Optimal hydration post-exercise: Study compares sodium and carbohydrate levels in sports drinks for effective rehydration

In a recent study published in the journal Nutrients, researchers in the United States evaluated the impacts of varying carbohydrate (CHO) and sodium (Na) content in sports drinks (SD) and oral rehydration solutions (ORS) for post-exercise rehydration.

Study: Post-Exercise Rehydration in Athletes: Effects of Sodium and Carbohydrate in Commercial Hydration Beverages. Image Credit: GaudiLab / Shutterstock

Inadequate fluid intake during sports can cause dehydration, impair performance, and increase the probability of heat sickness. Proper rehydration is critical between training sessions and tournaments, especially during brief recuperation periods. Rehydration involves stomach emptying, intestinal fluid absorption, and fluid retention to replenish bodily fluid compartments. Studies have indicated that individuals must consume 125% to 150% of their lost volume to rehydrate after exercise. Rehydration beverages, such as sports drinks, contain carbohydrates and sodium. The impact of varying sodium and carbohydrate content on rehydration beverages is unclear and requires further research.

About the study

In the present randomized, double-blinded, and placebo-controlled clinical trial, researchers compared beverages with varied CHO and Na content within sports drinks consumed for rehydration following exercise among athletes. The researchers hypothesized that higher sodium and lower carbohydrate content would promote the greatest rehydration.

The study aimed to assess the completeness of rehydration after 3.5 hours of beverage administration. The researchers compared water (W) as a placebo to an oral-rehydrating solution (45.0 mmol/L sodium and 2.5% carbohydrates) and a regular sports drink (18.0 mmol/L sodium and 6.0% carbohydrates). The team expected that rehydrating the ORS and SD would surpass the W.

The study included physically fit male individuals aged between 18 and 30 years who engaged regularly in moderate-strong exercise. Females were not included to evade the potential confounding influence of estrogen levels on the retention of water, which may influence rehydration comparisons in the testing duration. The study participants were healthy, without metabolic, cardiovascular, renal, or endocrine illness or malfunction, and followed a uniform diet. The peak oxygen intake was 50 mL/kg/minute.

Participants exercised during a 90-minute session consisting of three 25-minute periods of intermittent-intensity exercise performed indoors after a 2.0-minute warm-up. Sweat samples were collected during the second 25-minute phase of activity, and sodium concentration was determined to estimate whole-body sodium loss during exercise. No fluids were administered during the exercise-dehydration interval to achieve a 2.5% to 3.0% drop in body mass.

Participants were weighed and rested for 45 minutes before consuming a beverage volume equal to 100% of body mass loss. Beverages were consumed in six aliquots over 1.0 hours after the study. The study used urine samples to evaluate fluid retention and the quantity of beverages provided to participants for rehydration. The mass of urine voided at minutes 30, 60, 135, and 210 following beverage administration was used to evaluate fluid retention. After collecting urine, body mass was assessed at 60 and 210 minutes.

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 beverage by the amount of fluid swallowed. The drinks were purple-colored, grape-flavored, and served in opaque cups.

Subjects were provided similar meals for 24 hours and questioned about physical exercise and nutrition before each experiment to establish a consistent diet with equivalent calorie and salt intake. Exercise was performed on treadmills, stationary bikes, and elliptical machines; the usage sequence varied across individuals but was consistent across all trials. Every 25-minute phase included specific intervals of jogging (7.0 mph), running (10 mph), and walking (3.0 mph) or exercising with elliptical machines or cycles with equivalent intensity. A pilot study was conducted to evaluate how sodium balance influenced rehydration completion.

Results

In total, 20 individuals participated in three trials performed over 3.5 hours. The ORS and SD had similar and higher %FR at 3.5 hours, with the ORS having increased suppression of urine production in the first 60 minutes compared to the W. By 3.5 hours, the ORS and SD promoted greater rehydration than the W, but the pattern of rehydration early in recovery favored the ORS.

The water placebo showed more excretion of urine at the 30-minute timepoint than the SD trial, and at 60 minutes, it promoted more significant fluid loss compared to ORS and SD. By 135 minutes, W induced greater fluid loss compared to the ORS. The statistically significant interactions observed between 30- and 60-minute collections for SD versus the oral-rehydrating solution indicated that the ORS suppressed urine secretion to a greater extent than SD.

Compared to pre-exercise levels, there was a statistically significant reduction in body mass post-exercise. Non-significant differences were observed between the treatments for body mass in absolute terms. However, significant body mass alterations were observed between 60 and 210 minutes post-treatment, indicating that higher sodium may benefit hydration maintenance. The variation coefficient for the mean sodium content in sweat was 10%, indicating that improving sodium balance improves rehydration.

Overall, the study findings showed that beverages with higher sodium and lower cholesterol content promoted better rehydration completeness. The ORS and sports drinks rehydrated the athletes to a greater extent than the placebo. The ORS was 32% more effective than the water placebo, indicating that carbohydrates may offset the effects of lower sodium content. The ORS also promoted a more rapid recovery, as rapidly absorbed beverages with sodium and glucose osmolytes kept plasma osmolality higher and reduced urine excretion.

Written by

Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.