Grandpa Bill Talks About weight loss:

This is the result of a "famine response" within the body. There is not enough food (wholesome nourishment) in a food restrictive diet, so the body stores energy in the form of fat for a future day when no food at all is given it. When it finally becomes necessary, this fat is converted by the body into glucose for energy.

During calorie restrictive diets the body's adaptive response triggers a cascade of events. This is called "metabolic adaption":

After a certain point of nutrient depletion from dieting (or from a daily diet of processed food void of nutrients), the body goes into a "starvation mode", then seeks to protect itself against future hardships with more fat storage.

Intense exercise produces the same metabolic response as caloric restrictive dieting. Dieting plus exercise compounds the following cascade of events.