Exercising first thing in the morning before breakfast can significantly lessen the detrimental effects of an unhealthy diet.
Belgian researchers fed healthy active young men a high-fat, high-calorie diet for six weeks. Those who exercised before breakfast gained almost no weight and showed no signs of insulin resistance at the end of the study. In addition, men in this group were found to have burned the fat they were taking in more efficiently. (Training in the fasted state improves glucose tolerance during fat-rich diet. J Physiol. 2010 Nov 1;588(Pt 21):4289-302).