Using fat as fuel slows metabolic rate, reduces ATP production efficiency by increasing ROS, and slows Krebs cycle activity as an adaptive response to conserve energy during perceived starvation.

“when we are using fat as our fuel source this puts the brakes on our metabolic rate it puts the brakes on at the electron transport chain with a lot of reactive oxygen species production which if we produce enough of those it'll totally stop our ATP production by inducing and coupling and it also slows down the CP cycle activity now this is totally adaptive right if we Zoom back out and remember why you the larger context here when we're burning fat we want to conserve energy conserve fuel we want to turn our metabolic rate down”