The scale jumped
overnight. That's water,
not fat.
A 1 to 2kg jump on the scale in a day is almost always water, not fat. Gaining a kilo of fat takes roughly 7,700 surplus calories, which nobody eats by accident overnight. What moved is water, pulled in by two things on your plate: salt and carbohydrate. Salt-driven water usually settles within a day or two of your sodium coming back down, and the NHS caps adults at 6g of salt a day. Here is what pulls water in, how to tell it from fat, how fast it settles, and the signs that mean a GP should look instead.
Free download Tracks salt vs the NHS 6g cap iPhone
Why did I gain
weight overnight?
You can't gain a kilo of fat in a day, that would take roughly 7,700 surplus calories. What moved is water, and two things on your plate pull it in.
Salt holds water
Sodium makes your body hold extra water to keep blood concentration steady. A salty takeaway, a tin of soup or a sharing bag of crisps can park 1 to 2kg of water for a day or two. The NHS caps adults at 6g of salt a day, and most of us are over it.
Carbs hold water too
Your body stores carbohydrate as glycogen, about 500g of it across muscle and liver, and each gram drags roughly 3g of water along, so around 1.5kg of water is tied up in your carb stores. A big pasta or rice meal, or refilling after a low-carb stretch, tops it right back up.
And the usual suspects
Hormonal shifts in the days before a period, a poor night's sleep, a long flight, a hard workout, or simply being too hot all nudge water retention up. None of it is fat.
Water weight
vs real fat.
The tell is speed. Fat changes slowly; water changes by the hour.
Your weight swings 1 to 2kg across a normal day from food, drink, salt and the toilet, well before any fat has changed. Real fat loss is slow: a kilo of fat is about 7,700 calories, so it takes a sustained deficit over a week or more, not a morning.
So a number that spikes after a salty meal or a carb-heavy weekend, then settles a day or two later, was water the whole time. The fix isn't panic, it's understanding what moved it.
How to tell which one you are looking at. Three things separate them, and none of them need equipment. Speed: water arrives and leaves in hours to days, fat takes weeks. Size: a 1.5kg move overnight is water, because the calories for 1.5kg of fat do not exist in one day's eating. Pattern: water tracks what you ate and drank the day before, so it spikes after a takeaway, a big carb meal, a flight or a hot day, then comes back down on its own. Fat has no such link to yesterday.
This is why a single morning reading tells you almost nothing. Weighed on the same scales at the same time each day, the daily numbers zigzag by a kilo or more while the weekly average barely moves, and the weekly average is the only line that describes fat. If you have ever felt a diet stopped working on a Wednesday, you were almost certainly reading water.
How do I lose
water weight?
Six things that work, ranked fastest first. No detox teas, no diuretic pills. The big one is carbs.
The fastest lever is cutting carbs. Your body keeps roughly 500g of glycogen in your muscles and liver, and each gram drags about 3g of water with it, so that's around 1.5kg of water tied up in your carb stores. When carb intake falls, the body burns through that glycogen and the water goes with it. It is why the first week of any low-carb diet drops fast, and why the drop comes straight back when normal eating resumes. Up to 70% of the weight lost in the first few days of any diet is water and glycogen rather than fat, which is worth knowing before you read anything into it.
One honest caveat: this is water, not fat. Eat normal carbs again and your glycogen, and its water, refills. It's a fast way to debloat for an event, not a shortcut to fat loss.
Salt is the other half, and UK labels make it easy to miss. Two numbers get used for the same thing. Some packs print salt, some print sodium, and they are not interchangeable: sodium multiplied by 2.5 gives you salt. A label showing 1g of sodium is really 2.5g of salt, which is close to half an adult's day against the NHS cap of 6g. Read the wrong number and you can be a long way over while believing you are under.
The other trap is where it hides. Very little of a normal day's salt comes from the cellar on the table. It comes from bread, breakfast cereal, pasta sauce, soup, ready meals, bacon and cheese, in amounts that look small per portion and add up hard across a day. This is the part worth actually measuring rather than guessing, because the foods that carry the most salt are rarely the ones that taste saltiest.
