I think they used to think that raw or slightly cooked vegies were “unwholesome”, in that they could induce indigestion. If I ate a diet of bread and salt beef and boiled potatoes and beer, the occasional raw cucumber or slightly cooked carrot might disagree with me, too, because I wouldn’t be used to it. I think the concept carried over even to our parents’ time.
The 19th century fiction I like to read has occasional references to this - in particular, Louisa May Alcott’s works. LMA loved to preach about what sort of “wholesome” food one should eat.