Search "how to lose weight fast" and you get "5 kg in a week" or "10 kg in 7 days". It sounds tempting, and the scale can do something like it. Those numbers are mostly water. This article covers what a fast drop is made of, what pace makes sense, and how to start faster without paying for it over the next six months.
Short answer: The scale can drop several kilos in a few days, and almost all of that is water and glycogen. It comes back when normal eating returns. A kilogram of body fat is about 7,700 kcal (calories), so real fat loss is hundreds of grams a week. A sane pace for most people is about 0.5 to 1% of body weight a week. For a 70 kg person that is roughly 0.35 to 0.7 kg.
Why 5 kg in a week is possible, and why it should not excite you
Yes, the scale can fall by several kilos in a few days. Almost all of that loss is not fat. Cut carbs and calories and the body first burns stored carbohydrate (glycogen) in muscle and liver. About 3 grams of water are bound to each gram of glycogen. Add less food in the gut, and water shifts from salt and hormones.
The scale shows a nice drop. You feel motivated. Fat loss was small. Go back to normal eating and the glycogen and the water refill. The kilos return, and you did not "fail". That is just your body refilling its stores. More on day-to-day swings: Why your weight fluctuates.
How much fat can you actually burn? One kilogram of body fat holds about 7,700 kcal. Even with an honest calorie deficit we are talking about hundreds of grams of real fat a week, not kilos. Fast numbers on the scale are mostly water and glycogen. Treat them as feedback.
What going too fast takes from you
An extreme deficit looks great on paper. You pay for it. The harder you push, the harder hunger and fatigue push back, and the more likely the weight returns.
- Hunger and cravings. A sharp cut raises hunger and lowers fullness. You can hold it with willpower for a few days. Willpower is not infinite, and one evening it gives.
- Muscle loss. If the deficit is too fast and protein and lifting are missing, the body gives up muscle, not only fat. Muscle keeps you strong and shapes how you look, and it burns a little more than fat even at rest. More on keeping it: Build muscle in a calorie deficit.
- Fatigue. Too little energy means worse training, mood and sleep. It is hard to hold for long.
- The rebound. After a crash diet the weight usually comes back, and so does the frustration. That is yo-yo dieting, and it says more about the method than about you.
A pace you can keep
For most people a sane pace is about 0.5 to 1% of body weight a week. For a 70 kg person that is roughly 0.35 to 0.7 kg. It sounds slow next to "10 kg a month". It is the pace where most of the loss comes from fat and where you can keep going for months.
The more fat you have to lose, the faster the start usually is. A leaner person chasing the last few kilos has to go slower and be more patient. Do not compare yourself with someone who is somewhere else.
And do not judge one day on the scale. Weight moves with water, salt, the cycle, and whether you have been to the bathroom. Watch the average over a week or two. You do not have to ban foods. Flexible dieting lets you hit your daily total without a list of banned foods. To set your deficit, see Calorie deficit: how much to eat, or estimate it with the calculator.
When a faster start makes sense
A faster start is not banned. Sometimes it helps motivation. If you have a medical reason to lose faster, talk to your doctor. Very low calorie diets belong under medical supervision, and very fast loss raises the risk of gallstones. Do not take the pace from an article. Just plan the fast phase so it still holds a few weeks later.
- Enough protein. About 1.6 to 2.2 g per kg of body weight a day. If you carry a lot of extra weight, use your target weight instead. If you have kidney disease, check with your doctor first. Protein holds fullness and muscle. More in how much protein.
- Strength training. It tells the body "keep the muscle". Without it, a fast deficit costs you more muscle.
- Do not keep cutting deeper. Even on a faster start, leave room to cut more later when things slow down.
- Put a time limit on it. Treat the faster phase as a few weeks, then move to a pace you can keep.
You can lose weight quickly. The first kilos are mostly water and glycogen, not fat. Lasting fat loss runs at a pace you can live with: enough protein, lifting, a milder deficit, and patience with the scale. Boring, maybe. But it is a pace you can keep long enough to build habits that last.
Frequently asked questions
Can you really lose 5 kg in a week?
The scale can drop by 5 kg, and it is mostly water and glycogen, not fat. Real fat comes off in the order of hundreds of grams a week. When you eat normally again, the water returns. That is why the fast numbers do not last.
What pace of weight loss is sane?
For most people about 0.5 to 1% of body weight a week. For a 70 kg person that is roughly 0.35 to 0.7 kg. That pace spares muscle, keeps hunger tolerable, and can be held for months. Watch the weekly average, not one morning.
Why did the weight come back after a fast diet?
The water and the glycogen came back, and often the urge to make up for weeks of hunger came back too. That is the yo-yo. A plan this hard does it to almost anyone. What helps most is a pace you can keep long enough to build habits that hold after the diet.
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1) Figures in the article: about 3 g of water bound to each gram of glycogen; about 7,700 kcal in 1 kg of body fat
2) Pace in the article: about 0.5 to 1% of body weight a week; protein 1.6 to 2.2 g per kg
If you are pregnant or breastfeeding, under 18, take medication for diabetes or blood pressure, or have a history of an eating disorder, talk to your doctor before you cut calories. If dieting turns into bingeing, guilt or making up for food, talk to a doctor or a specialist.
This article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.