DispatchJuly 2026 · 8 min read
The First Week Is a Beautiful Lie
Every diet opens with a gift: the scale plunges, two or three kilos gone in days, and it feels like proof that something's finally working. Then it slows to a crawl and the doubt creeps in. The truth is that the fast part was mostly water — and the slow part is the only part that was ever fat.
By FG Samartsidis · Filed under: Nutrition, Physiology, Everyday Science
I. The dramatic opening week
It's the most motivating week of any diet, and the most misleading. You cut your eating, the scale drops fast — two, three, sometimes four kilos in the first seven days — and it feels like a triumph. Then, cruelly, it slows. Week two brings a fraction of week one; week three feels like pushing a boulder uphill. Many people read that slowdown as failure, decide the diet has "stopped working," and quit right at the point where it finally started doing the thing they wanted. The whole misunderstanding comes from not knowing what actually walked out the door in week one.
Almost none of it was fat. Here's what it really was.
Understanding this one mechanism changes your entire relationship with the scale — it defuses the early euphoria that sets you up for a crash, and it disarms the mid-diet despair that makes people give up. The number on the scale is not a pure measure of fat. It's a measure of everything your body is carrying at that moment, and a surprising amount of that is water that comes and goes for reasons that have nothing to do with getting leaner.
II. What actually left your body
Your body stores carbohydrate in a form called glycogen — a ready fuel reserve tucked into your liver and muscles. And here's the crucial fact almost nobody's told: glycogen doesn't sit there dry. Every gram of it is bound to roughly three grams of water. Your stores hold several hundred grams of glycogen, which means they're also holding a couple of kilos of water alongside it, like a sponge.
Burn through the glycogen, and its water pours out with it.
When you eat less — and especially when you cut carbs — your body turns to that glycogen reserve for fuel and starts draining it. As the glycogen goes, the water it was holding is released and flushed out. Empty a good chunk of your stores and you can shed one to two kilos of water almost immediately. On the scale it looks like dramatic progress. In reality you've wrung out a sponge, not burned fat.
Two more things pile on in that first week, and all of them are water or waste, not fat:
Insulin drops, and so does water retention
Eating fewer carbs lowers your insulin levels, and lower insulin signals the kidneys to release more sodium — and water follows sodium out. This is a big part of the 'whoosh' of early loss, and it's why low-carb diets in particular produce such dramatic first-week numbers. It's a fluid shift, not fat metabolism.
There's simply less in the pipeline
Eating less food means less material moving through your digestive system at any given moment. That's a small but real contribution to a lighter scale reading that, again, has nothing to do with body fat.
What the scale drop is made of
Same person, same diet. The kilos are bigger early — but look what they're actually made of.
Illustrative proportions, not a measurement — real numbers vary by person and diet. The pattern is the point: early loss is mostly water; real fat loss is the slow, steady part that comes later.
III. The unforgiving maths of real fat loss
Here's why fat loss can't be fast, no matter what the first week suggests. A single kilo of body fat stores an enormous amount of energy — on the order of 7,000 to 7,700 calories. To lose it, you have to run a genuine energy deficit of that size: eat that much less than you burn. And there's a hard limit to how big a daily deficit you can sustain safely.
Real fat loss lands around half a kilo to a kilo a week. That's the ceiling, and it's a success.
A steady deficit of 500 calories a day adds up to roughly half a kilo of actual fat per week; a larger, still-sustainable 1,000 a day, closer to a kilo. That's it. So when your loss slows from three kilos in week one to half a kilo in week three, you're not failing — you've just run out of water to lose, and what you're seeing now is the real, honest, slow burn of fat. The slow number is the true number. The fast one was mostly the sponge.
This reframing is the single most useful thing to carry out of this piece. The dramatic first week trains people to expect a pace that is physically impossible to maintain, because after the water's gone, no safe diet can shed fat that fast. When reality reasserts itself in week two or three, the unprepared dieter feels betrayed and quits. The prepared one shrugs, understands that half a kilo of fat a week is exactly what winning looks like, and keeps going. Same diet, same results — completely different outcome, decided entirely by whether you understood the water.
IV. Why low-carb "wins" the first week — and what that's worth
This also settles a long-running argument. Low-carb and keto diets are famous for spectacular early results, and that's often held up as proof they're superior for fat loss. But now you can see what's really happening: cutting carbs drains glycogen and its water fastest of all, so the first-week scale drop is dramatic — and largely water. It's not evidence of some magical fat-burning advantage; it's the sponge being wrung out harder and sooner.
