See ESC right:

Food, adequate

Relevant ICESCR Articles:

11(1): The States Parties to the present Covenant recognize the right of everyone to an adequate standard of living for himself and his family, including adequate food, clothing and housing, and to the continuous improvement of living conditions . The States Parties will take appropriate steps to ensure the realization of this right, recognizing to this effect the essential importance of international co-operation based on free consent.

Related Documents:

Basic Principles for the Treatment of Prisoners

Body of Principles for the Protection of All Persons under Any Form of Detention or Imprisonment

Standard Minimum Rules for the Treatment of Prisoners

Universal Declaration on the Eradication of Hunger and Malnutrition