Depends on the country. The Canadian government lists several special medical diets for prisoners. Food allergies is one but they list quite a bit like diets for diabetic or pregnant prisoners. For all these diets the prisoner must be diagnosed and can’t simply request it because they want it.

A diet of conscience is a requested diet for religious or moral beliefs. The Charter of Rights and Freedoms requires prisoners recieve a diet that conforms to their beliefs should they be able to adequately describe and demonstrate adherence to them.