Elves and human share many similarities, from body type to the ability to perform magic. However, elves retain a slim, lean build throughout their life, and are incapable of becoming obese. The biggest they could get would be that of a swimmer's body, toned and lightly muscled. This is true regardless of how much they consume. My original hypothesis for this was "because magic", but humans are able to match them in this regard. What biological reason would there be for elves not being able to become fat?
From a genetic heritage standpoint, the Elves evolved in Underhill, which is an ideal garden-like environment, free from both harsh winters and famine-inducing dry months.
As a result, their bodies never developed the ability to store calories for times when food was scarce. Food was always abundant, so fat elves had no survival advantage over thin elves.
and in the end, Shakespeare was right...
Let me have men about me that are fat,
Sleek-headed men and such as sleep a-nights.
Yon Cassius has a lean and hungry look,
He thinks too much; such men are dangerous.
The long eared folk have a metabolism just like that of humans. They don't usually get obese not because of faster metabolism, but because of a combination of factors:
- They have less adipose tissue. Meaning that they don't store fat as efficiently as we do.
- Their bowels naturally produce tetrahydrolipstatin. Trust me,.you don't want to see a chamber pot that has been used by an ellf.
- On top of that, they willingly and ritually eat tapeworm infested food. They insist that as part of nature, those tapeworms are symbiotes and not parasites.
And that's how they stay slim without making any exercise.
It is a pathological condition caused by over-activity of the thyroid gland. The symptoms include Weight loss, sometimes significant, may occur despite a good appetite.
There is a number of known causes for this condition in humans, several of which are genetic. However, simple inflammation due, perhaps, to the habitat is also a possible cause, suggesting that elves may gain weight if they abandon their forests rich in thyroid-irritating spores.
I don't think its a matter of biology and more just lifestyle and oversight by the creators. After all, how many fat people do you find in fantasy stories with elves outside of corrupt despots? It just doesn't happen.
Elves also tend to live a very healthy lifestyle. Being physically active and eating lots of organic healthy foods(cause thats whats readily available to them). Many writers make their elves vegetarian even although its still possible to get fat with that diet.
The human body stores fat becuase it burns carbohydrates first. If their diets have low carbs then their bodies burn fat first and they dont store it. So its just their meat and veggie diet.
Or maybe theres an internal mechanism in elves bodies where they burn fat first instead of carbs.
High metabolism helps too.
I'm not sure why an elf couldn't become obese. It depends entirely on what kind of elves and what universe you're setting it in - some worlds might give more detailed answers or reasons, but I'm assuming since it's in the Worldbuilding StackExchange that you have some control over how things work.
In that case, make elves fat if you want it. Alternatively, the spirit of the planet changes the individual's appetite when it begins to consume more calories than necessary to the point that they aren't interested in eating. Without knowing the rules in your specific story, it's hard to give any sort of satisfactory answer (especially since you'd need to wonder how many of the readers would really think twice about it).
Elves have a smart digestive system that only absorbs calories when they're needed. If they eat more than they need, the excess is simply passed through, undigested.
Note that this really isn't so far-fetched. Many animals do much the same, passing through a good part of the food they consume because they lack the means to digest it. But that material still contains plenty of calories, and other creatures - dung beetles are the archetypical example - make a good living off it.
I'd go back to magic as the solution. People seem to think of fat as a bad thing but it really is not. It is just a way to store excess energy so you do not starve later, which is a very god thing. A secondary function is to work as thermal insulator. It also helps absorb damage without serious injury but other than gladiators that is probably incidental and can be ignored.
So you need to provide the elves with some other form of storing energy and insulating themselves. It also needs to be significantly more efficient than fat to have replaced fat entirely despite elves being rather similar to humans. You could say that you need something magically good. Best way to provide magical solutions is to use literal magic.
The elves can store excess energy as mana in their personal magical field. When short of food or when needing energy rapidly for combat or magic they draw from that field. They probably have ability to interact with ambient mana as well. They can eat ridiculous amounts and it will just make local mana richer. They can go without eating for a long time but that risks depleting local mana. This is more convenient and flexible than fat, so it would probably replace fat based storage fairly fast, if available. In fantasy it would probably be a gift from the gods creating the elves, though.
Such innate magic field could reasonably protect elves from cold and heat. So they would not need fat for insulation either. It would probably also give protection from hostile magic and maybe even physical damage. Elves would also have superior ability to sense the surrounding magic. It might boost kinaesthetic sense as well.
This all fits common fantasy elves fairly well.
The simple answer is that in Tolkien's mythology (which seems to be really what you are talking about, at least in the sense that JRRT was the updater of the Old Tales for the modern age, and gave them a sort of list of canonical ideas, such as "there are elves, and they tend to be thin") there are several races of "little people." Elves, dwarves, hobbits, and orcs. They are interfertile, and interfertile with humans. In fact, orcs were said to have been produced by miscegenation, i.e. breeding with other races. In Tolkien's own works, there are three races of hobbits, who share characteristics of men, elves, and dwarves. They appeared in the Wild, in an area between two large orc-settlements, somewhere between prehistory and the distant past.
The obvious conclusion for me is that hobbits are orcs. To be specific, they are not pure orcs (double twist), but orc-slaves bred from captured members of other races, and at some point a large group of them escaped. That is the logical situation in Tolkien's paradigm. In the traditional (fairy-tale) paradigm, a more logical conclusion is that the little people (including kobolds, brownies, goblins, leprechauns, etc.) are all variations on the same race. Tolkien introduced his own ideas about higher and lesser races, and he imagined that there would be a corrupt race of orcs bred from a pure race of elves. Thus he made them tall and thin. Peter Jackson had to cast them from a pool of New Zealanders who would have made good clothing models. That idea is handy for creating heroes and villains in an adventure story. But in real life, things go in a much more circular fashion, and we all have pretty much the same DNA.
Though Tolkien himself was careful to separate elves from magic, especially in the LOTR stories. Only wizards (Istari) did actual magic. Sauron was of the same race as the wizards, and he made the magic rings that non-magical creatures later used. I think he was trying to make a helpful distinction, because in our world, only spirit beings do or claim to do magic. If an elf has powers, it's not an elf.