apple a day
A small preventive treatment wards off serious problems, as in He exercises regularly—an apple a day is his motto. This idiom shortens the proverb An apple a day keeps the doctor away, first cited about 1630.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.