"I'd have to take a dose of cod-liver oil"

Old-fashioned bottle of cod liver oil
Creative Commons AttributionOld-fashioned bottle of cod liver oil - Credit: User: Mattes
Cod liver oil, as its name suggests, is made from the livers of cod fish.

It is high in vitamin A and vitamin D, and has traditionally been given to children to keep them healthy.

Most people do not like the taste of cod liver oil. Nowadays it can be taken in tasteless gelatin capsules. When Heidi was written in 1880, it would have to be taken from a spoon.