Official recommendations for diet and nutrition is an area which is constantly undergoing change as different studies come out. Not surprisingly, as we are consuming a whole host of complicated substances in multiple forms, every day.

Much dietary treatment of illness involves trying to omit or restrict certain basic foodstuffs.

This is getting steadily harder to manage. Processed foods get more and more ingredients added to them - the trend is for every food to have every conceivable basic ingredient (to make the product easier to make or store).   It used to be that potato crisps contained nothing but potatoes, salt and cooking oil. Now, even many plain crisps (and most flavoured ones) contain wheat, used as a coating to improve the texture. Dairy products (mainly milk powder) get into the most improbable foods, mainly because milk powder is really cheap.

Three of the commonest foods implicated in intolerances are dairy, wheat and nightshades. All of these are commonly used in processed foods, and under so many different names that it's really difficult to tell when they're included. The new labelling regulations in the UK that say the 12 declarable allergens must be listed on the packet are a help, but nightshades are not declarable (the list is based on the likelihood of anaphylaxis, which is rare with nightshades). And there are many foods not covered by those regulations: almost anything loose-packed. The law says that anybody selling food to the public MUST be able to say if one of those 12 allergens is present in any specific product - but some food businesses just flatly ignore the law. Pretentious cafes and small makers of artisanal foods tend to be the worst.