People who have had kidney stones should drink 2.5–3 liters of water daily and keep salt intake under 6 grams

Cost: There is no cost at all. Simply make drinking water a daily habit. On days when you sweat a lot, you’ll need even more water — the hard part is remembering to do this every day.
In plain terms: Roughly one in four people who have had a kidney stone will develop another one within five years. In a study of 199 people who had just experienced their first calcium stone episode, one group was instructed to drink enough water every day. Over five years, 12% of this group had a recurrence, compared to 27% of the control group that received no special instructions. UK guidelines recommend drinking 2.5–3 liters of water daily while keeping salt intake under 6 grams. Exactly 99 participants took part in this trial.
Benefit: This is a five-year randomized controlled trial involving 199 patients with idiopathic calcium stones — meaning no underlying cause could be identified. Participants were split into two groups: one group drank at least 2 liters of water daily, while the other received no special intervention. After five years, 12.1% of the water-drinking group had a recurrence, versus 27.0% in the control group (P=0.008). A lower P value indicates a less likely chance that this difference occurred by random chance. The average time to recurrence was 38.7±13.2 months for the water group versus 25.1±16.4 months for the control group (P=0.016). Baseline 24-hour urine volume measurements showed that stone patients produced significantly less urine than healthy individuals: men produced 1,057±238 mL versus 1,401±562 mL in healthy men (P<0.0001), and women produced 990±230 mL versus 1,239±440 mL (P<0.001). NICE guideline NG118, section 1.8.1, offers several recommendations: adults should drink 2.5–3 liters daily, while children and teens should drink 1–2 liters based on age. Adding fresh lemon juice to water is encouraged, and carbonated drinks should be avoided. Adults must also keep daily salt intake under 6 grams.
Evidence grade: A
Sources:Borghi L, Meschi T, Amato F, Briganti A, Novarini A, Giannini A (1996). Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. The Journal of Urology, 155(3), 839-843. https://doi.org/10.1016/s0022-5347(01)66321-3；National Institute for Health and Care Excellence (2019). Renal and ureteric stones: assessment and management. NICE guideline NG118，第 1.8.1 条. https://www.nice.org.uk/guidance/ng118/chapter/Recommendations
Notes: This measure reduces recurrence rates by roughly 55% at zero cost, making it the most cost-effective recommendation in this section according to our ranking criteria. The key metric to monitor is 24-hour urine volume, which should reach at least 2–2.5 liters — not simply the amount of water you drink. People who sweat heavily or work in hot environments need to drink even more. This trial involved only patients experiencing their first idiopathic calcium stone episode; different types of stones — such as uric acid, infection-related, or cystine stones — require different treatment approaches. Anyone with a history of kidney stones should undergo a stone composition analysis and a 24-hour urine test to determine whether medication is needed. Do not cut calcium intake to prevent stones; see section 6, item 21 for more details. Severe flank or abdominal pain during a stone episode requires immediate medical attention — drinking more water alone will not resolve it. Non-painful blood in the urine may indicate other conditions; see section 1, item 27 for further information. 〔100〕〔100〕