The heart rate monitor doesn’t lie. For years, the instinct of many recreational runners, grab the bottle, take a long gulp mid-run, feel virtuous, seemed entirely sensible in May heat. But when the data started telling a different story, it turned out the problem wasn’t hydration itself. It was the way they were drinking.
Key takeaways
- Heart rate climbs in heat due to two simultaneous forces: fluid loss shrinking blood volume and your body redirecting blood to cool the skin
- Large gulps of water create gastrointestinal stress and fail to maintain plasma volume the way steady sipping does
- Overhydration with plain water dilutes electrolytes and can cause hyponatremia, a silent risk that mimics dehydration symptoms
What your heart rate is actually telling you in the heat
Cardiovascular drift is the phenomenon where cardiovascular responses begin a time-dependent “drift” after around 5–10 Minutes of Exercise in a warm environment, characterised by decreases in stroke volume and a parallel increase in heart rate. This means your heart rate climbs even though you haven’t picked up the pace, and on a May afternoon, when temperatures in the UK can shift quickly, this effect is amplified considerably.
The mechanics behind it are worth understanding. Exercising causes Dehydration from fluid lost through sweat, which directly decreases the plasma volume of your blood. When plasma volume decreases, there is less blood circulating in the vessels and less to fill the ventricles of the heart. Consequently, when the heart contracts, it pumps a smaller volume of blood, reducing stroke volume. If stroke volume drops, cardiac output, the amount of blood pumped per minute, also drops. The heart compensates by beating faster, which is exactly what you see on the watch.
A second force compounds this: when body temperature rises, the body sends a large amount of blood to the skin through cutaneous vasodilation to dissipate heat. Muscles and skin are effectively competing for the same blood supply. The rise in whole body temperature during exercise in the heat leads to a narrowing core-to-skin temperature gradient and a reflex increase in skin blood flow, which contributes to increasing heart rate. So the cardiac drift you see in your data has two culprits working simultaneously: fluid loss and thermoregulation.
Research in trail runners demonstrated that heart rate differences of nearly 20 beats per minute were recorded between well-hydrated and dehydrated participants at the end of a run and after 10 minutes of recovery. That is not a small rounding error, that is a physiologically significant gap that, repeated over weeks of summer training, affects your adaptation and your recovery.
The problem with chugging
Here is where the real nuance lives. Drinking mid-run is not the mistake. The pattern of drinking, gulping large volumes all at once rather than consuming small, regular amounts — is what generates problems, and your heart rate data captures the consequence.
A systematic review with meta-analysis found that, for a hydration protocol carried out before, during and after exercise, heart rate values during exercise were lower compared to controls by around 6 beats per minute. The key phrase is “before, during and after”, a consistent, distributed strategy, not a single large intake. When running in hot conditions with increased perspiration, fluids should be taken at regular 15–20 minute intervals. Chugging a large amount infrequently does not replicate this effect on plasma volume — it can cause gastrointestinal discomfort, sloshes around in the stomach, and may even temporarily spike blood pressure before being absorbed.
There is another layer to this that receives far too little attention: overhydration. Drinking too much water dilutes your blood and decreases electrolytes in your body, especially sodium, causing water to move into your body’s cells and making them swell. This condition, hyponatremia, is the silent risk on the other side of the equation. A study on runners completing the 2002 Boston Marathon found that thirteen percent finished with hyponatremia, and the strongest predictor was weight gain while racing, a sign of over-hydration. The runners who chugged the most were, paradoxically, at higher risk than those who drank too little.
The symptoms of overhydration are similar to, and can be easily mistaken for, dehydration, which leads people to drink more water, worsening their odds. Your heart rate data, in this scenario, may look superficially elevated not because you are dry, but because your electrolyte balance is compromised.
How to actually hydrate on a warm run
The practical shift is not dramatic, but it is specific. Start by arriving at your run already hydrated. Drinking around 6 mL of water per kilogram of body mass in the 2–3 hours before training in the heat is a widely cited general recommendation. This gives your kidneys time to equilibrate and means you begin with adequate plasma volume, a much stronger position than trying to rescue the situation halfway through.
During the run itself, small and regular beats large and infrequent every time. Drinking enough fluids helps maintain concentration and performance, increase endurance, and prevent excessive elevations in heart rate and body temperature. The goal is not to suppress thirst completely or to drink on a rigid clock; it is to keep fluid intake steady enough that plasma volume is never dramatically depleted. Athletes broadly use either programmed fluid intake, which prescribes fluid volumes based on estimated sweat rate, or thirst-driven fluid intake, which relies on internal cues. For most recreational runners out for 30–60 minutes, listening to thirst while sipping regularly, rather than waiting until you feel parched and then gulping, is a practical middle ground.
For longer efforts or particularly warm days, plain water has a limitation. Some athletes use sports drinks containing electrolytes and carbohydrates, and these may be useful if activity is moderate to vigorous in intensity for more than 60 minutes. The electrolyte component matters: sodium, in particular, helps retain fluid in the circulation and reduces the risk of dilutional hyponatremia that comes with large plain-water intake over time.
After the run, a simple habit borrowed from serious endurance sport is worth adopting: weigh yourself before and after. The difference between your pre- and post-exercise weight tells you how much water you have lost through sweat, giving you a precise target for recovery rather than a guess. Even mild post-exercise dehydration of around 1% of body weight increases resting heart rate, so the recovery window matters as much as the run itself.
One detail that consistently surprises runners: with a proper hydration protocol, the return to a more complex, parasympathetic heart rate state occurred significantly sooner during recovery, within the first 5 to 20 minutes, whereas without adequate hydration, some markers took the full 60-minute recovery period to return to resting values. That gap, invisible to most people, is the difference between arriving at your next session genuinely recovered or quietly depleted. The heart rate monitor was not delivering a verdict on fitness. It was flagging a plumbing problem, one that a steadier hand on the water bottle can largely fix.
As always, if you experience unusual symptoms during exercise, dizziness, confusion, or extreme fatigue, stop and seek medical attention. Consult your GP before making significant changes to your training routine, particularly if you have an underlying health condition.
Sources : pmc.ncbi.nlm.nih.gov | frontiersin.org