Background: Prolonged, moderate-intensity exercise training is routinely prescribed to subjects with obesity. In the general population, this type of exercise can lead to fluid and sodium imbalance. However, little is known whether obesity alters the risk of fluid and sodium imbalances.
Objective: This study examined physiological responses, such as core body temperature, fluid and sodium balance, in lean (BMI<25), overweight (25<BMI<30) and obese (BMI>30) subjects during prolonged moderate-intensity exercise.
Subjects: A total of 93 volunteers (24-80 years), stratified for BMI, participated in the Nijmegen Marches and walked 30-50 km at a self-selected pace. Heart rate and core body temperature were recorded every 5 km. Subjects reported fluid intake, while urine output was measured and sweat rate was calculated. Baseline and post-exercise plasma sodium levels were determined, and urinary specific gravity levels were assessed before and after exercise.
Results: BMI groups did not differ in training status preceding the experiment. Exercise duration (8 h 41 ± 1 h 36 min) and intensity (72 ± 9% HR(max)) were comparable across groups, whereas obese subjects tended to have a higher maximum core body temperature than lean controls (P=0.06). Obese subjects demonstrated a significantly higher fluid intake (P<0.001) and sweat rate (P<0.001), but lower urine output (P<0.05) compared with lean subjects. In addition, higher urine specific gravity levels were observed in obese versus lean subjects after exercise (P<0.05). Furthermore, plasma-sodium concentration did not change in lean subjects after exercise, whereas plasma-sodium levels increased significantly (P<0.001) in overweight and obese subjects. Also, overweight and obese subjects demonstrated a significantly larger decrease in body mass after exercise than lean controls (P<0.05).
Conclusion: Obese subjects demonstrate a larger deviation in markers of fluid and sodium balance than their lean counterparts during prolonged moderate-intensity exercise. These findings suggest that overweight and obese subjects, especially under strenuous environmental conditions, have an increased risk to develop fluid and sodium imbalances.