Marathon Training: Water & Hydration

The common fact that our bodies are 70% water is almost true. In reality, they can be up to 70%, but there’s a huge variation depending on body composition. For example, muscle contains about 70% water whereas fat contains around 50%; so a muscular person will carry more water than someone who is overweight and thus, the water requirements for those individuals may be different. Other variables include activity and ambient temperature.

An average adult (not exercising) requires around 2-3 litres of water a day. Drinking 3 litres of water a day sounds like a mammoth task, but don’t forget the requirement also includes water found in food, which can vary hugely e.g. a lot of fruit and veg is up to 90% water content. Water is also generated by the various metabolic processes of the body, which can be up to 25% of your daily requirement. With this in mind then, it’s not as difficult as first imagined to stay hydrated, just make sure you top up with a water bottle.

The main loss of water from the body is via the kidneys and bowels, skin and in expired air. The loss from expired air is increased in the cold and at high altitude and reduced in hot humid weather: Just think of those cold winter mornings when you can see your breath. Sweat loss accounts for around 0.5-1 litres a day however this can be increased over ten times in a well-trained person whilst exercising. Spin class anyone? Those bikes don’t wipe down themselves.

Hydrating During a Marathon

Water loss by sweating during exercise is obviously dependent on the type and intensity of the exercise, the surrounding temperature and the humidity. With this in mind, how much water do you need to take on board during a marathon? The amount of fluid you need to drink while training or during an event cannot be prescribed, but there are certain guidelines that can help absorption, e.g. by pre-loading some fluid into the stomach and fluid intake during exercise. The key to better and faster hydration, is dependent on the speed at which fluid is moved out of the stomach and into the small bowel – the quicker the better. This is where most of the water is absorbed into the body (the rest being in the large bowel). There are various factors that inhibit and promote this.

Factors That Affect Gastric Emptying

Can you drink too much water? Hyponatraemia

Drinking too much water can lead to a condition called hyponatraemia (low blood sodium concentration). Sodium is a vital electrolyte that is required for many bodily functions including fluid balance and nerve conduction. Hyponatraemia occurs when drinking more water than the body requires. That, along with the sodium lost in sweat during exercise, can reduce concentration of sodium to dangerous levels, causing swelling of the brain. Symptoms include: anything from headache, nausea, cramping and confusion, to pulmonary oedema (fluid in the lungs), coma and cardiac arrest. The most common occurrence of this is during endurance events, particularly in hot conditions – this is why monitoring your fluid intake during long events like a marathon, is important. The following is a basic guideline to ensure adequate hydration and a reduction in the risk of hyponatraemia:

1. Do not preload the body with liquids prior to an event
2. Prior to training, if you’re thirsty, drink a maximum of 150-300 ml of water 15 minutes prior to starting
3. During exercise, drink to thirst only. I'll say that again: Drink to thirst. The body is very good at telling you when it needs water
4. Avoid drinking commercial "sports drinks"
5. If you need carbohydrate in your liquids, aim for 8% concentration of sugar
6. Avoid turning up to an event without having some idea of your fluid requirements during exercise

So whether you see life as glass half full or glass half empty, just make sure there’s some water in the glass!

This blog is written by friend of Neat, Dr Nick Ambatzis MB BS, MSc (SEM), MRCGP.

Nick is a General Practitioner specialising in Sports and Exercise Medicine. He completed his medical degree at University College London Medical School in 2002. Nick worked for almost ten years as a junior surgeon and spent three years in Trauma & Orthopaedics. He attained a Masters in Sports and Exercise Medicine and subsequently trained as a GP practising in Paddington.

From an early age, Nick has been both a keen cross-country runner and water-polo player, having competed at college level. Nick is also an accomplished ultra-marathon runner, having competed in many cross-country and cross-alpine races, ranging from 50-100 miles. He has also been a Crossfit and Crossfit Endurance coach.