My question is why is it ok to push your body to produce most/all of your carbs from protein while it's ridiculous and unhealthy to allow your body to manufacture the fat you need from carbs? How are the systems fundamentally different?
To reframe the point - How is eating 20gm of fat different from eating enough fruit to have your body produce 20gm of fat?
Others can give you the scientific answer; I'll just give you my practical, real-life answer.
I happen to love fruit--seriously so. But if I eat more than 3 whole fruits in one day or have more than 1/2 cup of starchy food such as sweet potatoes or rutabagas, I feel like crap. And if I go over 3 pieces of fruit my blood sugar becomes volatile--still normal fasting or 2 hrs after eating, but MUCH higher post-prandial spikes.
When I stay under 80g of carbs per day, I have sky-high energy and great bg and I gradually lose body fat. BIG DIFFERENCE!
It's starting to seem like you have an anti-fat agenda here and I'm getting bored so I won't read any more questions like this one.
Converting the carbs to fatty acids is long down the chain. The body's not going to do that when it can use the carbs as is (especially if your glycogen stores are low). By the time it gets to the fatty acid stage, it's to store it because all your energy needs are met and it can't make more glycogen.
You're not "pushing" anything. It is a process that occurs. Ingesting carbs are not a requirement for life....fat and proteins are....thats the most basic truths without getting all up in it. BUT, will you be better off for "optimal health" with some carbs?...Jury is kinda out on that far as I'm concerned, so I don't argue that point since I think it is mainly dependent on previous health state, age, hormonal status, and so on and so forth.
First, I don't think anyone said that its ok to "push" your body to make glucose. I think the consensus is that you can make sugar if you need it, where as you can't make protein or fat if you need it. So carbohydrates aren't essential to life. That doesn't mean zero/low carb is optimal to life.
Both gluconeogenesis and de novo lipogenesis are things you body does to manage your sugar levels. If you don't have enough, your body can make some and if you have too much your body can get rid of it. I'd argue that neither is really "good" for you. They both take energy and your body would rather probably do something more useful. But if your brain (or something else) needs sugar, your body will make some. Likewise, if you gorge on fructose your body has to get rid of it. It's a highly reactive oxidizing agent in your body, so you need to do something to make it more stable. Step 1 is to store it as glycogen, but there's only so much of that you can store in your muscles and liver. Step 2 is to convert it into fat. Fat (particularly saturated fat) is a very chemically stable molecule and as a result lots can be stored very safely (from a chemical standpoint). Your body doesn't really care about the long term effects of having too much fat in your system, all it cares about is getting the highly reactive toxic sugar (particularly fructose) out. So that's why we have the ability to turn sugar into fat.
I'd argue that if your body is relying on gluconeogenesis you're not eating enough carbs; however, if you're doing de novo lipogenesis you're eating too many. Sugar is safe in the body only in a very narrow range, think of all of the feedback loops we have in our body to manage sugar levels.
Personally, from a chemical standpoint, I'd rather be a little low on sugar than high. Sugar is highly reactive and I don't like the thought of having extra floating around my body waiting to be turned into fat. I'd rather just make it on demand for the systems that must have it.
Eating twenty grams of fat does not expose you to the ill effects of glycation, and reactive oxygen species that burning carbs provides, or if you're getting it from fruit higher in fructose, remember that that fructose is 10x worse a glycator than glucose.
If you're getting it from fruit, it's not too terrible a hit on your liver, if you're getting it from sugar, since there's little fiber, it gets absorbed quickly and your liver has to dispose of the fructose half in the sugar rapidly.
Not to mention the insulin spikes if you go over the threshold.
In terms of neoglucogenesis, it's less efficient, which means that some of the protein is wasted in the process.
You're better off with the fat, if all you're worried about are the fat vs carbs macros.
Now, if you're doing a PWO, carb refeed, and that's what you're after, you can't eat fat instead as fat gets turned into triglycerides or ketones, so if you've depleted your muscle/liver glycogen, the only thing you can do is to eat carbs, or have your body produce them by catabolizing muscle, if not enough protein is being digested.