Quick answer
Yes, swimmers get dehydrated: they sweat in the pool, and the water hides it. Weigh your swimmer before and after one practice to learn their sweat loss, then plan around it: a bottle at the wall, sips at every break, and about 1.5 litres of fluid for every kilogram lost, afterwards. Water is enough for most practices; sports drinks are for long, hard sessions, and energy drinks are not for kids.
Halfway through a long meet in Victoria, I noticed Rafa's water bottle was still completely full. It was noon. She had raced twice, warmed up, warmed down, and the bottle had not been touched. When I asked about it she looked at me like I was crazy: "Dad, I'm literally in water all day. Why would I be dehydrated?"
That is the most common hydration myth in swimming, and it is wrong.
Do swimmers really sweat in the pool?
They do. A hard practice raises body temperature just like any other sport, and the body sweats to cool down. The pool washes the sweat away instantly, so swimmers rarely notice, and they rarely feel thirsty until they are already behind.
Many young athletes also start practice behind. In a study of 59 elite teenage male athletes from five sports, including swimming, training in warm weather, 76% arrived at practice already hypohydrated [1]. Starting behind makes it much harder to finish practice in good shape.
The American College of Sports Medicine's position stand on fluid replacement gives a clear target: drink enough during exercise to avoid losing more than 2% of body weight from water loss, because beyond that performance and comfort start to drop [2].
The weigh-in test: how much does your swimmer lose?
Sweat rates vary a lot between kids, pools and sessions, so a generic "drink X ml" rule is a guess. One weigh-in tells you what your swimmer actually needs.
- Before practice: weigh your swimmer in their suit, after a trip to the bathroom.
- Note the bottle: fill it to a known amount (most bottles are marked).
- After practice: towel dry, weigh again in the same suit, before they drink or eat anything else.
- Do the math: sweat loss = weight before − weight after + what they drank.
Example (metric): a 45 kg swimmer weighs 44.6 kg after a two-hour practice and drank 0.5 L. Sweat loss is 0.4 + 0.5 = about 0.9 L. Their 2% limit is 0.9 kg, and they ended 0.4 kg down, so they stayed well inside it. Without the bottle, they would have ended close to the limit.
Example (imperial): a 100 lb swimmer's 2% limit is 2 lb. If they come out of practice 1.5 lb lighter, they are cutting it close and need more at the wall.
Repeat the test once in a hot, humid pool or a longer session. Those are the days when needs jump.
Before, during and after practice
| When | What the ACSM guidance says | What it looks like for a 45 kg (100 lb) swimmer |
|---|---|---|
| Before | Drink slowly about 5–7 ml per kg of body weight at least 4 hours before; if urine is still dark, another 3–5 ml/kg about 2 hours before [2] | A glass (about 225–315 ml) with a meal a few hours out; a smaller top-up if needed |
| During | Drink enough to stay under a 2% body-weight loss [2] | A bottle at the end of the lane and sips at every break, sized by your weigh-in |
| After | About 1.5 L of fluid for every kg lost, spread over the next hours [2] | Down 0.4 kg after practice → about 600 ml on the way home and with dinner |
Two quick checks between weigh-ins: pale, straw-coloured urine is a good sign; dark yellow means catch up. And an unopened bottle at pick-up is the easiest red flag a parent can spot.
What should swimmers drink?
The American Academy of Pediatrics is direct about this for children and teens: water is the main drink for hydration, sports drinks have a limited place during long or intense activity, and energy drinks, with their caffeine and other stimulants, should not be consumed by children or adolescents at all [3].
| Situation | Best choice |
|---|---|
| Normal practice, about an hour or so | Water |
| Long or very intense session, or a hot, humid pool | Water, plus a sports drink the swimmer already knows |
| Meet day, between events | Water, with light snacks |
| Heavy sweaters (big losses on the weigh-in, salt marks on the suit or cap) | Talk with the coach or a sports dietitian about electrolytes |
| Energy drinks, high-caffeine drinks | Never for kids and teens |
Race day is not the day to try a new drink. Anything new should be tested in practice first.
Meet-day hydration plan
Meet days are long: six to eight hours at the pool for a few minutes of racing, often in a warm, humid building. The mistake is waiting for race time to think about fluids.
| Time | What to do |
|---|---|
| The night before | Normal dinner with a drink; pack the bottle with the suits |
| Morning of the meet | A drink with breakfast; check urine colour |
| Warm-up | Bottle on deck, sips between warm-up sets |
| Between events | Small, regular sips instead of chugging right before the race |
| After the last race | Refill and keep drinking on the way home, using the 1.5 L per kg rule if you weighed in |
Pack two bottles for a full-day meet, so a refill line never becomes the excuse. And skip the concession stand's sodas; most of what is sold there is sugar, not hydration. For food on the same day, see what to eat before a swim meet.
If you suspect a long, hot meet day is catching up with your swimmer, the back half of the last races is where it would show. After the meet, sign in and open Times → Splits in Gophin to compare the splits of morning and afternoon swims, then talk it through with the coach before blaming fitness. It is free.
Signs of dehydration to watch for
- Dark yellow urine, or not needing the bathroom for hours
- Headaches during or after practice
- Muscle cramps, especially in the calves or feet
- Unusual fatigue, or a swimmer who fades in the second half of practice
- Dizziness or light-headedness on the blocks
If these show up often, run the weigh-in test before anything else. Persistent symptoms are a conversation for your family doctor.
Build the habit early
Hydration works best when it is automatic: a bottle packed with the suit every day, a drink with breakfast, a bottle finished on the drive home. A well-hydrated swimmer recovers faster between sessions and feels better in the water, and sleep matters just as much for recovery (see how much sleep swimmers need).
Pack the bottle. When the meet is over, sign in to Gophin and open Meets to see every official time from the day once the results are published. It is free.
Sources
- Arnaoutis, G., Kavouras, S.A., et al. (2015). Fluid Balance During Training in Elite Young Athletes of Different Sports. Journal of Strength and Conditioning Research, 29(12), 3447–3452. doi.org/10.1519/JSC.0000000000000400
- American College of Sports Medicine; Sawka, M.N., et al. (2007). Exercise and Fluid Replacement. Position stand. Medicine & Science in Sports & Exercise, 39(2), 377–390. doi.org/10.1249/mss.0b013e31802ca597
- American Academy of Pediatrics, Committee on Nutrition and Council on Sports Medicine and Fitness (2011). Sports Drinks and Energy Drinks for Children and Adolescents: Are They Appropriate? Pediatrics, 127(6), 1182–1189. doi.org/10.1542/peds.2011-0965
