While recently rereading Reay Tannahill's awesomely informative Food in history, I came across the following paragraphs in Chapter 21, which I thought rather striking although not really surprising.
In view ... of the extravagant publicity given to artificial additives, this is perhaps the place for a reminder that quite a number of natural, healthy, real foods would not be on the market today if they were subjected to the kind of tests that have to be undergone by the additives of commerce.
Caffeine, the natural stimulant in coffee, is fatal to humans at a dose of about one-third of an ounce. Nutmeg is hallucinogenic. Two pounds of onions a day are enough to cause anaemia. Rhubarb and spinach contain oxalic acid, which builds kidney stones. Carotene, which puts the colour in egg yolks, sweet potatoes, mangoes and carrots, can result in jaundice. Cabbage in excess can help to cause goitre. Bran, promoted in the high-fibre diet thought to help prevent coronary and colon diseases, can in excess prevent absorption of iron and calcium. Red kidney beans, inadequately boiled, can be toxic. Watermelon seeds are claimed to damage the liver and kidneys. People have been poisoned by the solanin in green potatoes, the prussic acid in bitter almonds, the cynanide in lima beans.