How much hydration really matters for your training

You've probably heard that you need to drink more water when you train. That's true, but the useful question is how much, and when it actually changes what you can do in the gym.

How much fluid loss actually matters

Researchers usually talk about dehydration as a percentage of your body weight lost through sweat. Small losses, under about 1 or 2 percent, don't seem to do much. But once you cross that 2 percent line, endurance work starts to suffer. One review found dehydration in excess of 2% of body weight consistently impairs aerobic exercise performance. For a person who weighs 80kg, that's only about 1.6kg of sweat, which is easier to lose than you'd think on a hot day or a long run.

Weightlifting and short, hard efforts are a bit more forgiving. A review looking at strength and anaerobic work found that the critical level of water deficit affecting anaerobic performance is usually bigger than the deficit that hurts endurance, somewhere around 3 to 4 percent of body weight rather than 2 percent. So if you're doing a normal hour in the weight room and you're not sweating heavily, mild dehydration probably isn't the thing standing between you and a better set.

That said, it isn't zero. A small study on trained lifters put people through resistance training sessions at different hydration levels and concluded that hypohydration attenuates resistance exercise performance, with strength, power and endurance all dropping more as the fluid deficit got bigger. It was a small sample, so treat the exact numbers loosely, but the direction is consistent with everything else here: being noticeably dehydrated works against you, even in the weight room.

Mild thirst on a normal day at the gym probably isn't costing you reps. Real dehydration, on a hot day or a long session, probably is.

A simple way to check yourself

You don't need to weigh yourself before and after every workout, though serious athletes sometimes do exactly that to work out their sweat rate. For most people, a much easier check is urine color. Sports nutrition guidance points out that light colored urine is a sign of appropriate hydration, while dark urine is a sign you're behind on fluids. It's not a lab-grade measurement, but it's good enough to catch the big misses.

1. 1

   Drink through the day, not just at the gym Hydration status is built up over hours, not fixed by one bottle right before you train.

2. 2

   Check your urine color before a session Pale, lemonade-like urine suggests you're starting in reasonable shape; dark urine is a sign to drink more first.

3. 3

   Drink to thirst during shorter sessions For a normal hour of lifting or moderate cardio, drinking when you feel thirsty is usually enough.

4. 4

   Plan ahead for long or hot sessions On long runs, hot rooms or double sessions, sweat losses can climb past 2 percent of body weight, so it helps to drink before you start and keep sipping through the session.

Where hydration fits with everything else you eat

Hydration isn't a separate project from your calories and protein, it's part of the same picture of eating for training. If you're cutting calories, you're also often eating less watery food, like fruit, soup and vegetables, so it's worth paying a little more attention to fluids on those days. If you're in a bulk and eating more food overall, you're probably taking in more fluid from food too, without trying.

None of this needs to be complicated. Most people training a normal amount, in a normal climate, are fine drinking water through the day and paying attention when they're thirsty. The times to be more deliberate are hot days, long sessions, or when you notice your urine has gone dark and you haven't had much to drink. If you're dealing with cramping, dizziness or symptoms that feel bigger than normal training fatigue, that's worth raising with a doctor rather than guessing at fluid or electrolyte fixes.