Non-metabolizable components in meat proteins accumulate in elves that eat them, causing gradual physical and mental degeneration.
Lectins are cell surface glycoproteins. About 150,000 years ago there was an evolutionary bottleneck, from which the ancestors of humans emerged lacking certain lectins that are still present in our ape relatives. I am sorry I could not find full text. An abstract:
Varki A. Glycoconj J. 2009 Apr;26(3):231-45. Epub 2008 Sep 7.
Multiple changes in sialic acid biology during human evolution.
Humans are genetically very similar to “great apes”, (chimpanzees,
bonobos, gorillas and orangutans), our closest evolutionary relatives.
We have discovered multiple genetic and biochemical differences
between humans and these other hominids, in relation to sialic acids
and in Siglecs (Sia-recognizing Ig superfamily lectins). An
inactivating mutation in the CMAH gene eliminated human expression of
N-glycolylneuraminic acid (Neu5Gc) a major sialic acid in “great
apes”. Additional human-specific changes have been found, affecting at
least 10 of the <60 genes known to be involved in the biology of
sialic acids. There are potential implications for unique features of
humans, as well as for human susceptibility or resistance to disease.
Additionally, metabolic incorporation of Neu5Gc from animal-derived
materials occurs into biotherapeutic molecules and cellular
preparations - and into human tissues from dietary sources,
particularly red meat and milk products. As humans also have varying
and sometime high levels of circulating anti-Neu5Gc antibodies, there
are implications for biotechnology products, and for some human
diseases associated with chronic inflammation.
Evolutionary divergence events in hominids are associated with shuffling of the glycoprotein repertoire.
There are mutations in humans in which the afflicted cannot metabolize certain glycoproteins. Two are the mucopolysaccharidoses. Examples are Hunter's syndrome and Hurler syndrome. The accumulation of the glycoproteins in the body tissue produces characteristic deformities and developmental defects. I will not link images of the kids affected by these syndromes but they look different in a manner characteristic of their disease. Another name for Hurler syndrome is gargoylism.
So your elves: the divergence event that led them to speciate involved a loss of certain glycoproteins, and then also the metabolic capacity to handle these glycoproteins. Just as Neu5GC is in our meat animals but not in us, these glycoproteins are in other mammals, but not elves. When the elves ingest them (just as when human ingest beef) these now-foreign molecules persist in the body (as Neu5GC from meat persists in our bodies). A little bit is ok, but over weeks and months these substances accumulate, causing mucopolysaccharidosis- like physical deformities and mental disease in the elves who eat them. And they know now not to eat them.
This is more interesting, I think that just dropping dead if you eat a meatball. Elves in dire straits could eat some meat. They might even like it; probably they would. But a steady diet of meat will become obvious as the meat eating elf will become thicker featured and less sharp of mind...
In fact, there would be one terrestrial vertebrate whose meat these elves could eat with impunity. Perhaps this would be done for certain ceremonies. Deducing the identity of this animal is left as an exercise for the reader.