What happens if I you end in US jail or prison but with very serious food allergies?

I'm going with Joseph the second on this one.

Currently in a hospital setting, in the experience of many allergic folks I know, it can be very difficult to impossible to get safe food. They simply don't seem to have the experience needed to prevent cross-contamination from happening, or don't seem very allergy aware (like making food for someone with an allergen on it, and then telling them they can just 'pick it out' if they're allergic...which will not get all the allergenic proteins out and would likely cause anaphylaxis in the wrong person).

if a HOSPITAL has trouble making safe food, a prison would do worse, IMHO. Mild allergies, it would just cause sickness until you left. Someone with anaphylactic allergies? I don't think you would survive it, if it involved some of the common ingredients in prison food. And guards often treat allergic reactions as though they are minor nuisances, like hay fever, rather than potentially lethal issues.

As is shown in this article: https://www.prisonlegalnews.org/news/2014/apr/15/the-inadequacy-of-prison-food-allergy-policies/

-You don't come out Alive. :(

If those allergies are confirmed by a medical professional, the prisons will accommodate the dietary restrictions. It's the law.

Don't be silly. Of course the prison will accommodate that, just as it will accommodate vegetarians and religious food preferences, otherwise it will rightfully be condemned for not treating prisoners properly.

The gaol will accommodate the meals to medical necessity.
Depending on the crime, and duration, isolation confinement might be in order.