For three weeks the scale shows about 82.4 kg (181.7 lb) every morning, even though you stick to your plan. On Monday of week four it shows 81.2 kg (179.0 lb). More than a kilo vanished overnight, and you go looking for the reason.
Forums call this the whoosh effect: a sudden drop that follows a flat stretch. What is documented is that water in your tissues can hide fat loss for weeks, and the scale then catches up within a short time. The popular explanation, that emptied fat cells fill with water and release it all at once later, has no solid evidence behind it. A trend chart shows a whoosh clearly. A single daily value does not.
What the evidence supports and what it does not
Your body weight is the sum of fat, fat free mass, gut content and water. When you diet you lose fat, but the scale reports only the sum. If the water share grows, it can cover a whole week of fat loss.
The best studied mechanism is glycogen, the carbohydrate store in muscle and liver. Kreitzman et al. 1992 describe that glycogen is stored together with water, about three to four parts water per part glycogen, and that changes in glycogen drive short-term swings in weight. A day with a lot of carbohydrate or hard training refills the stores and binds water. A day with little carbohydrate empties them and releases water. Salt, your cycle and poor sleep also shift water, and the background is covered in Why your weight fluctuates every day.
One reading fits what people observe: fat leaves at a steady pace, water holds against it for a while and then gives way. Whether glycogen, salt, your cycle or stress is behind it in your case, a scale cannot separate. It reports only the sum.
A worked example
Say you lose body fat at 0.4 kg (0.9 lb) per week, which makes 1.2 kg (2.6 lb) in three weeks. At the same time water builds up in the same amount, and the scale stays at 82.4 kg. On day 22 the water lets go, you weigh 81.2 kg and stay there in the following weeks. The numbers are an illustration, not a measurement.
The trend, calculated here with exponential smoothing at a factor of 0.1, responds like this:
| Day | Daily value | Trend |
|---|---|---|
| 21 | 82.4 kg (181.7 lb) | 82.4 kg (181.7 lb) |
| 22 | 81.2 kg (179.0 lb) | 82.3 kg (181.4 lb) |
| 29 | 81.2 kg (179.0 lb) | 81.7 kg (180.1 lb) |
| 36 | 81.2 kg (179.0 lb) | 81.4 kg (179.5 lb) |
Each day the trend moves one tenth of the distance to the daily value. On the day of the drop it barely moves, and after two weeks it sits at 81.4 kg. During the three flat weeks before, the progress was hidden. The curve shows weight, not fat, and the weight was flat in that period.
How to read a whoosh in the trend curve
A chart shows two things: the daily values as dots and the trend as a line. A whoosh looks like this:
- The trend runs flat or nearly flat for several weeks.
- One dot falls clearly below the line.
- The following dots stay at the lower level, and the line bends downwards over the next days.
The raw data catches up with the line. Trendcurve shows you the trend weight and the 7 day change for this, both in the free version. The 7 day change is the weekly rate from the last 14 trend values. After a whoosh it grows gradually over one to two weeks, and that is what makes it reliable. More about the smoothing is in Trend weight instead of the daily value.
In Pro the app also detects a plateau: when the rate stays below 0.1 kg per week for several days, the Analysis tab shows "Plateau for X days" with an explanation for your goal mode. If you see three flat weeks before a whoosh, you will usually get that message, and it disappears once the trend turns down. A plateau in the app's sense describes the curve. It says nothing about your fat.
When a drop is not a whoosh
A sudden drop of more than a kilo has other causes too: less gut content, a day with little salt and little fluid, a different scale or a misreading. The difference shows afterwards. A whoosh stays, an outlier jumps back the next day or within three to five days.
If you recognise a value as a misreading, you can ignore it in Trendcurve (free). It stays visible in the history, struck through, but no longer feeds into the daily value, trend or rate.
Two limits belong to the topic. First, a flat phase does not automatically end in a jump. Sometimes there is none, the trend falls slowly and evenly, and that is a normal course. Second, you cannot trigger a whoosh. Anyone who pulls water out deliberately, through sauna sessions or days of fasting, sees a low that disappears after the next meal. If you gain or lose a lot of weight for no clear reason, or your legs or face swell, have a doctor look at it. The app is not a medical device.
What to do during a flat phase
A flat phase is the moment when many people change their plan. That usually makes things worse, because afterwards you no longer know what works.
- Leave the plan alone. If your trend was falling before, give it at least two weeks before you cut calories.
- Keep weighing at the same time. Regular measurements under similar conditions keep the noise small.
- Look at the trend, not the day. A single low value is a hint, the trend is the answer.
- Do not celebrate or overcompensate after the drop. A salty meal brings back part of the water, and the trend absorbs that.
- Judge only after three to four weeks. If the trend is still flat or slightly rising by then, you probably have a real plateau. What helps then is covered in How fast should you lose weight?.
Note the day the trend turned flat and set the date three to four weeks later as your appointment for the next assessment.