1. Carbs coming down
The clearest lever, because it empties the glycogen holding roughly 1.5kg of water. Refined carbs move it fastest: white bread, pasta, rice, sugary drinks and snacks. The weight returns when the carbs do, which is the part diet ads leave out.
2. Cut the salt
Aim under the NHS 6g-a-day cap (about 2.4g sodium). Most of it hides in bread, sauces, ready meals and deli meat, not the salt cellar. As sodium falls, the water it held follows within a day or two.
3. Drink more, not less
Counterintuitive but real: under-drinking makes your body cling to water. Steady water through the day, roughly 6 to 8 glasses, tells it to let go.
4. Top up potassium
Potassium balances out sodium and helps you shed the water it holds. Leafy greens, potatoes, beans, bananas and tomatoes are easy sources, no supplement needed.
5. Move and sweat
A workout burns glycogen, frees its water, and you lose fluid through sweat and breath. Even a brisk walk shifts fluid that's been pooling if you've been sat still.
6. Sleep, skip the nightcap
Poor sleep raises the stress hormones that hold water, and alcohol triggers a rebound where you retain more the next day. A good night and a dry evening both help it settle.
How fast does
it come off?
Faster than fat, which is the whole point.
Most salt- or carb-driven water weight settles in 24 to 48 hours once the cause eases, often a kilo or more. It's why a Monday weigh-in after a salty weekend reads high and a midweek one reads normal, with nothing else changed.
If you're tracking weight loss, weigh at the same time each day and watch the weekly trend, not the daily number. The day-to-day wobble is mostly water, and reading it as fat will drive you mad.
When to see
a doctor.
Everyday water retention is normal. Some swelling isn't, and that's a GP conversation, not a diet one.
See your GP if swelling is persistent, only on one side, leaves a dent when you press it, or comes with breathlessness, chest pain or a sudden large gain over a few days. The NHS lists these as signs that fluid retention may need checking, because it can point to a heart, kidney or other condition. NutraSafe is a tracking tool, not a diagnosis, so take anything that doesn't settle to a doctor.
Track the two
things that cause it.
Water weight is salt and carbs. NutraSafe reads both straight off the UK label, so you can see which one spiked.
Your salt, vs the 6g cap
Scan a UK barcode and we pull the salt figure off the label and total your day against the NHS 6g adult cap, so you can spot the soup, sauce or ready meal that's parking the water.
Your carbs, day by day
The diary totals carbs from every food you log, so a high-carb day that topped up glycogen and its water is easy to see next to the scale.
The trend, not the wobble
Log your weight and the diary shows the weekly trend, so you read fat loss properly and ignore the daily water noise.
Frequently
asked.
Sourced to the NHS and standard physiology.
Yes. Salt- or carb-driven water weight usually settles within 24 to 48 hours once the cause eases. It isn't fat, and you don't need to do anything drastic to lose it.
Yes. Sodium makes your body hold extra water to keep blood concentration steady, so a salty meal can add 1 to 2kg of water for a day or two. The NHS caps adults at 6g of salt a day.
They can. Carbohydrate is stored as glycogen, and each gram of glycogen holds about 3g of water. A big carb meal, or refilling glycogen after low-carb eating, brings water with it. It's the reason low-carb diets show a fast first-week drop that is mostly water.
Yes, oddly. Under-drinking makes your body hold on to more water. Drinking steadily through the day helps it release what it's retaining.
Commonly 1 to 2kg over a day or two, sometimes more after a very salty or carb-heavy stretch. It varies by person, diet and activity. If swelling is persistent or one-sided, see your GP rather than treating it as water weight.
Read
next.
More on this, sourced the same way.
Why calorie counting stops working
People in the landmark study reported 1,028 calories and ate 2,081. The four leaks to check before changing your target.
Balanced diet myths, and what UK guidance says
Carbohydrate is recommended at half of energy, juice counts once up to 150ml, and only 17% of adults hit five a day.
Your salt and carbs,
straight off the label.
Free download. Scan UK barcodes for salt and carbs, log your weight and read the trend. Up to 25 logs a day on the free tier; Pro £3.99/month or £34.99/year.
iPhone · iOS 17 · Cancel any time