לע"נ ר' אלכסנדר זושא ב"ר יוסף
The Rambam says [Shabbos 2/1] that we are mechalel Shabbos for a choleh based on a doctor. In Hilchos Yom Kippur the Rambam says that we may depend on the sick person himself who says he needs to eat. Apparently the distinction is that a person knows how hungry he is and how necessary it is for him to eat, but when it comes to chilul shabbos, how does a layman know when his situation is life threatening?? For that we need a doctor.