The purpose of this study is to learn if dietary habits can affect vasopressin secretion in patients with autosomal dominant polycystic kidney disease. Vasopressin increases the growth of kidney cysts and accelerates disease progression. Understanding how to control secretion of this hormone based on dietary habits may help to develop treatments to control this disease. The study will include about 60 patients from Tufts Medical Center. The study will last for 2 weeks. Blood and urine tests will be done 3 times during the study period. Subjects will be randomly assigned (by chance like flipping a coin), to one of two study groups. Group 1 will be given instructions to adjust their diet. This will include adjusting the amount of water, protein, and salt intake. Group 2 will have no adjustment of diet or water. The project has tremendous public health relevance, given the large numbers of people affected by autosomal dominant polycystic kidney disease and the substantial impact of the disease on morbidity, mortality, hospitalizations,dialysis or transplant, and societal costs of caring for those patients.

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease with an estimated 600,000 persons affected in the United States and 12.5 million persons worldwide. To date, no disease-modifying treatment has been approved for the treatment of ADPKD.

Arginine vasopressin (AVP) is a key player in cyst enlargement and disease progression. It has been established that patients with ADPKD have higher levels of AVP as compared to healthy controls. Suppression, blockade or elimination of AVP slows cyst progression. AVP-V2 receptor inhibition controls disease progression in both animal models and humans, as does genetic elimination of vasopressin in the Polycystic Kidney (PCK) rat. This evidence indicates that AVP could be a promising target for therapeutic intervention. Unfortunately, the only clinically tested medication that blocks the AVP-V2 receptor (Tolvaptan) is associated with side effects including hypernatremia, hyperuricemia and elevated liver enzymes. An ideal therapeutic approach to target AVP in patients with ADPKD would be safe, easy to administer and could be adopted early in the disease process to prevent permanent kidney damage. High fluid intake presents one such possible treatment, and has been shown to suppress plasma levels of AVP, and slow cyst progression in an animal model of polycystic kidney disease. However, adherence to a high fluid intake diet is difficult to maintain in clinical practice.

To address this adherence challenge, The investigators have developed a stepwise approach of combining a low osmolar diet (low protein and salt) with adjusted water intake, with the goal of lowering the amount of water intake needed to suppress AVP secretion. The major objective of this proposal is to evaluate whether this intervention can suppress vasopressin secretion in patients with early ADPKD. Vasopressin suppression will be assessed by measuring copeptin levels, which have been shown to be a reliable surrogate marker for the circulating AVP concentration.

The rationale for this proposal is based on the fact that part of the difficulty in sustaining a low AVP level with daily water ingestion is the consumption of a diet that generates a large number of osmoles; high osmolar load stimulates vasopressin secretion to maintain water homeostasis. Hence, combining a low osmolar diet with adjusted water intake might prove to be sufficient to suppress vasopressin secretion in the clinical setting. The investigators propose the following:

Specific Aim: To conduct a randomized controlled trial to evaluate the effect of a low osmolar diet and high water intake intervention on vasopressin secretion, urine osmolality, and daily solute excretion in adult patients with ADPKD. The investigators hypothesize that a low osmolar diet combined with adjusted water intake will decrease serum copeptin level and total daily solute excretion in patients with ADPKD as compared to the control arm.

To accomplish the research goals, the current proposal builds upon existing expertise at Tufts Medical Center in conducting controlled clinical trials in patients with ADPKD.

The expected outcomes include the identification of a relevant, safe, easily tolerated and affordable intervention that can suppress vasopressin secretion in ADPKD patients early in the disease process; the proposed stepwise approach of combining a low osmolar diet and adjusted water intake carries the premise of lowering the amount of water needed to suppress AVP secretion and potentially slow the progression of this devastating disorder.

The study long-term goal is to evaluate whether this therapeutic approach could be tolerated by patients over a longer period of time, and could impact clinical outcome measures such as kidney volume and kidney function progression.

Inclusion Criteria:

• Adults 18 to 60 years of age, who have ADPKD with an estimated glomerular filtration rate (eGFR) of 60 ml/min/1.73m2 or above

Exclusion Criteria:

1. Patients on chronic use of medications known to affect AVP secretion (Serotonin Specific Reuptake inhibitors (SSRI), Opioids, Tricyclic Antidepressants (TCA) and Tolvaptan)
2. History of diseases influencing renal concentration capacity, such as, diabetes insipidus, adrenal or thyroid deficiencies, present or prior use of lithium, or kidney diseases other than ADPKD.
3. Baseline hyponatremia (Na below 135 mEq/l)
4. Inability to comply with dietary or fluid requirements
5. Have physical or cognitive impairments which prevent participation
6. Pregnant women

Small sample size and short follow up duration.