(I probably should append “part 2” or “part 3” or something to the title, but I don’t recall what the number should be and I don’t think anyone will really care if I don’t look it up.)
The advice to eat carbohydrates and as little fat as possible in order to lose weight is very strange advice. I’ve talked about this before, and for people who have the dysfunction of insulin resistance (or worse) it’s downright insane advice. (I don’t use the term as hyperbole, but rather than a person who recommends people who have trouble processing carbohydrates, or worse, outright diabetics, eat primarily carbohydrates for energy is not meaningfully connected to reality. It is possible to be insane only when some subjects come up, rather than completely insane, i.e. insane about all subjects, such as the man who thinks he’s a poached egg and tries to sit, motionless, in an egg cup all day.)
(Before I proceed, I should note that there are a few caveats to what I’m saying, here, which primarily apply to athletes. If you need to maintain maximal athletic performance for competition while losing weight, you are in a specialized situation and specialized strategies will apply.)
The argument for eating primarily carbohydrates for energy when losing weight mostly come down to the observation that carbohydrates are less energy-dense than fats are. Carbs contain 4 calories per gram while fat contains 9 calories per gram. So carbs fill you up more than fat does, so you won’t be as hungry and want to eat more for the same calories!
First, this is a stupid satiety model which is entirely ignorant of how human satiety works. Anyone who has ever been to a large meal such as Christmas dinner is familiar with eating the main course until feeling complete stuffed and unable to eat another bite, then suddenly having plenty of room for desert a few minutes later, knows this. This sort of ignorance is entirely inexcusable; it would be like giving people gardening advice without knowing the plants need sunlight.
The second problem is that, even if one ignores the bad satiety model, it’s not even the right inputs. Stomach expansion is a matter of volume, not mass. Looking it up, olive oil has a density of .92 grams per cubic centimetre, while granulated sugar has a density of 1.59 grams per cubic centimetre. Thus 1 cubic centimetre of olive oil will have 8.2 Calories, while 1 cubic centimetre of granulated sugar will have 6.36 Calories. If you eat the same number of calories in olive oil and granulated sugar, the sugar will only take up 30.2% more space. (Granulated sugar, by the way, is not as dense as sugar can get, since the granules are not tightly packed.) It’s more space, but not by a lot.
A bigger problem is that it’s extremely doable to add bulk to food while adding minimal calories. 100g of butter plus 100g of baby spinach will have only a few more calories than 100g of butter (and mostly in protein, curiously), but will take up way more room in the stomach than 103g of sugar will.
The general defense of telling people to eat carbs not fat is that most people can’t handle the complexity of actual food-volume calculations. In an abstract way, this is true, but again a person is straight-up delusional if they think the average person can’t handle, “eat a certain number of calories and try to make them take up as much room in your stomach as possible.”
And then we come up to the issue of satiety-over-time. If you want to make your stomach full on few calories without concern for how long this lasts, just drink a glass of water.
The moment that we care about satiety over time, though, the fact that the human stomach takes many more hours to process fats than it does to process carbohydrates becomes relevant, even on a garbage model of satiety like pure-stomach-pressure.
When one takes a moment to consider all of the false assumptions required to make the carbs-not-fat recommendation work, it’s really quite astonishing that anyone ever had the temerity to propose it in public.