John saw a green soda labeled "Fitness Freak" with organic ingredients listed on the label. Ah, he thought, I can have a soda that's actually good for me. (John was the sort of guy who thought vegetarian pizza counted as health food, so his standards weren't really high.) He went to the clerk to pay for it. She ran an eye over him and said "excellent choice, you won't be disappointed."
John chugged the fitness soda before he even left the store. It felt good, amazingly good. The fat on his body began to melt away, although some of it migrated to his chest area. He was no longer huffing and puffing, but breathing deeply and evenly. Muscles became visible on his arms and legs. His waist narrowed, and his hips flared slightly, but now with bone and muscle rather than fat. His heart steadied as it pumped more efficiently in his smaller, leaner body. He felt energy he hadn't felt since he was a kid.
Of course, even more fundamental changes were happening. Her new breasts were contained in a sports bra. There was a brief, intense feeling of disorientation as her penis and testicles disappeared to be replaced by female genitals and her internal organs changed to match. Where the obese man had been stood a lean, chiseled woman who had barely worked up a sweat in her morning run.
"Wow" said the clerk. "You chugged the whole thing. Welcome to your new life, ma'am."
"I'm a girl?" asked a somewhat overwhelmed John.
"You certainly are, Ma'am. The effects of the soda when completely consumed are permanent and irreversible, as it says on the warning label."
John had never bothered reading a warning label in his life. She knew she should be really upset about being thrust into a new gender without being properly advised, but everything about her new body just felt so, so good.
"Cool!" she exclaimed. "I'll think I'll run another ten miles."