Devouring alien life was truly a technical craft, and one that demanded a very high level of skill.
Other than directly extracting spiritual power or using spells to transform matter, all energy transfer and absorption were determined by changes in material properties at the molecular level. Therefore, what a living creature could and could not eat had, in truth, been decided from the very beginning.
Even if the properties of carbon, hydrogen, and oxygen meant that sugars, Path of Living Beings Source substances, and fats throughout the universe possessed similar structures, the odds of encountering a species on another planet that you could digest were still extremely low.
Because digestion and absorption ultimately rested upon enzyme systems. Enzymes were highly selective substances. One enzyme could digest only one or several kinds of matter.
For living beings, "preparing enzymes for organic matter commonly found in one's environment" was far more efficient than "preparing countless enzymes to ensure one could digest anything."
Even if you truly encountered sugars similar in structure to those of life on your home planet, you still had to consider chirality, isotopes, and countless other factors.
If even one of them was wrong, then the substance you encountered could not be absorbed by your physical body. Instead, it would throw your physiological systems into disorder.
Most legends of "immortals not partaking of mortal food and smoke" had actually arisen this way.
Once immortals left their home planets, finding something to eat was truly not easy. Fortunately, immortals would not die from skipping meals. Otherwise, the leading cause of death among immortals in this universe would probably have been "starvation."
The trait of "not needing to eat" not only freed immortals from social production, but also ensured that they would not starve to death after leaving their home planets.