This chart was taken from the book "The Art and Science of Low Carbohydrate Performance, by Jeff Volek & Stephen Phinney. It show's the huge difference in fat burn during exercise when comparing athletes who were running on fuel driven by carbohydrate intake, versus, those who had switched their fuel intake to fats and ultimately ketones. It shows a comparison between the lowest, average and highest burners of fat during 60 minutes of exercise, with the highest fat driven athlete burning almost double the amount of fat.