"Nahrung" is presumably formed from "nähren" through an extremely productive verb-to-noun derivation, and there's plenty of information about this process.
What I'm puzzling over though is the loss of the umlaut. The introduction of an umlaut in this and many other derivations is well attested, but I've struggled to find any specific information on this phenomenon (if indeed it's not merely a one-off).
Certainly it can't be particular to "nähren", since there are a whole host of words derived from it which retain the umlaut; indeed there's also "Nährung" anyway which is formed in the way one would expect.
Of course, it may be that my initial assumption that "Nahrung" is derived from "nähren" is false!
Nevertheless, is this just a one-off, or is this but a single example of a broader class of words in which a derived form has a de-umlauted vowel?