Rare are the studies comparing water and oral rehydration solution (ORS) intake after dehydration induced by exercise in the heat and its effect on muscle cramps. A study by Lau W.Y., Kato H. and Nosaka K. tested the hypothesis that water ingestion after dehydration would increase muscle cramp susceptibility, but counteracted by ORS ingestion. 

The study utilized 10 men running downhills for two bouts in the heat (35°C–36 °C) till the reduction in body mass by 2%. Ten minutes following DHR, they were given either spring water or electrolyte water similar to ORS in a counter-balanced order on two different days of week 1 and 2. The participants were then subjected to-

• Muscle cramp susceptibility by a threshold frequency (TF) of electrical train stimulation to induce cramp before, immediately after (0), and 30 and 60min after the ingestion.
• Blood samples analysis for serum electrolyte concentrations, immediately and 80min after DHR.

They noted that-

• TF did not show any change from baseline to immediately after DHR for both conditions.
• TF reduced following water intake by 4.3Hz (30min) and 5.1Hz (60min post-ingestion), rather elevated following ORS intake by 3.7 and 5.4Hz, respectively.
• Serum sodium and chloride concentrations reduced following water intake but stabilized after ORS intake. 

They finally concluded that water intake after dehydration makes muscles more susceptible to electrical simulation-induced muscle cramp, possibly due to dilution of electrolytes, contrasting ORS consumption.Thus consuming ORS is superior to water after dehydration to prevent muscle cramp.

Source: BMJ Open Sport & Exercise Medicine 2019;5:e000478. doi:10.1136/ bmjsem-2018-000478