Saturday night was pizza, olives and a slab of cheese. On Sunday morning the scale reads 81.4 kg (179.5 lb), and on Friday it said 80.0 kg (176.4 lb). Your body did not build 1.4 kg (3.1 lb) of fat in one night. It held on to water.
The short answer: a jump after salty food is almost entirely water and glycogen. It usually fades within a few days, and your trend weight moves by only a couple of hundred grams.
What salt does to the water in your body
Salt binds water, so more salt means more water kept in the body. In a study by Rakova and colleagues in 2017, a higher salt intake reduced free-water clearance: the body saved water instead of passing it on through the kidneys. That water sits in tissue and blood, and the scale weighs it.
Salty food also makes you thirsty. You drink more than usual, and the drink counts on the scale for as long as it stays in your body.
Salty meals also tend to be big. A restaurant dinner brings more food, more drinks and more content in the gut, which only clears the next day. All of it lands on the scale without being body fat.
The second driver: carbohydrates and glycogen
Pizza, pasta, burgers and fries bring plenty of carbohydrates along with the salt. The body stores them as glycogen in muscle and liver, and glycogen holds water. Kreitzman and colleagues describe about three to four parts of water for each part of glycogen, and they show that changes in glycogen drive short-term swings in weight.
Salt and carbohydrates together explain why a hearty evening shows so clearly on the scale. Both effects reverse. If you eat and drink normally the next day, the body gives the water back within a few days.
Fat works differently. According to Hall (2008), one kilo of body tissue corresponds to roughly 7,700 kilocalories of surplus. Even a dinner 2,000 kilocalories above your needs adds about 0.26 kg (0.6 lb) of fat on paper. A jump of 1.4 kg cannot be fat.
A worked example: pizza on Saturday
Say your trend weight is 80.0 kg on Friday. On Saturday you eat salty and late. From Sunday you weigh yourself each morning and eat normally. The trend values below follow exponential smoothing with a factor of 0.1, the default in Trendcurve.
| Day | Daily value | Trend weight |
|---|---|---|
| Friday | 80.0 kg (176.4 lb) | 80.0 kg (176.4 lb) |
| Sunday | 81.4 kg (179.5 lb) | 80.1 kg (176.6 lb) |
| Monday | 80.9 kg (178.4 lb) | 80.2 kg (176.8 lb) |
| Tuesday | 80.5 kg (177.5 lb) | 80.2 kg (176.8 lb) |
| Wednesday | 80.1 kg (176.6 lb) | 80.2 kg (176.8 lb) |
The daily value jumps by 1.4 kg on Sunday and falls back close to the old level by Wednesday. The trend rises by 0.2 kg in that time and stays there. The Sunday reading is a real measurement, but it tells you something other than what you suspect: it measures the water balance of the night, not your fat mass. If you look only at Sunday's number, you see a setback. If you look at the curve, you see a spike that disappears.
How to read the spike in your trend curve
Trendcurve smooths your daily values into a trend weight. One high reading pulls the line only a little way, because each new day gets a small share of the trend. How the method works and which alternatives exist is covered in trend weight instead of the daily number.
Two features help after a jump like this, and both are free:
- Ignore a measurement. You can save Sunday's value as ignored. It stays visible in your history but does not feed the trend or the rate.
- A marker or experiment of the type nutrition. Under "Settings & more" → "Experiments and markers" you set Saturday as a marker. In three months you still know why the curve has a bump there. The before and after evaluation of an experiment is part of Pro, creating one is not.
You do not have to ignore the value, because the smoothing dampens the spike without your help. Ignoring pays off mainly when you do not want the rate and the forecast distorted by a single reading.
A week later, the bump in the daily values has shrunk, and the trend shows hardly a trace of it.
When more than salt may be behind it
A salt spike is a short swing after a clear cause. Something else is going on if:
- the daily value has not come down after four or five days of normal eating,
- the trend keeps rising over two to three weeks although no meal was especially salty,
- you are in the second half of your cycle, when many people hold more water anyway,
- the daily values drift upward and no longer scatter.
In that case, look at how much you eat overall and at what changed in your routine. Why the scale moves from day to day even without salt is covered in why your weight fluctuates daily.
Swollen legs, fast weight gain over several days or shortness of breath belong in a doctor's office. The same goes if you have to watch your salt intake because of your heart or kidneys. Trendcurve is not a medical device and does not replace an examination.
What to do the day after a salty meal
- Eat and drink normally. Skipping food or cutting fluids the next day tends to keep water in the body longer and distorts the next readings.
- Weigh yourself at your usual time. Same time, same scale, same conditions keep the daily values comparable.
- Keep weighing. A skipped day hides the spike, but also its decline. The values from Tuesday and Wednesday show you that the jump is over.
- Judge the trend after a week. Compare today's trend weight with the one from seven days ago. If it is 0.2 kg higher after a pizza, nothing has changed in your routine.
- Do not plan a correction. A salt spike needs no compensation through cutting calories.
If you see 1.4 kg more tomorrow morning, save the reading, add a marker if you like, and judge the situation on Wednesday using the trend.