: Which one is more harmful for your kidneys, drinking more or less water? Which is more harmful for your kidneys? Drinking more than enough water or having less than the minimum amount required?

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Background reading

Even though your fluid intake can be highly variable, the total volume of fluid in your body normally remains stable. Homeostasis of body fluid volume depends in large part on the ability of the kidneys to regulate the rate of water loss in urine.

Normally functioning kidneys produce a large volume of dilute urine when fluid intake is high, and a small volume of concentrated urine when fluid intake is low or fluid loss is large. The Anti Diuretic Hormone (ADH)/sometimes called arginine vasopressin controls whether dilute urine or concentrated urine is formed. In the absence of ADH, urine is very dilute. However, a high level of ADH stimulates reabsorption of more water into blood, producing a concentrated urine.

When water intake is low or water loss is high (such as during heavy
sweating), the kidneys must conserve water while still eliminating
wastes and excess ions. Under the influence of ADH, the kidneys
produce a small volume of highly concentrated urine. Urine can be four
times more concentrated (up to 1200 mOsm/liter) than blood plasma or
glomerular filtrate (300 mOsm/liter). The kidney is crucial in
regulating water balance and blood pressure as well as removing waste
from the body. Water metabolism by the kidney can be classified into
regulated and obligate. Water regulation is hormonally mediated, with
the goal of maintaining a tight range of plasma osmolality between 275
to 290 mOsm/kg.

Substianting the answer

In addition to regulating fluid balance, the kidneys require water for
the filtration of waste from the blood stream and excretion via urine.
Water excretion via the kidney removes solutes from the blood, and a
minimum obligate urine volume is required to remove the solute load
with a maximum output volume of 1 L/h.

The kidneys function more efficiently in the presence of an abundant
water supply. If the kidneys economize on water, producing a more
concentrated urine, there is a greater cost in energy and more wear on
their tissues. This is especially likely to occur when the kidneys are
under stress, for example when the diet contains excessive amounts of
salt or toxic substances that need to be eliminated. Consequently,
drinking enough water helps protect this vital organ. In cases of
water loading, if the volume of water ingested cannot be compensated
for with urine output, having overloaded the kidney’s maximal output
rate an individual can enter a hyponatremic state

Concluding remarks

Note: Is it very important to understand that both dehydration or overhydration both pose equally disastrous effects on health. While the former is associated with orthostatic hypotension, delirium and morbidities the latter has been reported to be associated with water intoxication and hyponatremia, proteinuria etc.

References

Water, Hydration and Health. Barry M. Popkin, Kristen E. D’Anci, and Irwin H. Rosenberg. Nutr Rev. 2010 Aug; 68(8): 439–458.doi: 10.1111/j.1753-4887.2010.00304.x.
Principles of Anatomy and Phisiology G. Tortora: Urinary system
Excessive fluid intake as a novel cause of proteinuria. F Clark et al. Available at: www.ncbi.nlm.nih.gov/pmc/articles/PMC2175005/