None of this means low-carb diets don't work — they can be an excellent way to eat, and for many people the appetite control they provide makes the all-important calorie deficit easier to sustain, which is what actually drives fat loss over months. The point is narrower: the early scale magic is water, not proof of superiority, and when a low-carb dieter eats carbs again and "gains it all back overnight," that too is water refilling, not fat returning. Judge any diet by what it does over a couple of months, not by the theatrics of week one.
V. The reverse trick: why you "gained 2 kilos" after one meal
The same mechanism runs backwards, and understanding it will save you a great deal of needless panic. You have one big meal — a pizza, a pasta dinner, a proper carb-heavy feast — and the next morning the scale is up two kilos. It feels like a disaster, like the meal undid weeks of effort. It didn't. You didn't gain two kilos of fat from one dinner; that would require eating something like fifteen thousand extra calories. What happened is that the carbs refilled some glycogen, glycogen pulled its water back in, and the salt in the meal held onto more fluid still.
Overnight weight swings are water. Fat doesn't move that fast in either direction.
The sponge works both ways: wring it out and the scale drops, soak it again and the scale rises — and neither has much to do with your fat. That two-kilo "gain" will melt away over the next day or two as the water rebalances, exactly as the two-kilo first-week "loss" did. Once you know this, a single scale reading loses its power to ruin your day, and you can stop riding an emotional rollercoaster driven entirely by your hydration.
"The scale doesn't weigh your fat. It weighs your fat, your water, your glycogen, your dinner, and your mood-crushing timing. Learn to read past it."
VII. How to actually measure fat loss
If a single day's scale reading is mostly noise, how do you know if you're really losing fat? You stop trusting any one number and start watching the trend. Fat loss is slow and steady; water is fast and chaotic. The tools below cut through the chaos.
Weigh over weeks, not days
Judge progress by the trend across two to four weeks, not day to day. If you weigh daily, expect it to bounce around — that's water, sodium, and glycogen doing their thing. A simple average over a week, compared to the week before, tells the real story that any single morning hides.
Use a tape measure and photos
Waist, hips, and other measurements, plus photos every few weeks, often reveal steady change even when the scale is sulking. Because fat loss is slow and body water fluctuates, these can show progress the scale is temporarily masking.
Notice how your clothes fit
One of the most honest, low-tech signals there is. Clothes getting looser is real change in body composition, and it doesn't care what the scale said this morning. Trust it.
Expect — and welcome — the slowdown
Reframe the week-two deceleration as the diet working, not failing. Fast early loss then a slow steady grind is exactly the correct, healthy pattern. If anything, a diet that kept dropping three kilos a week would be a warning sign, not a triumph.
VIII. So what should you actually do?
Different moments in a diet, different honest answers. Tap the one that fits.
Tap to find the real answer for your situation.
Enjoy the motivation, but calibrate your expectations so the coming weeks don't crush them. Most of that 3 kg was water and glycogen, not fat, and the loss will slow dramatically from here — that's normal and correct, not a sign anything's wrong. Bank the early momentum, and be ready to celebrate half a kilo a week from now on, because that slower number is the real fat loss you actually wanted.
This is almost certainly not failure — it's the diet finally doing its real job. Week one's big numbers were water; now that the water's gone, what's left is the honest, slow burn of fat, which tops out around half a kilo to a kilo a week. Don't quit here; this is the part that was always the point. Keep your deficit steady and judge yourself over weeks, not days.
Relax — you did not gain 2 kg of fat from one dinner; that's physically impossible without eating many thousands of extra calories. The jump is water: carbs refilled glycogen, which pulled water back in, and the salt held onto more. It'll rebalance and drop away over the next day or two. Don't let one morning's reading, driven entirely by hydration, derail an effort that's actually going fine.
Then stop weighing daily, or start treating the daily number as noise rather than verdict. Weigh once a week at the same time, or take a weekly average, and lean on measurements, photos, and how your clothes fit instead. Fat loss is a slow trend hiding under a lot of daily water static; the trend is the truth, and the single reading is mostly there to mislead you.
This closes the loop with the rest of these dispatches, and with the water piece especially: the scale, like the eight-glasses rule, hands you a single tidy number for something your body handles in a far messier, more dynamic way. Weight is not fat. The dramatic first week is a sponge being wrung out; the slow grind that follows is the real work; and the overnight swings in between are just water moving in and out. Learn to read past the number, and you stop being at its mercy — which, more than any diet, is what keeps people going long enough to actually succeed.
Filed under everyday science. The author weighs himself weekly, ignores the overnight swings, and trusts the tape measure over the scale.