. If your BMI drops below 18.5, you're at risk of becoming weaker and slower. WEIGHT LOST 5K 10K HALF-MARATHON MARATHON 900g 12.4 secs 25 secs 52 secs 1:45 2.3kg 31 secs 1:02 2:11 4:22 4.5kg

of Sports Medicine, which calls for 590-1180ml per hour.For the first time in its 107-year history, the Boston Marathon this spring provided all 20,000 runners with a fold-out pamphlet from the American Running Association and the American Medical Athletic

. "Reducing calorie consumption by just 100 calories a day will theoretically give you a 4.5kg weight loss by the end of the year," Clark says. "Drop 200 calories a day, and you'll lose 9kg (around one and a half stone)." Clark suggests cutting calories

medicine groups, which have generally advised endurance athletes to drink 600ml to 1,200ml per hour. This is more or less where things stood before the NEJM publication last spring. A trio of triggersAfter the NEJM article appeared on 14 April, I waited a

of the participants running a 62-mile race consumed 14,000 millilitres of liquid; another who ran only 26 miles, drank only 280 millilitres. The latter runner, not surprisingly, showed the most loss of vascular fluid, resulting in thickened blood plasma and impaired

1.041 50 1.128 60 1.222 70 1.324 80 1.494 Click here to use Ray Fair's age-and-pace calculator