1. The Mitzvah to eat hot food on Shabbos:
It’s a Mitzvah to insulate food for Shabbos in order so one can eat hot food on Shabbos, as [eating hot food] is [part of the mitzvah] of honoring and having enjoyment on Shabbos. [See Q&A 1]
The law by one who hot food is unhealthy for him: However, for one whom eating hot food causes him damage, it is permitted for him to eat [only] cold foods. However, one who does not believe in the words of the Sages and prohibits eating hot food on Shabbos, we suspect that perhaps he is a heretic.
It is a Mitzvah to insulate food on Erev Shabbos in the permitted ways to be explained so one can eat hot food on Shabbos.
 Admur 257:13
 As the apostate Tzedoki clan which did not believe in the Oral Torah held that one may never eat hot food on Shabbos. Thus, we suspect that one who follows this is part of this clan of heresy.