People who have to avoid certain wheat and grain products or have been born without the ability to digest gluten have

The correct answer is Celiac or Coeliac disease.

Celiac disease is when your body is unable to digest gluten. When people with this condition do ingest gluten, so for example bread, their body reacts by launching an immune response and attacking the small intestine. It is classed as an autoimmune disorder. It is hereditary but people can also develop it at any age.

fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept within healthy ranges. the core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in.