New question:
Good morning,
I hope you are well. I was looking at the explanation for this practice question that I believe is from your lectures, and I was a little confused at the explanation which describes this as a “SIADH syndrome from the patient not being able to excrete water taken in during the marathon”. Could you elaborate how you would know this is SIADH? My original thinking was that they were electrolyte depleted from the extended exercise as well as hyperglycemia, so having had an energy drink would have helped them.
My answer:
This is classic exercise induced hyponatremia.
The stress of extreme exercise (especially in people not in great physical condition) causes them to retain water via a non-osmotic release of ADH.
These patients actually gain weight during the marathon and are not electrolyte depleted.
A great study on this was published in the NEJM in 2005
Here is the abstract
BACKGROUND
Hyponatremia has emerged as an important cause of race-related death and life-threatening illness among marathon runners. We studied a cohort of marathon runners to estimate the incidence of hyponatremia and to identify the principal risk factors.
METHODS
Participants in the 2002 Boston Marathon were recruited one or two days before the race. Subjects completed a survey describing demographic information and training history. After the race, runners provided a blood sample and completed a questionnaire detailing their fluid consumption and urine output during the race. Prerace and postrace weights were recorded. Multivariate regression analyses were performed to identify risk factors associated with hyponatremia.
RESULTS
Of 766 runners enrolled, 488 runners (64 percent) provided a usable blood sample at the finish line. Thirteen percent had hyponatremia (a serum sodium concentration of 135 mmol per liter or less); 0.6 percent had critical hyponatremia (120 mmol per liter or less). On univariate analyses, hyponatremia was associated with substantial weight gain, consumption of more than 3 liters of fluids during the race, consumption of fluids every mile, a racing time of >4:00 hours, female sex, and low body-mass index. On multivariate analysis, hyponatremia was associated with weight gain (odds ratio, 4.2; 95 percent confidence interval, 2.2 to 8.2), a racing time of >4:00 hours (odds ratio for the comparison with a time of <3:30 hours, 7.4; 95 percent confidence interval, 2.9 to 23.1), and body-mass-index extremes.
CONCLUSIONS
Hyponatremia occurs in a substantial fraction of nonelite marathon runners and can be severe. Considerable weight gain while running, a long racing time, and body-mass-index extremes were associated with hyponatremia, whereas female sex, composition of fluids ingested, and use of nonsteroidal antiinflammatory drugs were not.
As part of this study they looked at sport drinks versus drinking pure water and it did not affect the risk of developing hyponatremia (which was 13% of Boston Marathon runners!):
Additional adjustment for female sex (P=0.20) or drinking 100 percent water (P=0.89) was not statistically significant and did not appreciably alter the coefficients of the remaining variables in the model.
Is this clear or do you need more?