Dietary fat contains lots of calories, just like carbohydrates.
But the metabolic response differs.
For example, for ingested fat, the body has a much lower insulin response.
Is all the fat you eat metabolised or stored, or does the body have a (protection?) mechanism to respond to large amounts of dietary fat by simply letting it pass straight through?
After ingestion of even large amount of fat (like 200 g/day), most of it will be absorbed and then either used as an energy source or stored as body fat. In the context of weight gain, a healthy human body has no effective mechanism to "ignore" fat once it is ingested. Ingestion of abnormally large amounts of fat at once could trigger diarrhea, though.
How do we know that most of fat is absorbed?
There is a fecal fat test in which you are instructed to consume 100-150 g fat per day for few days and then the amount of fat in your stool is measured for few days. The normal amount of fat in the stool in healthy adults is up to 7 g/day (Rochester.edu). So, from 100-150 grams ingested only up to 7 g can normally escape absorption. Larger amounts of fat in stool (steatorrhea) occur in malabsorption conditions.
How do we know that fat, once absorbed, is not excreted as fat?
When dietary fat is absorbed, it is either burned or stored as body fat. Where else could it go? A healthy human body can excrete unmetabolized nutrients via the stool, urine and skin. A minimal amount of fat is secreted via bile into the intestine, but most of it is reabsorbed. Fat is not excreted via urine in any significant amounts, and when it is (lipiduria), it can be a sign of kidney disease. Sebaceous glands excrete only few grams of fat per day.
Not all the fat we eat is absorbed, but if we eat more fat then the body will become more efficient at absorbing fats. This is because certain intestinal microbes in the small bowel that help us to absorb fats, will increase in numbers in response to an increase in the amount of fat available there. There must, of course, be a good reason why the feedback mechanism is positive instead of negative, as evolution can be assumed to have led to a well designed body. In Nature animals don't have an obesity problem, they need to survive on the sources of calories they can actually find. So, if there is a shift from carbs to fats in the diet, it makes sense that the animal will become better at absorbing fats rather than becoming worse at absorbing fats.
