hi deepa and anil...
actually the term diet is extremely under estimated by the world today. every one relates this term to starving, and putting yourself into under nourishment. but this is absolutely wrong. DIET actually refers to eating healthy and nutritionally balanced diet.
people only see DIET as a provision of calories. but they forget its most important function that is provision of important nutrients which are required for the normal functioning of a person's body. many are aware of the importance of nutrients as their deficiency causes severe medical conditions (ex. calcium- rickets, osteoporosis,vit A- nigh blindness) if not these, problems like protein energy malnourishment is very severe in many communities even in india.
there are many cases where the person being excessively obese cannot indulge heavily in workouts as there may be some physical limitations due to the weight, such people mostly suffer from pain in bone joints,knees and heels or back pain
it is advisable in such cases to first lose weight with the help of a gud diet regime until they actually can involve themselves in physical exercises. and it will be unfair if you forget the role that DIET plays in weight loss for the physically challanged.
Talking of practicality, every other person in the world can not work out at a pace that respected mr. micheal phelp does. and even he would agree to the role of specific nutrients in increasing stamina and prolonging point of exhaution.
so i say diet and exercise are both equally important and they go hand in hand .. comparing between both is like chosing between your left eye and your right eye.