Deficient

Some people who try vegetarianism but give up may be doing so because their body becomes deficient in the postulated essential fats or oils, says Yaseen. “They decide that this diet is not for them or gradually creep back into an omnivore diet. Some people might think they just don’t have the willpower.”

Another possibility, says Yaseen, is that the apparently vegetarianism-promoting gene variants affect people’s taste. “A lot of information about genesis yet to be known,” he says.
–New Scientist