In his column, John Berardi says “[because of a decline in metabolism during a drop in caloric intake,] after a diet, it’s the undereating itself that causes the problem and not the loss of lean mass.”
I wonder: does the metabolism drop as much when the caloric deficit is due to increased aerobic exercise as compared to when it’s due to a decline in caloric intake? In other words, is the drop in metabolism a response to the drop in caloric intake or to the shortfall?
No, creating the caloric deficit by eating less will downregulate your metabolism whereas creating the caloric deficit with aerobic exercise will not. Several studies have shown that resting metabolic rate does not decline as much or even at all when aerobic exercise is included. It is best to phase in cardio and modify carb intake if you are planning to use cardio as your main form of caloric deficit. Both John Berardi (older Appetite for Construction articles) and Bryan Haycock (thinkmuscle.com) have very good plans for using cardio and carb intake modification to lose fat.