Water. is the correct answer, with any other liquid being of secondary importance.

The kidneys are organs working with fluid volume.

Once you start drinking enough water on a consistent basis, urine remains diluted, there
is no chance of stone formation, because substances dissolved in the liquid are unable to
concentrate, and any infection will have zero grounds for development.

None of the specially created beverages or expensive fruit juices are capable of doing
what plain water can when consumed sufficiently.

It should be noted that the consumption of two to three liters of clean water per day by
a healthy adult person should remain a rule.

However, there are several liquids which can provide something useful besides hydrating
your body.

Citric acid found in lemon water helps prevent stone formation.

By increasing the level of citrate in urine, the liquid binds free calcium in it, thus
preventing its crystallization.

As far as people having tendency towards calcium oxalate stone formation are concerned, this is a rather effective remedy proven to work in practice.

If you suffer from frequent UTIs that can harm your kidneys, then unsweetened cranberry juice can become the remedy you need.

Full story:
https://www.quora.com/What-is-the-best-drink-to-flush-your-kidneys

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