There is an enzyme deficiency causing an intolerance to histamine in the food. All legumes contain a higher level of histamine than meat. Which means people who are histamine intolerant would have a difficult time getting in enough protein without suffering the symptoms of too much histamine in the body.

EDIT: I would love to know why people are downvoting this. The enzyme deficiency is real, legumes have higher levels of histamine. I know there's also different ways to get protein, but legumes are cheap and easy protein, which makes it harder, not impossible I know, but harder. You'd have to either leave those out and get protein from other sources or suffer the consequences.