title: Sodium replacement and plasma sodium drop during exercise in the heat when fluid intake matches fluid loss.
abstract: BACKGROUND
  Sodium replacement during prolonged exercise in the heat may be critically important to maintaining fluid and electrolyte balance and muscle contractility.
  
  
  OBJECTIVE
  To examine the effectiveness of sodium-containing sports drinks in preventing hyponatremia and muscle cramping during prolonged exercise in the heat.
  
  
  METHODS
  Randomized crossover study.
  
  
  METHODS
  Thirteen active men.
  
  
  METHODS
  Participants completed 4 trials of an exercise protocol in the heat (30 degrees C) consisting of 3 hours of exercise (alternating 30 minutes of walking and cycling at a heart rate of 130 and 140 beats per minute, respectively); a set of standing calf raises (8 sets of 30 repetitions); and 45 minutes of steep, brisk walking (5.5 km x h(-1) on a 12% grade). During exercise, participants consumed fluids to match body mass loss. A different drink was consumed for each trial: carbohydrate-electrolyte drink containing 36.2 mmol/L sodium (HNa), carbohydrate-electrolyte drink containing 19.9 mmol/L sodium (LNa), mineral water (W), and colored and flavored distilled water (PL).
  
  
  METHODS
  Serum sodium, plasma osmolality, plasma volume changes, and muscle cramping frequency.
  
  
  RESULTS
  During both HNa and LNa trials, serum sodium remained relatively constant (serum sodium concentration at the end of the protocol was 137.3 mmol/L and 136.7 mmol/L, respectively). However, a clear decrease was observed in W (134.5 +/- 0.8 mmol/L) and PL (134.4 +/- 0.8 mmol/L) trials compared with HNa and LNa trials (P < .05). The same trends were observed for plasma osmolality (P < .05). Albeit not significant, plasma volume was preserved during the HNa and LNa trials, but a reduction of 2.5% was observed in the W and PL trials. None of the volunteers experienced cramping.
  
  
  CONCLUSIONS
  The data suggest that sodium intake during prolonged exercise in the heat plays a significant role in preventing sodium losses that may lead to hyponatremia when fluid intake matches sweat losses.
authors.affiliation: Harokopio University, Athens, Greece.
authors.forename: Costas A
authors.initials: CA
authors.lastname: Anastasiou
authors.forename: Stavros A
authors.initials: SA
authors.lastname: Kavouras
authors.forename: Giannis
authors.initials: G
authors.lastname: Arnaoutis
authors.forename: Aristea
authors.initials: A
authors.lastname: Gioxari
authors.forename: Maria
authors.initials: M
authors.lastname: Kollia
authors.forename: Efthimia
authors.initials: E
authors.lastname: Botoula
authors.forename: Labros S
authors.initials: LS
authors.lastname: Sidossis
mesh_terms: D000328:Adult; D000450:Aldosterone / Q000032:analysis / Q000378:metabolism; D000704:Analysis of Variance; D001786:Blood Glucose / Q000032:analysis; D001810:Blood Volume; D001823:Body Composition; D015992:Body Mass Index; D001833:Body Temperature Regulation; D004326:Drinking; D005080:Exercise Test; D006339:Heart Rate / Q000502:physiology; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D007010:Hyponatremia / Q000517:prevention & control; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D009994:Osmolar Concentration; D012016:Reference Values; D012680:Sensitivity and Specificity; D012964:Sodium / Q000008:administration & dosage* / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*; D014883:Water-Electrolyte Imbalance / Q000517:prevention & control; D055815:Young Adult
chemical_list: D001786:Blood Glucose; D000450:Aldosterone; D012964:Sodium