@JulietEMcKenna And even if the intolerances didn't kill children, they were still recognized among adults. I'm thinking of all the 19th century books about people with dyspepsia, or how someone who was generally lucky or healthy was described as having 'good digestion'.
Then there's poor Mr Woodhouse, in Emma, who can't stomach anything other than very thin gruel. Dietary intolerances were definitely behind at least some of that, I'm sure, even if they were described with different words.