I read on the Fringe Wikia that they have developed technology in order to control their own evolution. it says:
The technology begins to form new ridges in the brain to enhance rational thinking, referred to as "controlled evolution", consequently destroying emotion centers and with it a person's sense of humanity. This loss of feeling relates to both emotion as well as senses like taste."
It also says on Wikipedia's Mythology of Fringe page that
A distinguishing trait is their diminished sense of taste, and it is often shown that they can only taste very spicy food.
perhaps they only eat very spicy foods just so they know and can feel what they are eating, and perhaps to let their stomach know that they are full. I'm just taking a guess, but it seems that they only eat spicy foods because it's all they can taste, why they NEED to taste I'm not sure.