Exercise and Dehydration During a Heat Wave: How to Stay Safe and Perform at Your Best
As UK summers become increasingly warmer, with temperatures regularly exceeding 30°C, more people are exercising in conditions that place greater demands on the body. Whether you're running, cycling, playing tennis, golfing, training in the gym, or simply enjoying a long walk, exercising in extreme heat significantly increases your risk of dehydration.
Dehydration doesn't just make you thirsty—it can reduce sporting performance, increase injury risk, delay recovery, and in severe cases lead to heat exhaustion or heat stroke.
In this guide, we'll explain how heat affects your body, how much water you should drink when temperatures exceed 30°C, and what you can do to exercise safely during a heat wave.
Why Does Exercising in Hot Weather Feel Harder?
Exercise naturally increases your body temperature. To keep you cool, your body produces sweat, which evaporates from your skin and removes heat.
When the air temperature rises above 30°C, this cooling system becomes much less efficient. The body has to work considerably harder by:
Producing more sweat
Increasing heart rate
Sending more blood to the skin for cooling
Working harder to maintain normal body temperature
This means the same workout that feels comfortable at 18–20°C may feel significantly more difficult during a heat wave.
How Much Water Should You Drink Above 30°C?
Hydration requirements vary depending on your body size, fitness level, exercise intensity and how much you sweat. However, sports medicine organisations provide some practical recommendations.
Before Exercise
Drink approximately 5–10 ml of fluid per kilogram of body weight around 2–4 hours before exercise.
For example:
60 kg person: 300–600 ml
70 kg person: 350–700 ml
80 kg person: 400–800 ml
Starting exercise well hydrated is one of the easiest ways to reduce heat-related fatigue.
During Exercise
When temperatures exceed 30°C, aim to drink:
200–300 ml every 15–20 minutes
Approximately 750–1,200 ml per hour
Athletes with high sweat rates may require even more.
If exercising for longer than 60–90 minutes, particularly during running, cycling or team sports, drinks containing electrolytes (especially sodium) can help replace minerals lost in sweat and improve fluid absorption.
After Exercise
Replace both water and electrolytes lost during exercise.
A simple method is to weigh yourself before and after exercise.
For every 1 kg of body weight lost, drink approximately:
1.25–1.5 litres of fluid
This allows for ongoing sweat and urine losses during recovery.
Good recovery drinks include:
Water
Milk
Electrolyte drinks
Smoothies
Water-rich fruits such as watermelon and oranges
How Does Dehydration Affect Sporting Performance?
Research consistently shows that performance begins to decline when dehydration reaches just 2% of body weight.
For someone weighing 70 kg, this is only 1.4 kg (approximately 1.4 litres) of fluid loss.
Even mild dehydration has measurable effects on both physical and mental performance.
1. Reduced Endurance
Your heart has to work harder because there is less circulating blood volume.
This means:
Earlier fatigue
Slower running pace
Reduced cycling power
Shorter exercise duration
2. Reduced Strength and Power
Dehydration can reduce:
Muscle strength
Explosive power
Sprint performance
Jump height
This affects sports such as football, tennis, rugby and gym training.
3. Reduced Concentration
Sport isn't just physical.
Even mild dehydration can impair:
Decision making
Reaction times
Hand-eye coordination
Accuracy
Tactical thinking
This is particularly important in sports requiring quick decisions such as tennis, cricket, football and golf.
4. Increased Risk of Muscle Cramps
Although muscle cramps have several causes, heavy sweating combined with electrolyte losses—particularly sodium—may increase the likelihood of exercise-associated muscle cramps in susceptible individuals.
5. Higher Heart Rate
When dehydrated:
Blood volume falls
Heart rate increases
Exercise feels harder
Many athletes notice they reach higher heart rates despite maintaining the same pace.
6. Poor Recovery
Dehydration slows recovery by reducing:
Nutrient delivery to muscles
Removal of metabolic waste products
Muscle protein synthesis
Glycogen replacement
Remaining hydrated helps your body recover more efficiently between training sessions.
Warning Signs of Dehydration
Watch out for:
Thirst
Dry mouth
Dark yellow urine
Headache
Dizziness
Fatigue
Muscle cramps
Reduced performance
Poor concentration
Feeling unusually hot
If symptoms worsen to confusion, fainting, vomiting or collapse, seek immediate medical attention as these may indicate heat exhaustion or heat stroke.
Tips for Exercising Safely During a Heat Wave
Exercise early or late
Try to train before 9 am or after 7 pm when temperatures are lower.
Reduce intensity
You may need to:
Slow your pace
Lift lighter weights
Reduce training duration
Remember that maintaining fitness is more important than pushing through dangerous conditions.
Wear suitable clothing
Choose:
Lightweight fabrics
Loose-fitting clothing
Breathable materials
Light colours
Plan your route
Choose shaded areas whenever possible and avoid prolonged exposure to direct sunlight.
Don't ignore thirst
Although thirst is not a perfect indicator of hydration, drinking regularly throughout exercise is more effective than waiting until you become very thirsty.
Use electrolytes when appropriate
For prolonged exercise lasting over an hour in temperatures above 30°C, replacing sodium can help maintain hydration and reduce the risk of heat-related illness.
Key Takeaway
Hot weather shouldn't stop you from exercising—but it should change how you exercise.
By starting well hydrated, drinking regularly throughout your activity, reducing exercise intensity when temperatures exceed 30°C, and recognising the early signs of dehydration, you can continue to train safely while protecting both your health and your sporting performance.
Remember, good hydration isn't just about preventing thirst—it helps your muscles work efficiently, supports your heart, improves concentration, speeds recovery, and allows you to perform at your best, even during a heat wave.