A dietary constituent to prohibit outright? (4)

I believe the answer is:

bran

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'a dietary' is the definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Maybe you can see a link between them that I can't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for bran that I've seen before include "Horse food stored in barn" , "Husks of grain sifted from the flour" , "Fibre cereal" , "Type of edible fibre" , "Refuse from grain" .)

I've seen this clue in the Evening Standard.

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