wednesday after fans criticized his slim figure following a photo leak of his new, Season 5 costume.
"It's a cool suit. That's a terrible photo that I was unaware was being taken, much less being posted," Gustin said of the leak.
"Some things need work and they will be worked on. We'll get there."
"As far as the body shaming," he added.
"I've had 20+ years of kids and adults telling me or my parents I was too thin. I've had my own journey of accepting it. But there's a double standard where it's ok to talk [expletive] about a dudes body."
Gustin explained he has a naturally slim frame and said his weight is also affected by stress.
"I do my best to stay in shape and add as much size as I can through out these seasons," the star said.
FOLLOW REALITY TV WORLD ON THE ALL-NEW GOOGLE NEWS!
Reality TV World is now available on the all-new Google News app and website. Click here to visit our Google News page, and then click FOLLOW to add us as a news source!
"I'm naturally thin, and my appetite is greatly affected by stress. Stress is something that ebbs and flows for me throughout a season. Thus, gaining weight is a challenge for me."
"I didn't cast as a slim actor as The Flash. I went to audition for a role I never dreamed I'd actually get. But, here I am 5 seasons later," he added.
"I'm happy with my body and who I am and other kids who are built like me and thinner than me should be able to feel the same way."
Photos of Gustin in the new suit surfaced this week. The new costume has a smaller emblem and no longer features a chin strap.
"Yikes the test photo suit doesn't look good," one fan responded, according to People. "I actually prefer the chin strap. Grant Gustin doesn't have a superhero chin or physique."
Copyright 2018 United Press International, Inc. (UPI). Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.