Drinking water every ten minutes during a summer run feels responsible. Conscientious, even. The problem is that it can be quietly making you worse, and the cramps seizing your calves at mile seven might be the clearest evidence of that.
The key insight That Changes Everything: sweat is not just water. Key constituents such as fluid and electrolytes are lost during sweating through the process of cooling the human body. You can lose one to two litres of sweat per hour in hot, humid conditions, and with that sweat goes sodium (ranging from 500 to 2,000mg per litre), potassium, magnesium, and calcium. When you top up relentlessly with plain water, you are replacing the fluid volume but leaving the mineral deficit untouched. Worse, you are actively diluting whatever electrolytes remain in your bloodstream.
Key takeaways
- A coach’s sweat test exposed why one runner kept cramping despite drinking constantly
- Plain water alone replaces fluid but deepens the mineral deficit your body is losing through sweat
- Individual variation in sweat sodium loss is so extreme that generic hydration advice fails most athletes
What the sweat test actually reveals
Athletes lose water and electrolytes as a consequence of thermoregulatory sweating, and it is well known that the rate and composition of sweat loss can vary considerably within and among individuals. Many scientists and practitioners conduct sweat tests to determine sweat water and electrolyte losses during practice and competition. This is the test that changes a runner’s perspective. A small patch is applied to the skin during a training session, and the collected sweat is analysed for sodium concentration. The results are rarely what people expect.
Research has shown that individuals who are prone to cramping tend to lose more salt in their sweat compared to athletes who do not cramp, and salt replacement interventions have been successful at preventing cramping in some athletes. Sweat rates and sweat sodium concentration vary considerably from person to person. This individual variation is the reason generic hydration advice so often fails. Some runners are “salty sweaters” who lose far more sodium per litre than average, and no amount of plain water will compensate for that biological reality.
One commonly held myth is that muscle cramping in active individuals is due to the loss of potassium. The amount of potassium in sweat is likely too low for this to be the culprit. Muscle cramping due to electrolyte imbalance is more likely associated with the loss of high amounts of sodium through sweat, making sodium replacement important to maintain electrolyte balance for physically active individuals who experience high losses through sweating.
The counterintuitive danger of drinking too much
Overhydration is one of the less-discussed hazards of summer running, and it has a clinical name: exercise-associated hyponatraemia. Hyponatraemia occurs when blood sodium concentration falls to an abnormally low level, prompting a rapid and dangerous swelling of the brain that can result in seizures, coma, and death. This is not a theoretical risk reserved for elite ultramarathon athletes.
Athletes competing in endurance events will often drink free water in excess either from fear of becoming dehydrated or in accordance with an arbitrary “drinking schedule.” A major indicator of this problem is that these patients will have maintained or even added to their pre-race body weight during prolonged efforts. Drinking to a rigid schedule, rather than to thirst and electrolyte need, is precisely the pattern that produces cramps and, in more extreme cases, dangerous sodium imbalances. The dilutional model of hyponatraemia proposes that sustained overconsumption of hypotonic fluids in the setting of impaired water clearance results in euvolemic or hypervolemic hyponatraemia and is associated with weight gain.
Research indicates that muscles become more prone to cramp by drinking plain water, but more resistant to muscle cramp by drinking electrolyte water. That single finding reframes the entire hydration conversation for anyone who has been reaching for plain water every time the discomfort starts to build.
Building a smarter hydration strategy for summer running
The practical response is not to drink less, but to drink better. Athletes are recommended to commence exercise at least well-hydrated and ingest fluids containing sodium during long-duration or high-intensity exercise to prevent body mass loss over 2% and maintain elevated plasma osmolality. For runs lasting under an hour in moderate heat, plain water is generally sufficient. Beyond that threshold, or when conditions are genuinely hot and humid, sodium needs to be part of the equation.
When the sweat rate is elevated in hot conditions, runners should aim for 300–600mg of sodium per hour. That figure is a useful starting benchmark, but the individual variation matters. Many scientists and practitioners conduct sweat tests to determine sweat water and electrolyte losses of athletes during practice and competition, though unstandardized methodological practices can produce inconsistent results. A formal sweat test, which can be arranged through many sports medicine clinics and some running coaches, removes the guesswork entirely.
For those who want a simpler rule of thumb without a lab visit, thirst remains a surprisingly reliable guide. Any increase in weight, or even a constant weight, during exercise is clear evidence of overhydration. Drinking to thirst is probably the most effective method of avoiding overhydration. Paying attention to urine colour before a run, aiming for a pale straw shade rather than colourless, also gives a reasonable read on starting hydration status.
The levels of sodium contained in many sports drinks are typically insufficient to match sweat sodium losses in high-intensity or long-duration scenarios. Reading labels matters more than most recreational runners appreciate. Many mainstream sports drinks contain far less sodium than their marketing suggests, which is why some athletes find they still cramp despite using them. Electrolyte tablets or capsules with a known sodium content offer more precise control, particularly on hot days when sweat losses are high.
One detail worth knowing before your next summer race: heat acclimatisation actually changes the composition of your sweat over time. Sweat rates are higher and sodium concentrations are lower in acclimatised compared to unacclimatised individuals. This means that as your body adapts to running in the heat over several weeks, it becomes more efficient at conserving sodium, losing more fluid but proportionally less salt. The cramps that plague the early weeks of a summer training block often ease naturally as that adaptation takes hold, though electrolyte replacement remains worthwhile throughout.
Please consult your GP or a qualified sports medicine professional before making significant changes to your hydration or supplementation, particularly if you have any underlying health conditions.
Sources : jscimedcentral.com | pubmed.ncbi.nlm.nih.gov