Heather asks on the forums:
Still not fully getting it, can dietary fat be stored as bodily fat?
Yes and no, but mostly yes.
Start thinking of dietary constituents as like money or commodities- amino acids and fatty acids are fungible. You can't say that this or that molecule becomes this or that thing, and it does not matter anyway. The more important thing is the "account balance" of the amino acid or fatty acid commodities
When you eat fats as TGs (triglycerides), the fatty acids are absorbed and either re-assembled into TGs for storage, or they are burned as fuel, or some of them may be used to make hormones or parts of cell membranes, etc.
There seems to be a predominant central misunderstanding , that this or that thing "turns into" or can't "turn into" something else in the body. Think of amino acids or fatty acids as like pounds sterling or euros and dollars - the body does not care where the currency it needs come from like your bank does not care where the dollars or euros or pounds in the vaults or on its digital ledgers came from. A molecule is a molecule, and there is no need to account for whether it came from last night's meal or one that occurred months ago, as it makes no difference.
It is like opening a window in your house for ventilation, and asking whether the oxygen in your next breath came from outside or was already in the house. Or let's say you have a light on inside the house to help you read on on overcast day. Which photons are illuminating your book? Or you accelerate your car after a stop light. Which gas did you just burn, the gas when you refueled an hour ago, or petrol that was already in the tank?
All these questions have the same epistemological status as asking "what happened to the fat molecules I just ate"?
The human body is a non-steady-state open system, both thermodynamically and in terms of conservation of mass. That is also why saying things like "calories in equal calories out" or quoting the first law of thermodynamics only proves you understand neither biology or physics at a basic level.
As far as fat you eat being stored, think of it like this: What you eat, including the fat or carbohydrate or protein is one set of parameters that affect hormone levels that in turn affect fat storage. Your body does not decide what to do with each lipid molecule just like your bank doesn't decide what to do with each dollar you deposit. It does not "decide" to store fat because you ate too much of it. Instead, amounts of macronutrients you eat are one set of factors that influence the balance of fats in storage.
The question of whether the fatty acids you ate were stored or whether existing fatty acids were not released is meaningless becuase fatty acids are fungible- just like the bank reserves growing if money does not leave is the same as money coming in - think BALANCE or EQUILIBRIUM.
Let me explain further.
Net fat storage is just the difference between fats stored and fats released - both are happening at the same time. Just like the patron count in a busy night club can grow or shrink with people coming in and people leaving at the same time, depending on whether more come in than leave.
The hormones that are influenced by what you eat don't work by locking the door or closing the nightclub and kicking everyone out. They work by changing the relative ease of entering or leaving the building. So think of fat storage in fat cells the same way. The same way patrons can leave and enter a nightclub simultaneously in opposite directions, fat is constantly being stored and released at the same time - the question is not "on or off" but what is the ratio of the two processes. Insulin is like a bouncer at the door - maybe he lets the prettier young women in, and maybe he tosses some obnoxious drunks. Maybe he is neutral when not many patrons are in the bar, maybe he turns you away if the joint is at capacity. But the door is not ever locked, and people come and go even as the number of drinkers grows and shrinks throughout the evening. And as you can see, other factors besides the doorman or bouncer affect the rate of patrons coming or going (time of day, the band is no good tonight, etc.) just as insulin's action to promote fat storage is always in the context of other factors.
If you have just been released from a POW camp on starvation rations, and you start eating 5000 calories a day of nothing but fat and protein, I can guarantee that you will start to store fat.
Alternatively, if you have lowered your caloric intake due to reduced hunger on a low carb diet, and have plenty of fat stores, and your body is seeing less glucose than it is used to, you will liberate and burn body fat under influence of the Randle cycle and lower insulin levels.
Is insulin involved? Yes. Do higher insulin levels, all other things being equal, shift the equilibrium towards storage and away from fat release? Definitely. Does any of this mean you cannot store fat without eating carbohydrate or that you cannot burn fat with insulin present? Of course not. You always have some level of insulin present if you are alive and healthy. What effect it has is contextual, as is the case with every hormone.
Insulin levels are an important factor in fat storage but they are not the only factor and IT IS NOT AN ON/OFF SWITCH. Insulin is ONE hormone that affects the storage/release equilibrium.