Hydration and cramps
Everything in the library studied for it, medical and holistic together. Within each kind, what was shown to help comes first, strongest evidence first. What was tested and not shown to help stays on the list.
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Medication
Vomiting in children with stomach bugs
Shown to raise the share of children who stopped vomiting and to cut the need for IV fluids by about half against placebo in children with gastroenteritis.
Compared with placebo [4][5][6]
The numbers and sources
- Effect
- Oral ondansetron raised the share who stopped vomiting (RR 1.34, NNT 5), and lowered IV rehydration in the emergency department (RR 0.41, NNT 5) and immediate hospital admission (RR 0.40, NNT 17) [4]. A later review found fewer oral rehydration failures (RR 0.5, 95% CI 0.37 to 0.69), less IV rehydration (RR 0.45, 95% CI 0.31 to 0.63) and fewer admissions (RR 0.53, 95% CI 0.29 to 0.97), with no difference in vomiting cessation after 4, 24 or 48 hours [5]. A single dose lowered oral rehydration failure (RR 0.43, 95% CI 0.34 to 0.55), IV hydration (RR 0.44, 95% CI 0.34 to 0.57) and hospitalization within 8 hours (RR 0.49, 95% CI 0.32 to 0.75) [6].
- Population
- 1,020 children in seven trials [4]; 1,215 children in 10 trials [5]; 2,146 children and adolescents in 13 trials [6].
- Sources
- [4], [5], [6]
- Caveats
- Return visits to the emergency department did not differ (RR 1.14) [5][6]. Diarrhea was reported as a side effect in four of five ondansetron studies [4].
Supplements
Muscle cramps
Not shown to reduce idiopathic leg cramps in older adults. Pregnancy cramp trials conflicted, and none tested exercise cramps.
The numbers and sources
- Effect
- For idiopathic cramps, mostly nocturnal leg cramps in adults aged about 62 to 69, magnesium did not change the percentage change in cramps per week at 4 weeks (MD -9.59%, 95% CI -23.14% to 3.97%) or the number of cramps per week (MD -0.18, 95% CI -0.84 to 0.49). The share with a 25% or better reduction did not differ (RR 1.04, 95% CI 0.84 to 1.29, high certainty) [10].
- Population
- 11 trials, 735 people, including pregnancy-associated and idiopathic cramps [10].
- Sources
- [10]
- Caveats
- Pregnancy cramp trials were conflicting and could not be pooled. No RCT tested exercise-associated cramps [10].
Muscle cramps
Limited evidence: no trial of potassium for cramps was located, and cohort studies found athletes who cramped had higher, not lower, blood potassium after races.
Compared with athletes who did not cramp [18][19]
The numbers and sources
- Effect
- A 2026 meta-analysis of 9 longitudinal studies (11,274 participants) found higher post-race serum potassium in people with exercise-associated cramps (moderate evidence). Apart from a history of cramps, no hydration, electrolyte or other factor was consistently linked to cramping [18]. An earlier review found that support for the electrolyte depletion explanation of exercise cramps came mainly from anecdotes, case series totaling 18 cases and one case-control study of 10, while four prospective cohort studies did not support it [19].
- Population
- 11,274 participants in 9 longitudinal studies of athletes [18]. Studies of exercise-associated cramps [19].
- Sources
- [18], [19]
- Caveats
- Observational studies of blood levels, not trials of taking potassium. The authors of [18] rated most findings as limited or very limited evidence. Cramps in other settings, such as at night or during dialysis, were not covered.
Therapies
Exercise performance, IV fluids against drinking
Small crossover trials disagreed: one found IV rehydration sped a 40 km cycling time trial by 3.5% against drinking, and another found drinking gave a faster time trial.
Compared with drinking fluids, sports drink and water [5]; drinking water [6]
The numbers and sources
- Effect
- After dehydration by 4% of body weight, rehydrating half with IV saline cut 40 km time trial time by 3.5% against oral rehydration, and oral glycerol by 3.7%. Combining IV fluid with oral glycerol restored the most plasma volume and did not improve performance over either alone [5]. During 90 minutes of cycling in 35°C heat, 25 mL of saline infused every 5 minutes left people thirstier and hotter, with a slower 12 km time trial (19.6 minutes) than drinking 25 mL of water every 5 minutes (17.7 minutes, P = 0.05) [6].
- Population
- 9 endurance-trained men [5]. Recreationally active men, mean age 29, number not given in the abstract [6].
- Sources
- [5], [6]
- Caveats
- Two small crossover trials with different designs. Neither tested recovery on the following days.
Lab markers
Sodium at the upper end of the range in midlife
One US cohort linked midlife sodium above 142 mEq/L to 39% more chronic disease, and above 144 mEq/L to 21% more early deaths, over 25 years.
The numbers and sources
- Effect
- Sodium above 142 mmol/L in middle age was associated with a higher risk of developing chronic diseases (HR 1.39, 95% CI 1.18 to 1.63) and with higher odds of a biological age above chronological age (OR 1.50, 1.14 to 1.96). Sodium above 144 mmol/L was associated with a higher risk of premature death (HR 1.21, 1.02 to 1.45) [14].
- Population
- 15,752 adults aged 45 to 66 at enrollment in the Atherosclerosis Risk in Communities study, followed for 25 years [14].
- Sources
- [14]
- Caveats
- One cohort, observational. The authors used serum sodium as a proxy for hydration habits and state that intervention studies are needed to confirm a link between hydration and aging. These values sit inside both laboratory reference ranges at the top of this page [14].