I am writing a novel about anatomically plausible superhumans (a new kind of man) who had to evolve in conditions of some food deficiency, living, for example, in high-altitude conditions or semi-desert countries where most of his diet would have to consist of dry vegetation almost devoid of moisture (like what camels eat), empty salads (ordinary leaves) and any carrion available to them.
Over the past time, I have already managed to think a little about how their digestive system could adapt to such a low-calorie diet and learn how to extract energy from it more efficiently. So if I understand correctly, the process of splitting food in the stomach is extremely energy-consuming, and the amount of energy consumed in this process depends on the caloric content of the incoming food (so within a few hours after a snack, energy consumption increases to 90%). People who consume healthy foods and have a balanced diet need only 10% of the total daily energy for digestion. When a large percentage of fat is received, it increases to 20%.
Therefore, I thought that before the food digested by them (the nutrients contained in it) would have to get into the blood, it would be fermented for a long time in a special cavity of the stomach. So ruminants such as cows have a complex multicameral stomach adapted for processing empty salads for greater digestive efficiency, they even regurgitate part of the food for re-chewing.
My people could use their intestines as a sieve, filtering the same food in it many times. Each time the nutritional value decreases, it returns to the stomach, where it is fermented further for a long time. Food is excreted from the body only when the nutritional value of working out becomes negative (we spend more energy than we get). Being on fermentation in a special pocket of the stomach, the food is "stored". So you can save a nutritious resource until you need it — a person can have a hearty breakfast, and get hungry only for dinner the next day. This will save them from heavy and long intestines. Such a mechanism will allow them to eat both empty salads and carrion.
The stomach of this type of person consists of two types of cavities: primary - here the food undergoes primary treatment with pepsin, which breaks down proteins to amino acids, and secondary - a blind pocket into which food enters from the intestine.
Food passes through the primary pocket of the stomach and intestine many times until the proteins in it are fully worked out. After that, the food enters the secondary pocket, where it is mixed with bile for further release of sugars and fats. This is a long process — so the body stretches the receipt of nutrients, as well as spends less effort.
The small intestine is thick and strong enough to distill food back into the stomach many times. It contains taste buds that determine its composition. If the food no longer has nutritional value, it is not sent to the stomach for fermentation, but thrown away. In the long (instead of our large) intestine, the remains of food are completely processed, and only after that they are removed from the body.
Food gets from the larynx to the first ventricle, where it is on primary fermentation. Depending on the taste of the food (analysis for the content of sugars and proteins), gastric acid is prepared. It is often extremely strong, and therefore the walls of the stomach are extremely thick and are regularly restored.
Next, the food enters the near intestine, where it is absorbed into the blood. In the case of carrion or "empty" salads, the food returns to the stomach in the second ventricle. The acid is softer there, the walls are smaller. Here the food is re-fermented and resorbed. This happens until the taste buds of the stomach stop registering a sufficient concentration of nutrients or their ratio to residual toxins becomes too low. After that, the junk food goes into the rectum and leaves the body.
Thanks to the second ventricle, where food can be fermented repeatedly, a long intestine is not needed — it goes "back and forth" until it is completely worked out. In addition, this allows you to make a short supply of food for a short time - sophont can have a good dinner, and the next time he eats only for lunch. In addition, in case of poisoning, food quickly leaves the body — it does not need to overcome the path of the long intestine and give away toxins.
However, how else could the process of extracting energy from low-calorie food be made more efficient (so I heard that the mitochondria of birds are more efficient than ours)?