This is a kavvanah (intention) for anyone in a desperate circumstance of needing to eat or drink for their mortal health, to do so with the full confidence that they are fulfilling a mitsvah required for them in the Torah, to preserve their life. The kavvanah was related by Rav Yitschok Zilberstein in his Toras haYoledes (1983), chapter 52, section 10, p. 357 (pp. 331-332 in the bilingual edition 1989), “הועתק ממחזור עתיק” (as “copied from an old maḥzor”). Unfortunately, we can’t provide a more direct reference to this maḥzor. If you know, please leave a comment or contact us. . . .