I’ve heard that during endurance races water stops rehydrating you after 40 minutes. Is this true?
Simon Carter, by email

Water won’t stop hydrating you if you drink it during a race, but the rate of absorption may slow down. This can occur if you drink a large volume of water in one go instead of small regular sips or if you drink it ice-cold (warmer water may not seem as refreshing, but it gets into your bloodstream much faster).

If you feel you need to hydrate better, try an isotonic drink instead of water. These contain small amounts of carbohydrate and electrolytes, which maximise the rate of absorption. Taking on fluid is an important part of race training, so experiment with different brands and get used
to drinking while exercising.