Water is essential for health and fitness, training comfort, and athletic performance. The catch is that hydration advice online often leans on dramatic stats and ultra-specific drinking rules that don’t hold up across different people, sports, and conditions.
This version is written for Australian fitness and personal training contexts. It focuses on essential hydration tips you can teach clients: how to stay hydrated, how to monitor hydration status without getting clinical, and how to replenish what’s lost through sweat.
Key takeaways
- Many gym-goers start a training session under-hydrated, especially in hot weather, early mornings, or when they’ve been busy and not drinking much water throughout the day.
- Around ~2% body weight loss from fluid can measurably affect performance for many people—most consistently endurance performance, particularly in the heat.
- Hydration and fluid balance affect heart rate, body temperature control, and perceived exertion, so intense workouts can feel harder than they need to.
- For most sessions under ~60 minutes in mild conditions, arriving reasonably hydrated matters more than following strict sip schedules.
- Both extremes can cause problems: clients can dehydrate, and clients can drink too much. For prolonged exercise and long events, avoid noticeable weight gain during exercise unless advised by a clinician.
Why proper hydration matters for performance
Water helps support blood volume, sweat-based cooling, and normal kidney regulation of fluid balance (including urine output). When a client is under-hydrated, cardiovascular strain tends to rise, body temperature control gets harder, and perceived effort increases.
As a rough average, men are ~60% water and women ~50–55% (varies with age and body composition). That’s one reason the amount of water someone needs to drink can shift quickly with heat, activity level, sweat rate, and training volume.
The relationship between hydration and training results (safe, coachable claims)
- Small fluid deficits can matter. For many people, around ~2% body mass loss from fluid can impair endurance performance, especially in the heat.
- The bigger the deficit, the bigger the hit to pacing, comfort, and heat tolerance. Exact percentages and “performance drop” numbers vary widely by task and conditions, so avoid hard claims unless you’re citing a specific study.
A practical “dehydration” check without getting clinical
Personal trainers shouldn’t diagnose medical conditions. Stick to education and simple monitoring.
Simple self-checks clients can use
- Urine colour (trend, not perfection): pale yellow often indicates adequate hydration; darker urine can suggest lower hydration and be a cue to increase fluids. (Note: some supplements, especially B vitamins, can brighten urine colour.)
- Body weight change (for heavy sweaters): weigh before and after a session to estimate what’s lost through sweat.
- How the session feels: unusually high perceived effort, headaches, or feeling overheated can be a cue to review hydration strategies.
Cramps: keep the message accurate
Muscle cramps are multifactorial. Fatigue is a major driver. Fluid and electrolytes can contribute for some people, so hydration is worth checking, but it’s rarely the only cause. If a client reports frequent muscle cramps, treat it as a coaching prompt to review sleep, load, and hydration and fluid habits, and refer out if symptoms are persistent or concerning.
Understanding individual hydration needs
Generic hydration advice doesn’t fit everyone.
The biggest drivers are:
- Body size and body weight
- Sweat rate (varies a lot between people and conditions)
- Session duration and intensity
- Heat and humidity
- Clothing, indoor airflow, and acclimation
Sweat-rate method (simple and useful)
- Weigh the client before training (minimal clothing, after bathroom).
- Track how much water they drink during the session.
- Weigh after training.
Example:
- 80.0 kg before
- 79.2 kg after
- 0.5 L consumed
Estimated fluid loss:
- 0.8 kg body mass loss ≈ 0.8 L
- plus 0.5 L consumed
- total ≈ 1.3 L lost through sweat (and other exercise losses)
Use this to guide post-session rehydration and to set realistic “during-session” sipping habits.
How much water should I drink? Use ranges, not rigid rules
Clients will ask “how much water should I drink?” Give them a starting point, then adjust based on sweat rate, fitness level, climate, and how they feel during a training session.
Daily baseline (as a starting range)
A common starting range is 30–35 ml/kg/day from all fluids. Some of that can come from food with high water content, which can contribute to your overall hydration.
If a client prefers a simpler heuristic, “glasses of water” can be a useful prompt, but keep it framed as a starting point rather than a rule.
Before training
For longer or hotter sessions, many people do well with:
- ~400–600 ml of water 2–3 hours before exercise
- ~200–300 ml closer to training if needed (and tolerated)
For short sessions, the main goal is simply arriving reasonably hydrated.
During training
- Under ~60 minutes (mild conditions): many people don’t need much water intake during the session if they arrive hydrated.
- Longer, hotter, or heavy-sweat sessions: encourage regular sips. A commonly tolerated range is ~150–250 ml every 15–20 minutes, adjusted to sweat rate and gut comfort.
- If a client feels sloshy or bloated, or is urinating excessively, scale back and reassess.
Avoid prescribing a single “everyone must drink X every Y minutes” rule. The water you need to drink depends on the person and the conditions.
After training
If they’ve clearly sweated a lot, a useful guideline is to replace ~125–150% of the fluid lost over the next few hours. This is most relevant when they need to drink enough water to recover quickly (for example, training again soon).
Practical version:
- If they lost ~1.0 kg across a session, aim for ~1.25–1.5 L of fluids above baseline over the next few hours, split into smaller drinks.
Water or a sports drink? Keep it simple
When plain water is usually enough
- Most gym sessions under ~60 minutes
- Low to moderate intensity
- Weight-loss focused training where extra liquid calories don’t help
When a sports drink (or sodium-containing option) can help
- Long sessions (often 90+ minutes)
- Hot/humid training with heavy sweating
- Multiple sessions close together
Keep the advice label-based:
- “Choose a drink with sodium (check the label).”
Sugar content and common drink choices
Clients often ask about sugar content.
A simple coaching line is:
- “If the session is short, water is key. If it’s long and sweaty, a sports drink can help. If it’s mostly sugar, it’s usually not the best fit for most goals.”
Coconut water and other options
Coconut water can be a preference-based option for some people. It still counts as fluid, but it may not match sodium losses for heavy sweaters. Pair it with food (or a salty snack) if needed.
Juice, soft drink, cordial
These still provide fluid, but they’re often not ideal during training:
- high sugar
- low sodium
- can cause GI upset
Common mistakes to coach (without fear-mongering)
- Waiting until you’re thirsty: thirst is useful, but it can lag behind needs during exercise, especially in heat. Encourage clients to drink water regularly and to sip regularly throughout the day.
- Over-correcting: some clients try to “catch up” by chugging. That can cause stomach discomfort, and in long events, large volumes of plain water can raise hyponatremia risk.
Overhydration: the simple safety message
Exercise-associated hyponatremia exists, especially in long events when people drink large volumes of plain water.
PT-safe guidance:
- In long events, avoid noticeable weight gain during exercise unless advised by a clinician.
- Symptoms overlap with dehydration. If symptoms are severe, unusual, or worsening, stop exercise and seek medical help.
What to teach in a session (quick scripts)
- “Bring a water bottle and take a few sips during breaks.”
- “If today’s hot and you sweat a lot, plan to sip regularly rather than waiting for thirst.”
- “Check your morning urine colour trend. If it’s consistently dark, drink more water earlier in the day.”
- “If you’re training again tomorrow morning, take post-session rehydration seriously and replenish what you lost through sweat.”
Scope note for Australian personal trainers (Certificate III/IV)
This is general education for Australian personal training (including Certificate III and IV in Fitness), not medical advice. For clients with medical conditions (kidney disease, heart failure, hyponatremia history, pregnancy complications, eating disorder history, etc.), refer to an appropriate clinician for individualised guidance.