Water Retention after a Long Run

I have an odd issue with water retention after a long run or a race, that I don't understand. I have run seven half marathons in the past three years and I have noticed that I bloat up really badly after the race (gain 3-4 lbs of water). Is this a problem? Should I be concerned?

I use gels, and electrolyte replacement drinks during the race and drink lots of water, it also does not matter if it is a cool or hot day, happens to me regardless of weather.

What could be causing this?

It does not happen to me on shorter runs; in fact I lose water and am lighter after a short run. I just ran a half on Sunday and today is Wednesday and I am just finally getting back to normal.

– Misty, Saskatoon, Saskatchewan

Answer by Dom: Misty, thanks for such a clear description. I have not experienced it personally. However, I have friends who it has happened to. It has always been temporary and not a massive cause of concern. Your pattern (long races only, back to normal in about three days, any weather) points to a few very ordinary causes, and I'll walk you through them:

1. Why long races make you hold extra water
2. What is normal, and when to get checked
3. Adjusting your drinking and fuelling during the race
4. What to do in the 72 hours afterwards

Why Long Races Make You Hold Extra Water

The biggest reason is glycogen. That is the carbohydrate your muscles store as fuel, and every gram is stored with roughly 3 g of water. A half marathon empties a good part of that store. When you eat and drink afterwards, your body refills it and pulls water in alongside. Refilling mostly brings you back towards your pre-race weight, but it can leave you noticeably heavier on the scale than straight after the finish.

The second reason is muscle damage. Running 21 km leaves tiny tears in your muscle fibres, and your body sends fluid to the area to repair them. That swelling peaks around 48–72 hours after the race. It matches your Sunday-to-Wednesday timeline almost exactly.

Third, sodium. Gels, sports drinks and the salty food most of us eat after a race (pizza, crisps, noodles) all add sodium, and sodium makes you hold water. This explains why you see it after 21 km but not after a 5 km jog. Short runs don't drain your glycogen, damage much muscle or need any fuel at all. Weather doesn't change any of this, so it makes sense that it happens in cool races too.

Hormones can add a little extra water at certain points of the month, so if your race lands in that window, it may feel worse. It isn't the main driver for you.

What Is Normal, and When to Get Checked

A gain of 3–4 lb (roughly 1.4–1.8 kg) for a few days after a half marathon, then settling by day three or four, is within the normal range from what you describe. This isn't fat gain, and it will leave without you doing anything dramatic.

There is one thing I'd want you to rule out, and it matters more than the bloating. If you drink heavily during a long race, you can dilute the sodium in your blood. This is called hyponatraemia (low blood sodium). It is uncommon, but it is the real risk of "drinking lots of water". The warning signs are nausea, a headache, confusion, and marked swelling of your hands and feet.

Here is a simple test. Weigh yourself just before and just after your next long run, and again once or twice in the days after. Losing a little weight during the run is normal. Gaining weight during the race itself is the warning sign, because it means you drank more than you lost. Gaining afterwards is the glycogen and repair process I described above.

See a doctor if the swelling lasts beyond four or five days, appears on one side only, is painful, or comes with breathlessness or unusually little urine. If you have concerns about persistent bloating or swelling, speak with a healthcare professional.

Adjusting Your Drinking and Fuelling during the Race

You told me you drink "lots of water", plus gels and electrolyte drinks. My advice to you is to drink to thirst instead of drinking lots. For most runners that works out at roughly 400–800 ml per hour, with the lower end for smaller runners on cooler days. At a half marathon pace that is a few sips at each aid station, not a full cup every time.

Sodium matters as much as volume. Your electrolyte drink and gels do help here, since electrolytes (sodium, potassium and magnesium) are the salts you lose in sweat. I'd recommend checking the sodium content on the labels, and taking your gels with a mouthful of fluid rather than a big drink of plain water on top. Plain water at every station, on top of gels, is the combination I'd cut first.

Don't swing the other way and under-drink, either. Starting a race properly hydrated and drinking to thirst is the sensible middle. If you want a quick check, a pale straw colour in your urine the morning of the race is about right.

What to Do in the 72 Hours Afterwards

Keep moving, gently. Walk for ten to fifteen minutes after you finish, and again later that day. Then eat normally. Don't cut carbohydrates or skip meals to "get rid of the water", as you need the glycogen refilled, and trying to avoid it just slows your recovery.

Do watch the very salty food on race day, and choose real meals with some fruit and vegetables over a bag of crisps. Elevating your legs helps with swelling: lie on your back and prop your legs up on a pillow or cushion for 15–20 minutes. By the way, this a massive tip for dealing with muscle soreness after a race as well. I try to do it after every race; it makes my legs feel less heavy. Professional football players swear by it!

Compression socks or tights can help with circulation in your legs, and some easy stretching is worth doing once the first stiffness fades. I've put together a page on running stretches for more info. Keep your next few runs short and at easy pace until you feel normal again.

Finally, stay off the scale for three to four days after a race. If the number is making you second-guess your training, it is worth reading about why runners hit a weight-loss plateau, so you can tell temporary water weight apart from an actual stall in fat loss.

Misty, seven half marathons in three years is a great record, and this bloating is a sign your body is doing its repair work. Weigh yourself around your next race, ease off the plain water, and let me know how it goes :)

Kind regards,
Dom

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