1. Exercise physiology diet & nutrition
2.1.3 Hydration & Electrolyte Balance
Learning Objective:
To understand electrolytes, hypotonic, hypertonic and isotonic solutions and their importance in maintaining hydration and performance

2. Fluid loss during exercise
How much fluid we lose during exercise is dependent on a few things:
Duration and intensity of exercise.
Temperature and humidity of the environment.
Body size and fitness levels.
Around 60% of our body mass is water.
At rest we lose some water through urine and evaporation (sweat) but it is easily replaced through regularly sipping water.

3. Fluid loss during exercise
During exercise, as our body works harder to meet the increased demands, we produce increased amounts of water and heat.
To prevent overheating our body transports water to our skin where it is evaporated (sweat) to maintain optimal body temperature.
This difference is even greater in hot conditions when the body has to sweat more in an effort to keep us cool.
There is also increased water loss during expiration as our breathing rate is higher when we exercise.
At the same time our kidneys decrease urine flow in an attempt to retain some water.
If the amount of water lost is greater than the amount taken in or retained there is a net loss of water (dehydration).

4. Fluid loss during exercise
The loss of water increases osmotic pressure as electrolytes become more concentrated in body fluids.
The electrolyte concentration can only return to normal by replenishing water content.
Thirst is a reactive mechanism – it kicks in when we’re already experiencing dehydration – so water should be consumed before you feel thirsty.
In high intensity, long duration exercise (e.g marathon) individuals can lose 6-10% of water content, which has to be replaced.
Individuals can expect to lose 1 litre in an hour’s exercise, possibly up to 2 litres in hot, humid conditions.

5. Water – vital for the body because:
It regulates body temperature.
It carries nutrients and oxygen to all cells in the body via blood plasma.
Helps convert food into energy and absorb nutrients.
It is required for expiration.
It removes waste.
It protects and cushions vital organs.

6. Problems of Dehydration
Water loss causes thickening of blood (reduces blood flow, increases HR).
This effects the body’s ability to cool itself (vasodilation becomes difficult).
Rises in core body temp can lead to heat stress, collapse, and death.
Fluid loss above 3-5% of bodyweight reduces aerobic performance, impairs reaction time, affects judgement, concentration and decision making.

7. Problems of Electrolyte Loss
Caused by sweating (and/or poor diet).
Leads to drowsiness and poor decision making.
Causes muscle weakness and fatigue.
Causes muscle cramps.
Can result in abnormal heart rhythm.
As electrolytes are dissolved in water any change to water levels will effect electrolyte levels and so the two are inter-related.

8. A summary to this point!

9. Sports Drinks
There is an important term you need to understand before the role of different sports drinks can be clear.
BBC Panarama – Gatorade v Water
Hypotonic sports drinks have a lower osmolality than the blood (4% glucose concentration). They are absorbed faster than water so replenishment is quicker.
Isotonic sports drinks match blood (5-7% salt and glucose) concentrations. They are absorbed at the same rate as water but also provide energy (glucose) and replace salts lost through sweating.
Hypertonic sports drinks contain higher concentrations of glucose (up to 20%) so they are a great source of energy. This can be useful before exercise or to replenish stores during recovery. However, due to their higher osmolality than water, they take a lot longer to absorb than water so they must be consumed minutes before exercise.
The difference between hypotonic, isotonic and hypertonic fluids (youtube)
Website link

10. Define the terms isotonic, hypertonic and hypotonic in the context of sports drinks. (3)
Isotonic is when the glucose osmolality of the drink is the same as blood (1).
Hypertonic is when the glucose osmolality of the drink is greater than the blood (1).
Hypotonic is when the glucose osmolality of the drink is lower than the blood (1).

11. Sports Drinks
There are 3 types of sports drink: hypotonic, isotonic and hypertonic. These terms refer to the percentage of glucose in the drink. Having said this, sports drinks may also contain electrolytes, sweeteners, protein etc. In which situations would we want to make use of each of the 3 types of sports drinks?

12. Hypotonic – The Thirst Quencher
Lowest glucose osmality (concentration). Lower glucose percentage than blood.
Emptied into stomach quickest so good for quick rehydration and quick energy release
Small amount of glucose means a large amount would need to be consumed to replace energy stores
Therefore good for sports at low intensities or those which require quick energy but not in high quantities. Examples of sport include gymnastics
Excellent for rehydration and getting electrolytes into the body but if used in sports such as marathon running you would need to take alongside another energy source.

13. Hypertonic – The Energy Booster
Highest glucose osmality (concentration). Higher glucose percentage than blood.
Emptied into stomach slowly so not good for rehydration and could cause stomach problems.
Good for post-workout replenishment of energy levels.
Potentially good for events such as ultra-marathon with high energy demands
Could be used alongside another drink to ensure rehydration.

14. Isotonic Similar glucose percentage than blood.
Fluid taken in at about the same rate as water
Preferred choice for most athletes including middle/long distance running and team sports due to the balance of energy supply, speed of energy supply and rehydration capacity.

15. Water compared to sports drinks
A very good option for rehydration, although not as quick as with hypotonic drinks)
Perfectly adequate for most recreational sport (especially if it is less than 1 hour)
However, excessive water intake can cause inbalance in electrolyte levels (fluids from sweat are replaced but salts aren’t)
Also provide no energy boost so would need to be used alongside an energy source
Can cause bloating, increased urination and lacks the taste of sports drinks (so you won’t drink so much)

16. Assess the merits of hypotonic drinks for a marathon runner (8)
Reward acceptable answers. Responses may include, but are not limited to the following. • Hypotonic drinks have the lowest glucose osmality (AO2). • They will be emptied from the stomach the quickest (AO2). • The greater the glucose osmality of a sports drink, the slower the gastric emptying and the greater the likelihood of a stomach upset (AO3). • Hypotonic drinks provide the fastest glucose benefit for the marathon runner (AO2). • A large volume of hypotonic drinks would have to be consumed in order to obtain a significant amount of glucose (AO3). • These drinks are most beneficial when working at a low intensity, requiring quick energy but not in great quantities (AO3). • Hypotonic drinks offer optimal hydration (AO2).