The jump

Two trips in one week. Meals out, a few drinks, no routine, and the morning you get home the scale says you're a kilo (just over 2lb) up. Your brain does the maths instantly: a month of careful work, gone in a week.

Except that isn't maths. It's panic wearing a calculator. The actual maths is much friendlier, and once you've seen it, a post-holiday jump stops being frightening and starts being predictable.

The fat maths

Body fat stores a lot of energy. Gaining a kilo of it takes roughly 7,700 surplus calories, the metric cousin of the old 3,500-calories-per-pound estimate (Wishnofsky, 1958), and modern modelling keeps that figure in the same ballpark for fat tissue (Hall, 2008).

So to gain a genuine kilo of fat in a week, you'd need to eat about 1,100 calories a day over maintenance, every day, for seven days. That is a big, sustained overshoot: think an extra large pizza on top of your normal food, daily. A holiday pattern of nicer meals and a few drinks usually lands at a few hundred extra calories a day, which works out at 0.2 to 0.4 kg (half a pound to a pound) of fat over the week.

The scale said a kilo. The energy you ate can only account for a fraction of that. So what's the rest?

Glycogen: the water sponge

Your muscles and liver store carbohydrate as glycogen, and glycogen is stored wet: every gram holds roughly three grams of water with it (Olsson & Saltin, 1970). If you'd been eating in a deficit before the trip, your glycogen stores were part-empty. A week of bread, pasta, chips and cocktails refills them, and the water comes along for the ride.

Refill 300 to 500 g of glycogen and you've added over a kilo, more than 2lb, on the scale without a single gram of it being fat. This is the same effect, in reverse, behind the dramatic first-week losses on low-carb diets, and researchers flagged how much it distorts people's reading of their own progress decades ago (Kreitzman et al., 1992).

Salt, and what's still in transit

Restaurant and holiday food carries far more sodium than home cooking, and your body responds to a salty stretch by holding extra water while it rebalances (American Heart Association). Add travel's usual effect on digestion (long sits, different food, disrupted routine) and there's simply more food in transit through you than in a normal week. Both show up on the scale. Neither is fat.

Weekends have a rhythm

None of this is unique to holidays. People who weigh daily show a reliable weekly wave: up over the weekend, down through the weekdays (Orsama et al., 2014). A holiday is just a longer weekend, so the wave is bigger. And the definitive holiday study, tracking thousands of people through Christmas in three countries, found the average festive gain was about half a percent of body weight, with roughly half of it gone shortly after without heroics (Helander et al., NEJM 2016).

This is why single readings can't be trusted with your mood. The signal lives in the 7-day trend; the daily number is mostly noise about water.

If you're on a GLP-1

The stakes feel higher when you're paying for Mounjaro or Wegovy and watching for proof it's working. A post-holiday jump reads like the medicine failing, and that thought makes some people skip doses or quit. It isn't failing. The drug changes your appetite; it doesn't suspend the physics of glycogen and water. Give the trend a week of normal life to answer, and it usually answers well.

What to actually do

Very little, done calmly:

• Go straight back to your normal routine. Not a punishment week. The water leaves on its own schedule, usually within three days to a week.
• Keep weighing, keep logging. Skipping the scale for a fortnight because you're scared of it just deletes your data at the exact moment it's interesting.
• Judge nothing until the trend settles. Look at your 7-day average a week after you're home. That number, and not the airport-morning spike, is the truth about your trip.

Raspberries and 0% Greek yogurt for breakfast this week isn't repentance. It's just a decent breakfast. The numbers don't need you to feel guilty; they need you to keep collecting them.

The questions everyone asks

How long until it comes off?

The water and glycogen portion typically settles within about three days to a week of normal eating. What remains after that is the real change, usually a fraction of the original jump.

Did I undo my progress?

Run the maths: 1,100 surplus calories a day for a week is what a kilo of fat costs. If you didn't eat like that, you didn't gain that.

Should I eat much less this week to compensate?

No. Crash-compensating trains a swing between restriction and blowouts, and the scale will mislead you all week while the water shifts anyway. Normal routine, normal portions, patience.

Healthcount plots your 7-day trend automatically, so a holiday spike shows up as exactly what it is: a wobble on the way down. Start tracking free or read why weighing food raw vs cooked changes your numbers.