Peta Bee on how much water you actually need to drink while exercising

I have been pedalling hard on a stationary bike for 40 minutes. My thighs are burning and my calf muscles throbbing while sports scientists probe the hi-tech sticking plasters dotted around my body that are designed to assess the quality and quantity of my sweat. Eventually, I am peeled from my saddle to be weighed by Dr Susan Shirreffs, a world-renowned expert in fluid research from Loughborough University’s school of sport and exercise science. What might be expected for someone like me – reasonably fit but far from athletic – is that I would be a pound or two lighter than when I started due to the fluid my body expelled during the torturous workout. In hot conditions, it is possible to lose 5lb-8lb in fluid but the scales aren’t registering any weight loss in me.

Sweat analysis tests like this are now de rigueur for top athletes. All of Britain’s Olympians have undergone a similar assessment prior to Beijing, and footballers from Premiership clubs view them as an essential part of their preparation. According to Shirreffs, the aim of such tests is to determine the rate of fluid lost through sweat so that sports people, and even Joe Joggers like me, can accurately pinpoint the amount they need to drink during exercise to avoid dehydration. “No two people have the same sweat rate,” Shirreffs says. “It can vary as much among top athletes as the rest of the population.”

My results, which showed that I lost a paltry 315ml an hour of sweat – compared with the 1-2 litres of fluid shed by some people – and a tiny amount of salt (1g in total) suggest I am in the camp that can physiologically cope with drinking very little. Perhaps I was particularly well-hydrated before beginning the trial, it was suggested. But, no, I had run for 30 minutes at lunchtime and drunk only two black coffees and an orange juice before taking up the sweat challenge. Not that the revelations come as a complete surprise.

I have run several times a week since I was 12 and have never needed to carry a water bottle. Only when I have plodded around marathons have I required extra fluid and carbohydrate to keep me going in the later stages.

Periodically my inability to drink and exercise simultaneously (I invariably get a side stitch) has caused me concern because so much emphasis is placed on avoiding dehydration. Yet now there is proof that I never needed much. I wonder how many others are swallowing more than their bodies require? “The majority of people will need up to 500ml of fluid per hour after the first 45 minutes of intense activity,” Shirreffs says. “For those who are prone to getting dehydrated, not taking a drink could be disastrous.” But, she adds, there is a simple way to tell if you are one of them. Weigh yourself before and after exercise. For every pound you lose, you need to drink around two 10 fl oz glasses (around half a litre) of fluid.

Checking the quality and quantity of your urine can help to tell if you are adequately replacing water loss. Dark and scanty generally suggests it is concentrated with metabolic waste and you need to drink more, although your urine may be darker if you take vitamin supplements (especially vitamin C), so volume is often considered a better indicator.

Staying well-hydrated undoubtedly affects sport performance, and isotonic sports drinks, containing tiny particles of easily-digested carbohydrate that enhance fluid uptake in the gut, are certainly effective. “If the average adult loses 3-4lb of fluid, their performance is seriously impaired,” says Louise Sutton, principal lecturer in health and exercise science at Leeds Metropolitan University. “If they lose 7lb, which is possible in the heat, they are likely to get cramps, nausea and experience a 20-30% drop in endurance capacity.”

However, in recent years sports scientists have discovered that it is just as risky to drink too much during exercise. Indeed, in many endurance activities, hyponatraemia – or fluid intoxication – is more prevalent than dehydration. Caused by sodium levels and other body salts (or electrolytes) becoming dangerously dilute, hyponatraemia can result in dizziness, vomiting, respiratory problems and fatigue. “During intense or prolonged exercise, the kidneys are unable to excrete fluid as efficiently as normal,” Sutton says. “In extreme cases, water is retained, especially in highly absorbent brain cells, and the pressure causes the body to shut down its primary functions, such as breathing and heart rate. Treatment involves a small volume of highly concentrated salt solution. But it can be fatal.”

After the 2003 London Marathon, 14 of the runners taken to hospital had hyponatraemia, and a study by Harvard University researchers found that 13% of competitors in the Boston marathon drank enough to cause fluid toxicity. And despite what bottled water and sports drink manufacturers (sports drinks are as likely to cause water toxicity as water) would have us believe, many top athletes drink only small amounts. According to Dr Dan Tunstall-Pedoe, the emeritus medical director of the London marathon, “it’s surprising how little elite runners do drink … they are able to run 26.2 miles at speed with very little fluid on board.”

In the British Journal of Sports Medicine, Professor Tim Noakes, from the University of Cape Town and the leading researcher into exercise hyponatraemia, criticised the sports drink industry for positioning their products to the exercising public as “a medicine that must be ingested to prevent heat illness and optimise sports performance. I believe that the body is adapted for conditions of mild dehydration.

“We evolved from hunters – we had to run and chase animals on the hot African plains. We didn’t have time to pause for a drink,” he says. “Physiologists developed an unproven hypothesis that to become even the slightest bit dehydrated during exercise would kill you. The sports drinks industry then used this bad science to market their products.” Runners have died from hyponatraemia, but Noakes says he “has yet to find a death from dehydration in the history of competitive running”.

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