Our focus is hydration or in more simple terms how much to drink.
Your body needs fluids for various functions. Body cells and tissues are filled with fluid, the nervous system requires fluid and the fluid component of your blood (known as plasma) is also affected by your drinking habits. Exercise leads to a loss of body fluids via sweating and breathing and this loss of fluid can eventually lead to what is commonly termed dehydration.
What happens when we drink?
When you put fluids into your stomach, they pass through the stomach wall into your blood vessels and effectively become plasma. As your blood stream can pretty much reach any part of your body, any tissue or any cell, this fluid can be transferred from the blood stream into the tissues or cells.
How does fluid actually pass from one place to another?
To get the fluid from your stomach into your blood stream or from your blood stream into tissue cells requires a process termed ‘osmosis’. Salt acts like a magnet drawing fluid towards it and the concentration of salt in your blood and tissues determines the shift of fluid around your body.
When you take a drink of water it reaches your stomach and waits excitedly to pass through the wall into your blood stream. Your blood is saltier than the water in your stomach and due to the higher level of salt in the blood, the water is drawn from the stomach, through the wall and into the blood. This water effectively becomes blood plasma and travels around your body. If it finds muscle tissue which has a higher salt concentration, the magnetic pull of the salt within the muscle will draw the fluid from the blood into the muscle.
In simple terms, when something is dehydrated, it becomes more salty. By becoming more salty it’s magnetic pull increases in power and it attracts water towards it. That’s how fluid shift and hydration works within the body, that’s ‘osmosis’.
What happens when you dehydrate?
When you dehydrate your tissues and blood have less fluid thereby making them more salty, in the hope that they can attract fluid towards them. Your blood becomes thicker as you still have the same amount of ‘blood cells’ but the fluid component is reduced, thereby making it more concentrated. Not only does the blood become thicker (making flow more difficult), the absolute amount of blood is also reduced so you have to pump the smaller blood volume more quickly around the body, thereby increasing heart rate.
Most text books will recommend somewhere between 1 – 1.5 litres per hour depending upon individual sweat rates, but it is unlikely that this amount can actually be absorbed when you are exercising. As each litre of fluid weight 1kg in weight, it is possible to calculate (very roughly!) fluid loss by taking weight before and after.