This is not medical advise, only opinion. Take with salt. The question was asked with no other way to contact:
"I just started studying a related problem. I need to know the extent of
fat"reuse". The body is continuously breaking down cells and replacing
them. Are the cell fragments broken down and reused? or are they broken
down and excreted? or both."
The body has only three was to get rid of anything it absorbed. The largest by far is to burn it for energy, biological energy or directly to heat. The body can pass small amounts through urine, but if you are passing sugar, fat, or protein in anything belong trace amounts, you have another problem. Third, small amounts of toxins can be excreted through the bile duct back to "outside the body" into the intestine. That is the three choices.
To get fat as FFA (free fatty acids) out of fat cells, insulin must be low, Triglycerides break into FFA and FFA diffuse out of the fat cells into blood system. Glycerides goes off as glycerides, glycogen and glucose and returned to the liver and kidneys for further reduction to glucose. The only way the body can get rid of fat, essentially, is to burn it. Good Calories, Bad Calories has a description of this.
Now the cell walls are broken down and reused or replaced one protein at a time. What is it reused for? Proteins and peptides can be reused if undamaged peptides or the liver can make glycogen out of them through gluconeogenesis. It must be a highly effective process, for once again, if we pass much glucose, fat or protein, we have a problem of some kind. Effective, not efficient, as energy is required for this process.
The body can use intakes three major ways, as energy, directly as heat, or process to storage. Tiny amounts, weight wise, are separated and used directly as they are (proteins, peptides, vitamins, minerals). The division of calories in fat, protein, and carbohydrates is different for each. Fat is slightly higher in energy and heat, protein is higher in heat, likely due to reuse and gluconeogenesis, and carbohydrate is higher in storage.
The philosophy for weight loss should be something like, reduce carbohydrate to some low level, 0 to 100 gm/day depending on your basil insulin production, no more protein than you need to satisfy ghrelin and protein requirements, and adjust the fat by weight of fat loss. The problem is all those other factors that prevent us from doing that. Eating is largely non rational, while the philosophy is totally rational. Other factors are food addiction, and addiction like behaviors, emotional maladaptive behaviors, temptations (low impulse control), environment (living at a smorgasbord), physical issues, desires, cravings, habits, likes and dislikes in foods, societal pressures, willful overeating, lack of concern about weight (no desire to change), etc.
Note that a calorie is a unit of heat, not bio-available energy. The conversion is not consistent from one person to the next, nor for one food to the next. But then, what do I know?