The usual connotations of dieting, these days, relate to losing weight.
Not so in Shakespeare's time. Indeed, most uses of the verb diet then are to do with feeding someone up to a satisfactory level.
This "fattening" sense is required when Alencon says, of the English, "they must be dieted like mules" if they are to fight well (Henry VI Part 1, I.ii.10) or Innogen says to Pisanio, "Thou art all the comfort The gods will diet me with" (Cymbeline, III.iv.182).
And the sense of a steadily increasing regime is present when Iago soliloquises about wanting "to diet my revenge" (Othello, II.i.285).
The modern sense is in the wings, however.
When one Lord says to the other, of Bertram, "he is dieted to his hour" (All's Well That Ends Well, IV.iii.28), the sense is "limit, restrict".