Are there any health risks of consuming large quantities of water at a time or throughout the day?
The primary risk of excessive water intake is hyponatremia (low sodium level in the blood). While a healthy person who drinks 6-8 liters of water daily is unlikely to suffer significant hyponatremia, people who drink this much often have psychiatric illness that is accompanied by poorly understood hormonal changes that may indeed cause hyponatremia and its attendant symptoms.
Many patients with a variety of psychiatric disorders drink more water than normal. This is termed psychogenic polydipsia. The cause is unknown.1
Sodium levels correlate closely with serum osmolality, since sodium is the primary cation contributing to osmotic pressure. The body maintains serum osmolality within a narrow range via regulation of antidiuretic hormone (ADH, a.k.a. arginine vasopressin), a hormone that is secreted by the posterior pituitary gland (a.k.a. neurohypophysis) at the base of the brain in response to a rise in serum osmolality. The hormone circulates in the blood and acts in the kidneys to increase free water resorption, thereby lowering serum osmolality. In the case of excess free water, ADH secretion will be maximally suppressed.
In a healthy person, ADH suppression results in urine diluted to a concentration of about 60 mmol/kg,4 which (assuming a normal glomerular filtration rate) corresponds to a fluid intake of about 28 L per day. Only above that level with serum osmolality be significantly compromised.
Unfortunately, many people can not suppress ADH maximally. As a result, urine may be ‘inappropriately’ compromised in the face of falling serum osmolality. This is called, creatively, Syndrome of Inappropriate Diuretic Hormone (SIADH). For reasons that are incompletely understood, many psychiatric patients with psychogenic polydipsia (see below) have SIADH.3 Some of the reasons that are understood:
- Medications including antidepressants (primarily SSRIs) and antipsychotics are well-established causes of SIADH.
- Schizophrenia itself appears to be associated with SIADH.
- Chronic hyponatremia may lead to a ‘reset osmostat’ whereby the pituitary's threshold for ADH secretion is changed.
The constellation of symptoms associated with hyponatremia is termed hyponatremic encephalopathy.3 It includes: headache, blurred vision, weakness, muscle tremor and cramps, nausea and vomiting, diarrhea, restlessness, confusion. This can progress, in severe cases, to seizures, coma, and death. It would be exceedingly rare for a person with purely psychogenic polydipsia to progress to this point (in part because one is likely to develop depressed consciousness and stop drinking such that the kidney can fix the problem prior to dying).
One important point is that the level of hyponatremia required to cause these symptoms is largely dependent on the rapidity of change. Acute hyponatremia can cause symptoms at a level of 128-130 mEQ/L (normal 135-145 mEq/L). On the other hand, patients with chronic SIADH may walk around with sodium 120-125 mEq/L without symptoms.
1. M Biswas and J S Davies. Hyponatraemia in clinical practice Postgrad Med J. 2007 Jun; 83(980): 373–378.
2. Dundas B, Harris M, Narasimhan M.Psychogenic polydipsia review: etiology, differential, and treatment. Curr Psychiatry Rep. 2007 Jun;9(3):236-41.
3. Illowsky BP, Kirch DG .Polydipsia and hyponatremia in psychiatric patients. Am J Psychiatry. 1988 Jun;145(6):675-83.
4. Robertson GL. Chapter 340. Disorders of the Neurohypophysis. In: Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 18e. New York, NY: McGraw-Hill; 2012.