Why a salty dinner shows up on the scale, and a carb-heavy one shows up more
You ate ramen. This morning the scale is up 1.2 kg. You did not eat 9,000 extra calories, so it is not fat — and it is worth knowing exactly what it is, because the two main culprits behave differently.
Carbohydrate: stored wet
Your body stores carbohydrate as glycogen, and glycogen is stored with water attached — the ratio conventionally given is around three grams of water per gram of glycogen. That is the arithmetic behind an entire genre of dieting claim.
The classic paper on this is titled, bluntly, “Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition.” That is the whole phenomenon in one title. Cut carbohydrate hard and you drop weight fast in the first week, and it is water leaving with the glycogen. Eat carbohydrate again and it comes back, at the same speed, and people call that “gaining it all back.”
Practically: a large carbohydrate meal after a low-carb stretch can put a kilogram or more on the scale within a day, and none of it is fat.
Salt: the slower, stranger one
Salt is the other half, and the physiology is less obvious than the folk version. Ten men were kept under fully controlled conditions in two space-flight simulation studies of 105 and 205 days, with salt intake set at 12, 9 or 6 grams a day and everything else held constant.
The result contradicted the standard story. Raising salt by 6 g/day increased how much the body had to excrete, but reduced free-water clearance — the body conserved water by concentrating the urine. The endogenous water surplus that resulted actually reduced how much the men drank at the highest salt intake.
So a salty meal does not simply make you drink more and weigh more the next day. It changes how much water your body holds onto, over days rather than hours. The scale reflects that.
Why this is not a licence to ignore the scale
The point is not that weight readings are meaningless. It is that a single reading confounds fat with water, and water moves faster and further than fat ever does. Measured directly over two weeks of ordinary living, 84% of a unit change in body weight was fat-free mass, not fat.
The way out is not to weigh less often. It is to average. Seven days of readings contains the same weekday-weekend pattern for everyone, and averaging cancels most of the water noise while leaving a real trend intact. The calculator on this page does exactly that.
Reading the common cases
- Up 1 kg after a big carbohydrate meal. Glycogen and its water. Gone within days if your eating returns to normal.
- Up after a salty meal. Water retention, over days, not a single overnight swing.
- Down 2 kg in the first week of a low-carb diet. Mostly glycogen water. This is why week one flatters every low-carb diet, and why week three feels like failure when it is not.
- Up on Monday, down by Friday, every week. A documented group-level pattern, not something you did.
None of these is a reason to change what you are doing. All of them are reasons to read the average instead of the reading.