You sweat more than you think in hot sessions, and guessing your fluid needs leaves you either underperforming or drinking more than you need.
Berry Limitless Coaching Team · November 2, 2026
Evidence level — General guidance
Practitioner experience and broad consensus rather than a specific trial.
Most hydration advice gives you a number. Eight glasses. Three litres. A bottle every half hour. The problem is that none of those numbers know anything about you.
A review of water and electrolyte needs during exercise makes the point clearly. Sweat losses vary widely between people and between sessions. They depend on how hard you work, how hot and humid it is, what you wear, how big you are, and how well adapted you are to the heat. Two people in the same class can lose very different amounts of fluid.
So the useful question is not "how much water does a person need" but "how much do I lose, and how much of it do I need to put back?"
You do not need a lab. You need scales and a bit of honesty.
Do this two or three times in different conditions, such as a hot morning run and a humid gym session. You will quickly see your own pattern. That pattern is worth more than any generic rule.
A review of fluids and hydration in prolonged endurance exercise notes that performance tends to suffer once fluid losses reach around 2% of body weight, and that the effect is greater in the heat. For a 80 kg person, that is roughly 1.6 kg.
Treat that figure as a rough guide, not a cliff edge. The practical aim is simple: keep losses modest, especially in long or hot sessions. You do not need to replace every drop as you go.
For a short session of under an hour, most people do fine by arriving hydrated and drinking afterwards. For longer sessions in the heat, drinking during the session starts to matter more.
The goal is not to drink as much as possible. The goal is to lose less than would hold you back, and to replace it sensibly.
This gets less attention than it should. The same endurance review highlights the risk of drinking far more than you sweat out, particularly with plain water over long events. Overdrinking can dilute the sodium in your blood, a condition called hyponatraemia. At its mildest it makes you feel bloated and unwell. At its worst it is a medical emergency.
Your scales help here too. If you weigh more after a session than before, you drank more than you needed. Scale it back next time.
Sweat is not just water. You lose sodium as well, and some people lose a lot more than others. If you finish sessions with white streaks on your clothing or a gritty feeling on your skin, you are probably a salty sweater.
The 1999 review on water and electrolytes points out that replacing fluid without the electrolytes lost in sweat is incomplete. In practice:
If you have been told to limit salt for blood pressure or any other reason, do not change your sodium intake without speaking to your doctor first.
One of the more encouraging findings comes from research on heat acclimation. A study in the Journal of Applied Physiology found that a period of repeated training in the heat improved exercise performance, not only in hot conditions but in cooler ones too. Part of the adaptation involves changes in how the body handles fluid and regulates temperature.
The practical lesson is patience. The first hot days of the year will feel harder than they should. That is normal. Build up gradually:
Respecting this process is part of looking after the body you have been given. Pushing through the first week as if nothing has changed is how people end up dizzy, sick or worse.
Most of what we have covered concerns single sessions. It is worth knowing that repeated heat stress and dehydration matter over longer periods too. A review of chronic kidney disease of unknown cause, seen in several hot regions around the world, discusses repeated heat exposure and dehydration among labourers as one of the factors under investigation.
That research concerns heavy manual work in extreme conditions, not your Saturday run. But it is a sober reminder that chronic under-hydration in the heat is not a harmless habit. If you train hard in hot weather and also work outdoors, take daily hydration seriously, not just your session plan.
Hydration is only part of heat safety. Stop training, cool down and get help if you notice:
These are not things to push through.
It is not complicated. It just needs to be based on your body rather than someone else's number.
Adults who train outdoors or in hot gyms, and anyone doing longer sessions in summer heat who wants a practical way to stay hydrated without guesswork.
If you have kidney disease, heart failure, or a doctor-advised fluid or sodium restriction, or you take diuretics, blood pressure medication or other drugs that affect fluid and salt balance, speak to your physician before changing your intake. The same applies if you have a history of low blood sodium.
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Medical disclaimer. This article is for education only and is not medical advice, diagnosis or treatment. Berry Limitless coaches are not physicians. Talk to a qualified healthcare provider before changing your training, nutrition, supplementation or medication — particularly if you are pregnant, managing a health condition, or taking prescription drugs.
