In a recent study published in the journal Dr nutrientsResearchers in the United States evaluated the effects of different carbohydrate (CHO) and sodium (Na) contents of sports drinks (SD) and oral rehydration solutions (ORS) for post-exercise rehydration.
Study: Post-exercise rehydration in athletes: effects of sodium and carbohydrates in commercial hydration drinks.. Image credit: GaudiLab/Shutterstock
Inadequate fluid intake during sports can cause dehydration, impair performance, and increase the likelihood of heat illness. Proper rehydration between training sessions and tournaments is important, especially during short recovery periods. Rehydration involves emptying the stomach, reabsorbing intestinal fluids, and retaining fluid to replenish body fluid compartments. Studies have shown that individuals need to consume 125% to 150% of their lost volume to rehydrate after exercise. Rehydration drinks, such as sports drinks, contain carbohydrates and sodium. The effect of varying sodium and carbohydrate content in rehydration drinks is unclear and requires further research.
About the study
In the current randomized, double-blind, and placebo-controlled clinical trial, researchers compared beverages with different CHO and Na contents to sports drinks consumed for rehydration after exercise in athletes. The researchers hypothesized that the high sodium and low carbohydrate content would promote maximum rehydration.
The aim of the study was to assess the completeness of rehydration 3.5 hours after drinking. Researchers compared water (W) as a placebo with an oral-rehydrating solution (45.0 mmol/L sodium and 2.5% carbohydrate) and a regular sports drink (18.0 mmol/L sodium and 6.0% carbohydrate). The team expected that rehydrating ORS and SD would outperform W.
The study included physically fit male subjects aged 18 to 30 who engaged in regular moderate-vigorous exercise. Women were not included to avoid a possible confounding effect of estrogen levels on water retention, which could affect rehydration comparisons during the test period. Study participants were healthy without metabolic, cardiovascular, renal, or endocrine diseases or defects and followed a uniform diet. Maximum oxygen uptake was 50 ml/kg/min.
Participants exercised during a 90-min session consisting of a 2.0-min warm-up followed by three 25-min intermittent high-intensity exercises performed indoors. Sweat samples were collected during the second 25-min phase of activity and sodium concentration was determined to estimate whole-body sodium loss during exercise. No fluid was given to induce a 2.5% to 3.0% loss in body mass during the exercise-dehydration interval.
Participants were weighed and rested for 45 minutes before consuming a drink equal to 100% of body mass loss. Drinks were taken in six aliquots after 1.0 h of the study. The study used urine samples to assess fluid retention and the amount of drink given to participants for rehydration. Urine mass at 30, 60, 135 and 210 min after drinking was used to assess fluid retention. After urine collection, body mass was assessed at 60 and 210 min.
A portable sodium analyzer was used to test sodium levels in sweat. Sodium consumption during post-exercise rehydration was determined by multiplying the sodium content of the drink by the volume of fluid swallowed. The drinks were purple in color, grape flavored and served in opaque cups.
Subjects were fed a similar diet for 24 h and questioned about physical exercise and nutrition before each test to establish a consistent diet with equivalent calorie and salt intake. Exercise was performed on treadmills, stationary bikes, and elliptical machines; The order of use varied between individuals but was consistent across trials. Each 25-minute phase included fixed intervals of jogging (7.0 mph), running (10 mph), and walking (3.0 mph) or exercise with the elliptical machine or cycle at equal intensity. A pilot study was conducted to evaluate how sodium balance affects rehydration completion.
In total, 20 individuals participated in three trials lasting over 3.5 hours. ORS and SD had similar and higher %FR at 3.5 h, with ORS having increased suppression of urine production in the first 60 min compared to W. At 3.5 h, ORS and SD promoted more rehydration than W, but the initial rehydration pattern of recovery favored ORS.
Water showed greater urinary excretion at the 30-min timepoint than in the placebo SD trial, and at 60 min, it promoted significant ORS and greater fluid loss than SD. At 135 min, liquid loss was greater than that of W ors. Statistically significant interactions between SD versus oral-rehydrating solution at 30- and 60-min collections showed that ORS suppressed urine output to a greater extent than SD.
Compared to pre-exercise levels, there was a statistically significant decrease in body mass post-exercise. Non-significant differences between treatments were observed for body mass in absolute terms. However, significant changes in body mass were observed between 60 and 210 min after treatment, indicating that higher sodium may benefit hydration maintenance. The coefficient of variation for the average sodium content of sweat was 10%, indicating that improving sodium balance improves rehydration.
Overall, the study results showed that higher sodium and lower cholesterol beverages promoted better rehydration completion. ORS and sports drink rehydrate athletes more than placebo. ORS water was 32% more effective than placebo, indicating that carbohydrates may offset the effects of low sodium content. ORS promoted more rapid recovery, as rapidly absorbed beverages with sodium and glucose osmolytes kept plasma osmolality high and reduced urine output.