The role of sodium in exercise
Sodium regulates intracellular and extracellular fluid volume, controls osmotic pressure and facilitates the transmission of the nerve signal for muscle contraction. During exercise, it is lost through sweat in amounts that vary widely from one individual to another: from 230 mg to 1,700 mg per liter of sweat (average of ~1,000 mg/L according to ACSM).
Hyponatremia: the danger of drinking only water
In long races, athletes who drink a large volume of water without replacing sodium can develop hyponatremia (serum sodium below 135 mEq/L). Symptoms range from nausea and mental confusion to seizures in severe cases. The irony: exercise-associated hyponatremia kills more athletes than dehydration in ultras and long-distance triathlons.
Hew-Butler et al., "Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference" (2015)
How much sodium should you replace?
For exercise lasting more than 60 to 90 minutes in a hot environment, ACSM recommends a replacement strategy based on sweat rate and individual sodium concentration. As a starting point: 300 to 600 mg of sodium per hour in prolonged activities.
Practical sources: electrolyte gel (80 to 150 mg per unit), sport-specific salt tablet (300 to 700 mg per tablet), isotonic sports drink with sodium.
The sweat rate test
ACSM method: weigh yourself before and after 1 hour of training in race conditions. The difference in grams plus the volume ingested is the sweat rate in mL/h. Repeat in different conditions of temperature and intensity to build an individual map.
Sodium in the athlete's daily routine
Athletes with a high training volume may need more sodium in their diet than the general population. The sodium restriction recommended for sedentary people with hypertension does not apply the same way to endurance athletes with a high sweat rate.
Exercise-associated hyponatremia (blood sodium at or below 135 mmol/L) is more common than it seems in long races. In the study by Almond and colleagues published in the New England Journal of Medicine in 2005, 13% of the 488 runners with a usable sample in the 2002 Boston Marathon finished hyponatremic, and 0.6% in critical condition (at or below 120 mmol/L). The factor most strongly associated with the problem was not lack of water, but weight gain during the race (odds ratio 4.2), that is, drinking in excess, along with finish times above four hours (odds ratio 7.4).
In long-distance triathlon the pattern repeats. In a series from the Ironman European Championship published in NEJM in 2016, 115 of the 1,089 athletes (10.6%) arrived hyponatremic, with 1.6% in severe condition. Sodium loss through sweat varies widely between people, somewhere between 200 and 2,000 mg per liter (average near 950 mg/L), which explains why elite athletes turn to the sweat test from companies such as Precision Fuel and Hydration to estimate individual replacement instead of following a single number from a table.
The takeaway
The most dangerous scene in a long race is not the dehydrated athlete, it is the one who drank too much water believing it was self-care: in the 2002 Boston Marathon, 13% of runners finished hyponatremic, and the factor that weighed most was not lack of fluid, it was weight gain during the race. Sodium is not a supporting player in the hydration plan, it is the variable that decides whether water becomes an ally or a risk. There is no magic dose in milligrams per hour, there is your sweat rate, measured on the scale before and after training, and the sports dietitian who turns that data into a protocol. The rule that remains: drink by thirst and by sodium, not just by the clock.
