i found a bug, if you have any saves at all from a previous playthrough of doing the submissive ending the game thinks you're still doing it and makes you still gain weight even if you only go to the gym.
That would be a weird bug (although the nature of bugs is that they do weird things) since the only time the game checks persistent data (ie variables that were not created during the particular game instance in progress) is at the very end of a run.
SPOILERS FOLLOW
Seeing your answer to IlLuco27's question below, though, I suspect it's because you're not prioritizing high protein foods. You can only get on the super-fit arc if you hit the gym regularly and take in a certain amount of protein as well - it's not just how much you eat, but what you eat, that matters. Does it put you on the fat arc if you visit the gym, avoid high-carb / high-sugar food, AND each protein rich (burgers, chicken, eggs) stuff?
when i did the gym run the first time i never ate once, i only went to the gym, when i did the same again after i had saved data from the submissive ending i tried to start a new game to test something, i ate nothing and only went to the gym, and yet it still made me gain weight.
i tried swtiching the save pages to blank ones, but it didn't work, what should i do?
You get 'wg points' for the crippling hunger, super expensive 'starving to death' cutscene. I ~think~ you could still pull it off if you did some other activities that fake as protein intake - walks in the park, for example, add a +1 to the hidden Protein counter, so you could theoretically go the fit route without manually eating anything if you did enough of that kind of thing.
Try a gym-heavy run while eating protein. Chicken and eggs are safe bets. There's no 'fat gain' from things like the early salads or cucumbers either, so don't feel like you have to starve Trisha to get swole.