Quote from above link:
"When a subject in weight equilibrium reduces energy intake without changing voluntary energy expenditure (e.g., by increasing or decreasing physical activity), a period of negative energy balance follows that draws upon energy stores.
Assuming the subject is ingesting a low calorie macronutrient-balanced diet, weight loss will proceed in two distinct phases; a rapid weight loss phase during the first few days or weeks followed by a slower weight loss phase lasting up to two years7,8.
Lost Weight ≠ 3500 kcal/lb and Energy Output is Not Constant
The early weight reduction phase lasting several days or weeks7,8 is characterized by relatively rapid loss in body mass consisting of a small carbohydrate (glycogen) pool, protein, and to a less extent fat as sources of energy.
Water balance is also negative during this period as carbohydrate and protein coupled with associated water are released with their oxidation and fluid balance readjusts with changes in dietary sodium intake.
Water is also a byproduct of carbohydrate and protein oxidation.
The high fluid content and low proportion of weight loss as fat during the evolving early weight loss phase is accompanied by an energy content of weight change that is thus not constant and substantially less than 3500 kcal/lb7,8.
As a contemporary example, men and women participants in the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE I) Study at Pennington Biomedical Research Center prescribed low (25% below baseline energy requirements) and very-low (890 kcal/d for three months followed by weight maintenance) calorie diets had intensive monitoring of actual energy intake with doubly-labeled water and dual energy X-ray absorptiometry body composition measurements over the 24-week weight loss phase8,9.
At week 4 the measured energy content of weight change was (X±SEM) 4858±388 kcal/kg (2208 kcal/lb), far lower than Wishnofsky’s value of 7700 kcal/kg (3500 kcal/lb).
Although the timing of metabolic adaptations with low calorie dieting is not exactly clear with respect to the early phase of weight loss, there develops over time hormonal and neural regulatory mechanisms that trigger reductions in resting energy expenditure, protein turnover, and other metabolic processes10, 11.
A reduced energy intake also leads to a lowering of the thermic effects of feeding and perhaps to levels of non-exercise activity thermogenesis10.
Taken collectively, exhaustion of the available glycogen pool and metabolic adaptations reduce the rates of protein catabolism and energy expenditure with a shift to increasing levels of fat oxidation7.
The combined effects of these processes slows the rate of weight loss and leads into the second slower weight loss phase.
The second weight loss phase extends for months or years, although very few supervised studies go beyond six months to one year that can be used to critically evaluate theoretically derived energy balance relations7.
Since glycogen is largely depleted, oxidized carbohydrate comes mainly from the diet and glucogenic amino acids in protein.
Nitrogen (i.e., protein) balance approaches zero, the steady-state level depending on energy and protein intake12.
Adipose tissue triglycerides constitute the main energy source during this period with the rate of weight loss substantially reduced from the early diet period.