James Whitaker
Start With Your Sweat Rate, Not Your Thirst
Thirst is a lagging indicator. By the time your brain nudges you towards a water bottle, you may already be 1–2% down on body mass, which is the point at which perceived effort climbs, coordination dips and strength endurance starts to slide. For personal trainers working with clients, and for active adults training on their own, relying on thirst alone is the single most common hydration mistake.
The fix is simple and unglamorous: measure it. Weigh yourself in minimal clothing before a session and again afterwards, towel off first, and note how much you drank. Every kilogram lost equals roughly one litre of fluid. Add back what you drank and you have your sweat loss for that session. A 70kg client who drops 1.2kg across a 75-minute spin class while sipping 500ml has actually lost around 1.7 litres — a rate of about 1.35 litres per hour. That number is far more useful than any generic guideline.
Low-Intensity or Short Sessions: Keep It Simple
For easy sessions under about 45 minutes — a steady walk, a mobility class, light resistance work — water is perfectly adequate. Aim for 300–500ml in the hour beforehand and sip 150–250ml every 20 minutes if conditions are warm. There is no need to reach for electrolytes here, and there is no performance benefit in forcing extra fluid down.
- Plain water is enough for the vast majority of short, low-intensity work.
- Avoid drinking more than you are losing; over-hydration dilutes blood sodium and can cause bloating, nausea and headaches.
- If a client arrives having had nothing since morning coffee, a pint of water before the warm-up is a sensible reset.
Moderate Intensity, 45–90 Minutes: Build a Rhythm
This is where steady sipping beats heroic gulping. Aim for 400–800ml per hour, taken in small amounts every 15–20 minutes rather than a large volume at the halfway point. A client doing a 60-minute circuit or a brisk outdoor run in British spring weather will typically sit at the lower end; the same session in a warm studio pushes towards the upper end.
Electrolytes become worthwhile if the session is sweaty, if kit is visibly soaked, or if the client is a heavy, salty sweater. Look for sodium in the region of 300–700mg per litre. For most recreational sessions, a small pinch of salt in a squash bottle does the same job as a branded product.
High Intensity and Sessions Over 90 Minutes
Once you pass the 90-minute mark, or work at genuinely high intensity — repeated sprint intervals, long metcons, endurance rides — fluid alone starts to fall short. Carbohydrate and sodium both matter.
- Volume: 600–1000ml per hour, adjusted to your measured sweat rate and the temperature.
- Carbohydrate: a 4–8% solution (roughly 40–80g per litre) supports blood glucose without slowing gastric emptying.
- Sodium: 300–700mg per litre helps maintain plasma volume and reduces cramp risk in salty sweaters.
- Practicality: set a timer. Sipping 150ml every 15 minutes is far easier to tolerate than 600ml every hour.
For personal trainers running back-to-back sessions, your own day is often a four-hour endurance event. Treat it that way, with a bottle you actually enjoy drinking from.
Heat, Humidity and Indoor Environments
Temperature and humidity change the maths more than most people expect. Above roughly 25°C, sweat rates can double, and high humidity reduces evaporation so you sweat more for the same cooling effect. Heated studios, packed classes and poorly ventilated gyms can feel hotter than the forecast suggests — a spin studio at 28°C is a different hydration problem from a cool outdoor run.
Cold weather is not a free pass either. Respiratory fluid losses rise, cold-induced diuresis increases urine output, and people simply feel less thirsty. Winter training often produces quiet, creeping dehydration.
A Practical Routine Before, During and After
- Before: 5–7ml per kilogram of body mass 2–4 hours out — around 400–500ml for a 70kg adult. Check urine colour: pale straw is the target, dark amber means start earlier.
- During: begin sipping from the 20-minute mark rather than waiting for the halfway point.
- After: replace 125–150% of the weight lost over the following two to four hours, with sodium included, since plain water alone drives urination and slows rehydration.
- Include: tea, coffee, milk and watery foods such as fruit, soup and yoghurt. They all contribute meaningfully to daily fluid intake.
- Watch for: persistent headaches, dark urine, unusual cramping, or a morning body mass that is consistently lower than baseline — all signs the daily total, not just the session, needs attention.
Hydration is not a single decision made mid-workout. It is a habit built around measured sweat rates, sensible electrolytes and a bottle within arm's reach. Get the fundamentals right and clients will notice it in their energy long before they notice it on the scales.
