Weight loss as a cinematic plotline is a complicated thread, to say the least.

Hollywood still exists in a culture of exploiting body perfection and playing that classic makeover fantasy where we can somehow fit into a smaller pair of jeans. Meanwhile, society outside of La La Land is moving toward a body-positivity ethos that encourages us to value our overall health instead of fixating on fitting into those cursed jeans.

So it’s not surprising if filmmakers tackle body-transformation stories with a degree of trepidation. That’s how it was for Paul Downs Colaizzo when the playwright set out to write his first screenplay, “Brittany Runs a Marathon,” about his former roommate who transformed both her body and her life by taking up distance running. He knew he’d have to approach the fraught subject of her metamorphosis from party girl to marathoner with the utmost tact and sensitivity.

“哦,你们ah, it’s totally loaded subject matter, to have a movie about a girl who obsesses over her weight, even if she eventually realizes that she’s focused on the wrong thing — like I did,” says Brittany O’Neill.

O’Neill is the real-life inspiration for Brittany Forgler, the titular figure in the comedy that opens in theaters Friday, Aug. 30. With the former “Saturday Night Live” writer and comedian Jillian Bell (“Rough Night,” “22 Jump Street”) in the title role, “Brittany Runs a Marathon,” which Colaizzo also directed, avoids Hollywood stereotypes like the fat sidekick. Instead, Bell says, it deals “with what really happens when you start making changes on the outside, but then realize you haven’t dealt with working on yourself on the inside.”

The film tells an inspirational, relatable story about someone taking her own goals seriously for the first time in her life and realizing she has been measuring her worth by the wrong metrics, obsessively getting on a scale to see whether a number has changed.

“I had so many discussions with Paul, as one of my best friends, even before he was writing a movie about me, just thinking about, ‘Why do I feel like being big is so bad?’ ” O’Neill said.

She and Colaizzo met as undergraduates at New York University and were living together in Queens when O’Neill — who had been living a hard-partying and unhealthy lifestyle — decided for the first time to get in shape.

In the film, this comes after Brittany visits a doctor hoping to score an Adderall prescription. The doc ends up telling her she needs to lose 55 pounds — “That’s the weight of a Siberian husky,” Brittany says in the film. She tries to join a gym, reacts to the sticker shock of the monthly fees, and decides to lace up her Converses and try jogging outside — for free. It begins with a jog to the end of her block.

运行的公关ecept of focusing on “just one step at a time” becomes the perfect metaphor throughout the film for Brittany slowly, with a lot of sweat and tears and laughter, turning her life around. She builds up her mileage and her confidence, eventually running the New York City Marathon (which O’Neill did in 2014).

O’Neill, now 35, lives in Brooklyn and works raising money to resettle refugees for the International Rescue Committee. Reflecting back on the period of her life depicted in “Brittany Runs a Marathon,” she said that a medical imperative didn’t motivate her to get healthy. “It was just overwhelming dissatisfaction with myself. I started running for a simple reason: I wanted to like myself more.”

Bell admitted this isn’t the first time she has been asked to read a script dealing with body issues, but she was pleasantly surprised this time that it avoided cliches like the goofy sidekick. “I read it and thought, ‘Wow, this is a such a wonderful, realistic take, layered and emotional and hysterically funny on what it’s like to make a monumental change in your life.’ ”

Although it wasn’t required for the role, Bell decided to lose 40 pounds herself by taking up running before filming began.

“It started with searching for ‘Couch to 5K’ on Pinterest,” she said. “I wanted to go through Brittany’s physical transformation because it informs her emotional journey.”

O’Neill said watching the film, with audiences going crazy for Bell’s unvarnished portrayal of her own personal travails, gives her the odd sensation of “being inspired by a story that was inspired by me. Every time I see it, I learn something new about myself. I highly recommend having somebody write a movie about you. You’ll learn so much about yourself. I tell people it’s better, in a way, than therapy.”

“Brittany Runs a Marathon” (R) opens Friday, Aug. 30, at Bay Area theaters.