Hormonal Control of Hydration and Sodium Balance during the Badwater Ultramarathon

Researchers: Jeff Lynn, Ph.D. Slippery Rock University, Lisa S. Bliss MD, and Jim Roberts, Ph.D. Edinboro University

At the 2007 Badwater Ultramarathon we conducted a follow-up investigation to the research we completed at the 2006 race. In 2006, we studied 4 runners and found that their body weights remained stable (<2% change) and their serum sodium concentrations remained within the normal range. However, we observed marked decreases in sweat sodium concentration and increases in urine sodium levels as the race progressed. Therefore, the principle aim of the study at the 2007 race was to gain a better understanding of how hormones that control fluid balance and sodium fluctuate during the race.

We recruited 12 race participants (8 men, 4 women) to take part in a study investigating the hormonal control of hydration and sodium balance. We asked the runners and their crews to record all food, beverage, and supplement intake to the best of their ability throughout the race so that we could assess fluid and sodium intake. We measured body weight, serum sodium, sweat sodium, urine sodium, urine volume, and urine specific gravity. We also measured the following hormones in the blood, which are known to influence hydration and sodium balance in the body: Arginine Vasopressin (AVP), Aldosterone, and Atrial Natriuretic Peptide (ANP). Data were collected at 3 times: 1) before the start, 2) Stovepipe Wells (SPW), and 3) the finish line.

Eleven of the study participants finished the race. We were unable to gather complete data in all 11 finishers due to challenges implicit in this type of field research. Data analyses are ongoing. Based on the data collected and analyzed, we can report the following observations:

• Fluid and sodium intake varied greatly between participants.
• Body weights tended to decrease between the start and SPW, but then trended slightly upward from that point to the finish. The changes in weight were all 5% or less.
• Serum sodium concentrations were normal for all participants at the start line, but tended to decrease between the start and SPW. At that point, 2 runners (both women) were slightly hyponatremic (Na,130-134). There was no trend in serum sodium concentrations from SPW to the finish, but 3 runners were slightly hyponatremic at the finish line (all 3 were women). None of the participants needed medical treatment.
• Sweat sodium concentration decreased in each participant as the race progressed.
• No consistent trend was observed in urine sodium concentrations as the race progressed.
• Arginine Vasopressin (AVP) appeared to increase as body weight decreased and vice-versa (as expected).
• Neither Aldosterone nor ANP levels were consistently what would be expected considering the hydration status of the runners at the time they were measured. We are continuing to investigate this finding.

The studies over the past 2 years demonstrate that runners maintain near normal hydration and serum sodium levels across a great range of sodium and fluid intakes.  Although it is still not clear how the hydration hormones are acting during the extreme challenge of running the Badwater Ultramarathon, we have some interesting data that we will continue to analyze and gain a better understanding of how the body responds to survive during this remarkable challenge. The next step is to combine what we’ve learned from the Badwater Ultramarathon studies in 2006 and 2007 and publish our findings.