Maintaining a good fluid balance in the body is important since water is used in numerous metabolic processes including energy production, electrolyte balance, temperature regulation and detoxification pathways. It’s also essential for cognitive function.
Water stored in the body is categorised as two different fluid compartments; “intracellular” (fluid inside the cells) and “extracellular” e.g. blood and blood plasma, the spaces in between cells, the lymphatic system, saliva, fluid in the eye, glandular secretion, spinal cord fluid and fluid excreted from the kidneys and skin.
Our kidneys are responsible for controlling water balance within the body as well as acting as a filler; removing waste products and reabsorbing things we need from our blood. We excrete around 1-1.5 litres a day though the kidneys, which when added to sweat and water vapour losses through breathing equates to roughly 2-2.5 litres per day. Therefore, it’s important to drink around this much daily. Bear in mind that these figures are before any exercise or environmental conditions are taken into account, which could see this demand increase.
Dehydration through exercise will affect performance since as you become dehydrated your body will work harder to retain the water it has left. This can reduce your sweat rate, which will limit your ability to regulate temperature. As water makes up around 55% of your blood (and 92% of blood plasma), When you’re dehydrated your total blood volume can be severely reduced. This means you have less total volume to pump around to your muscles, and your blood become thicker, meaning your heart has to work harder to ensure adequate oxygen supply to the working muscles. You can often see this as a more than normal increase in heart rate towards the end of an endurance event. (Note, a slight increase in average HR is normal as a result of natural fatigue, and reduced muscular efficiency).
Dehydration can have a knock-on effect in endurance events, long enough to require refuelling, since with even as little as 3% loss in body mass through water loss will significantly slow down gastric emptying rate (the rate at which food is emptied from the stomach). This means that’s as an athlete in a dehydrated state tries to refuel, it will take longer for that fuel to reach the working muscles. Linked to this slower rate of emptying, is the increased gastric distress that may occur as a result of more food and liquid being retained in the stomach.
Electrolytes are minerals (e.g. magnesium, calcium & sodium) that our body uses for hundreds of processes in the body, and are involved in muscle contractions, control of ph levels and the amount of water in your blood.
If electrolyte levels fall, through sweat, and not enough are replaced from drinking, your body will not only absorb water more slowly, but less will actually reach the working muscles. When water is taken with electrolytes, it is absorbed much more quickly since the higher concentration of electrolytes in the drink will naturally pass into the areas of weaker concentration within the body.
Drinking too much can be worse than not drinking enough! Hyponatremia is caused though a relative extreme low electrolyte imbalance, through replacement of too much water (hyper hydration), and can have serious health consequences.
Through training and experience, most of us should have a rough idea of how much water we drink on a long run, or from previous runs, and be able to extrapolate that over a longer distance. Use this as a starting amount, and then know where the aid stations are and take small additional sips as needed depending on the weather and how you’re feeling.