ONE of the world's biggest long-distance running races is fast approaching (no pun intended), the Virgin Money London Marathon. On Sunday 13 April around 35,000 runners of all abilities will start the 26.2-mile race in Greenwich Park ("sarf" of the river) and finish on The Mall, with the winner expect to arrive in just over two hours. If you're in marathon training or pounding the streets for any other race, even if it's a 5k run, then this article from Runners World will help keep you on track about hydration. No one doubts that proper hydration is necessary for optimal functioning. But a debate has raged in recent years about how to determine when you need to drink, and there are at least 13 hydration-assessment methods. The result is more confusion than clarity. Two camps have received much publicity in running circles. On one side, precision-minded scientists advise you to weigh yourself before and after workouts to determine your sweat loss, and hence your hydration need. On the other side are those who simply say _let thirst be your guide_. A new paper from a team of heat-and-hydration experts at the University of Connecticut concludes that morning thirst is an accurate indicator of two per cent dehydration, which is considered the point at which endurance performance begins to deteriorate. According to head researcher Prof Lawrence Armstrong it is the first study to show that thirst is a reliable dehydration guide, at least in the morning, upon waking. It_s important to note that the study was not an exercise study. The 29 subjects were all young, college-age men who worked out several times a week, but were not training for any specific purpose. They were examined twice: one morning in their normal hydrated state, and another after a 19-hour period in which they were instructed to eat their normal foods but to drink nothing. Both mornings they were asked to complete a dehydration scale from one to nine, with higher numbers indicating more thirst. When the subjects didn_t drink, they lost about two per cent of their normal weight. It also caused subjects to rate their thirst as a _seven_, equivalent to _very thirsty._ On mornings when they were normally hydrated, they chose _two_ as their thirst rating. Armstrong, a lifelong runner, says that runners can use morning thirst along with urine colour and daily body weight fluctuations to monitor their hydration status. He advises that urine colour should be checked every time a runner voids. _You can do it multiple times a day. Sensing your morning thirst when you wake up gives you another tool,_ he said. The research team also measured urine volume of the hydrated or dehydrated runners after they drank varying amounts of water. The dehydrated runners always produced less volume.