meaning of "a dose of prunes"

[KuaiLe]

I read this from "Too Much Happiness", a short story by Alice Munro.

"They know that their brother has a wonderful brain, that he is a great man, but they know also that he must have a dose of prunes every day, because of his sedentary occupation."

This piece of description is about German mathematician Karl Weierstrass and I'm confused about what "prune" means here.
It seems prune as a noun only means "a dried plum". But when I tried to look it up as an idiom, I found that "full of prunes" means "full of nonsense".
So, is this saying that Weierstrass needed to talk nonsense as a way of entertaining himself because his job as a mathematician was boring?
Or, does this simply mean that he loved to eat prunes everyday?

 

[5jj]

He needs a portion of prunes every day to ensure bowel regularity.

 

[Rover_KE]

Prunes are well-known as a purgative for relieving constipation. The author is comparing a handful of prunes to a dose of a laxative medicine or a couple of senna tablets.