There are 2 factors that determine how well you are able to get fluid to where it's needed during a race - Gastric Emptying (how quickly the fluid leaves the stomach) and Intestinal Absorption (how well the small intestine is able to absorb fluid).

Studies have shown that a larger volume of fluid in the stomach as well as the presence of carbohydrate and electrolytes improves the uptake of fluid.

In regard to carbohydrate concentration, the optimal amount is around 2.5g per litre of water (about 4-5% glucose). Higher concentrations tend to slow the process down, however for endurance athletes the benefits of higher energy replacement may be more preferable.

Interestingly, fructose (sugar from fruits) tends to absorb more slowly and may slow down the uptake of water.

If you can calculate your rate of loss of fluid per hour then you can easily assess your fluid requirements. To do this you should try to simulate your race conditions when you exercise and weigh yourself before and immediately after you complete your training - and with no clothes as they tend to absorb sweat.

Knowing exactly how much fluid you're going to need will help you understand your body more and improve your overall performance

The problem then comes down to organising your equipment and the race plan so you can get the fluid you need. A lot of runners are using running belts with small 100ml bottles - however while this may be OK for training or cross-country or ultra distance events, it can hamper you during a race over a shorter distance.

For this you will have to weight up a risk versus gain strategy and take the time to get the fluid you need. A 'slow down' in a couple of sections to take on more fluid and electrolytes may pay big dividends in terms of completing the race at your best as opposed to slowing down with running cramps.

Note: Thirst is not a good indicator of your level of hydration - by the time you a thirsty your body is signalling a state of dehydration and if you are in a prolonged event, chances are you will not be able to get back 'in front' of your hydration levels. Your chances of suffering a muscle cramp and fatigue will increase.

As an example, I would call myself a recreational runner who occasionally competes in half and full marathons a couple of times per year. When I do this I get much better results when I plan a hydration/carbohydrate strategy.

I know I need about 900 ml per hour and a 4% glucose solution (with natural saccharides as well as protein) sports drink with plenty of electrolytes gives me enough sustained energy to maintain an 80% pace for the whole race.

Overall, I would always recommend a well formulated carbohydrate/electrolyte drink that will increase hydration and reduce the likelihood of leg cramps and muscle cramps caused by exercising over a continuous period in a hot, humid environment.