How does diabetes insipidus affect fluid balance?

Diabetes insipidus primarily affects fluid balance by causing an inability of the kidneys to concentrate urine, which leads to excessive urination (polyuria) and increased thirst (polydipsia). This occurs due to a deficiency of the hormone vasopressin (also known as antidiuretic hormone, or ADH) or due to a resistance to its effects in the kidneys. As a result, individuals with diabetes insipidus excrete large volumes of dilute urine, which can lead to significant fluid loss over time.

This loss of fluid can result in dehydration if the individual does not adequately compensate for it by increasing fluid intake. The excessive thirst is the body's mechanism to encourage fluid intake to counteract the losses experienced. Therefore, the relationship between diabetes insipidus and fluid balance is characterized by the significant fluid loss and subsequent risk of dehydration if fluid intake does not meet the increased output.

In contrast to this, the other options describe effects that do not align with the mechanisms of diabetes insipidus. Fluid retention and increased fluid absorption are not characteristics of this condition, and claiming no significant effect on fluid balance ignores the profound impact diabetes insipidus has on urine output and thirst.