Anecdote time: My internship was served under a coach that mostly worked with football players looking to either get to or get back to the NFL. A few were NFL guys for a game or three, but mostly they were guys that would be a training camp body at best.
Still, phenomenal athletes, faster and stronger than 99% of the population. Our equipment guy's wife always packed him a care package of sweets, since he would be away from home for three-four months a year. Cookies, brownies, cakes, she could bake. Too much for him to finish, so he'd share with the training staff, equipment staff, and coaches.
One of the athletes came into the weight room early, a day before schedule and sat down in the office. He was a receiver, D2 school, undrafted, hoping to stay in shape in case some NFL team needed a punt returner for the preseason.
He was offered a brownie. He turned it down. Not because it would cover up his six-pack, but because he thought it would slow him down.
Now, we could argue the impact a 300 calorie brownie has in the grand scheme. I'm not one to deny myself one of my wife's cookies or a burger from time to time. It is not about the nutritional value of a brownie, it is about the psychological value of thinking about your goals.
When you find that intrinsic motivation, it all falls into place.