As you've probably seen in Sonic Rainboom, Pinkie Pie dips her hoof into a rainbow poo and licks it. She then I'd overcome by its spiciness and humorously saying it tasted hot!

Now looking back at Friendship is Magic Part 1; When she ate the cupcake she drenched with hot sauce ( that sent Twilight into her room with jet mouth blazing from a sip.) She was unaffected by the sspiciness. Meaning that Rainbows must be one of the spiciest things within Equestria.