Why Diets Fail 

There would be only one single diet and not hundreds of them, if the way in which the body stores energy and consumes energy were properly understood.

The lack of understanding of these natural processes is the main reason why diets fail.

There are other reasons such as the fact that  most people lose muscle. Muscles produce energy in the form of heat and physical strength and this energy producton consumes calories. When you lose muscle you will produce less heat and less physical energy and you will consume fewer calories. People gradually get fat after they have first lost muscle. At first they do not gain weight while they are starting to get fat because the weight they lose in muscle is replaced by weight in fat.  People lose on average a pound of muscle per year after age 25. This loss of muscle has nothing to do with aging it has everything to do with a decrease of physical activity after age 25 and the lesser use of muscles. This is why they say "use them or lose them". If people were to prevent the loss of muscle they would not gain that fat.