A 2 kg a week diet: what the evidence says

If you searched for a ‘2 kg a week diet’, you probably have something specific in mind, a wedding or a holiday, or you simply want to see results at last. That is entirely understandable. The numbers show, however, that 2 kg of fat a week is not a realistic pace for most people.

The arithmetic

A kilo of fat corresponds to about 7,700 kcal. A 2008 model showed that this figure per kilo lost roughly fits people with more than 30 kg of body fat. People with less fat also lose more fat-free mass, which holds less energy per kilo, so each of their kilos needs a smaller deficit. But this is mass you do not want to lose, and your muscles are part of it.

The UK's National Health Service (NHS) says that a woman uses about 2,000 kcal a day on average and a man about 2,500. For 2 kg of fat a week, a woman with this energy use would not get there even without eating at all, and a man would have to eat about 300 kcal a day (example).

And that is the favourable version. The 7,700 kcal rule overestimates weight loss, because a body uses less energy as it gets smaller. In 7 studies where food was fully controlled or measured, participants lost about 9.1 kg on average, while the rule predicted 12.5.

Then why did I lose 2 kg in the first week?

Because not all of it was fat. Carbohydrates are stored in the muscles and liver as glycogen, and every gram of glycogen holds at least 3 g of water with it. When you eat less, and especially fewer carbohydrates, glycogen falls and the water leaves with it.

An old study measured this in 6 people with obesity, over 10 days on 800 kcal a day:

The study is small and old, so the percentages are of low certainty. The mechanism is fairly certain. So if you lost 2 kg in the first week, you did nothing wrong, and nothing went wrong when the scales dropped less in the second week. The same water also explains why your weight goes up and down from day to day, as we write in the article on why the scales have stopped going down.

What the guidelines say

• NHS: steady weight loss of 0.5 to 1 kg a week.
• US guidelines (2013): a deficit of 500 to 750 kcal a day. Diets under 800 kcal only in limited cases and with medical supervision, because of the fast rate and possible complications.
• NICE (2025): diets under 800 kcal only in specialist services, when there is a medical reason for rapid weight loss, for up to 12 weeks. Before starting, the patient is told about risks such as constipation, tiredness and hair loss.

The cost of rushing

Gallstones. An analysis of 5 studies found that the risk rises sharply above 1.5 kg a week, and the authors suggested not going beyond this average rate. In a Swedish programme, the 500 kcal diet had about three times the risk of gallstones needing hospital care compared with the 1,200 to 1,500 kcal diet. These are observational studies, so the findings are of low certainty.

Fat-free mass. In a trial of 57 adults, a very strict 5-week diet and a gentler 12-week diet gave similar weight loss, about 8 to 9 kg. With the strict diet, however, a larger share of fat-free mass was lost, and those who lost more of this mass regained more weight. In a small trial of 24 athletes who lifted weights, those who lost 0.7% of their weight a week gained muscle mass, while those who lost 1.4% did not.

What does not hold. It is often said that whatever is lost quickly is sure to come back. In a trial of 200 adults, fast weight loss over 12 weeks and slow loss over 36 weeks led to similar regain after about 3 years. The reasons to prefer a gentler pace are safety, muscle and how easily you can keep it up, not a rule that weight lost quickly always comes back.

A pace that lasts

Instead of a fixed number of kilos, think in terms of a percentage of your weight. A review for athletes suggests 0.5% to 1% a week, so that muscle is kept. It concerns athletes, so for the general population it is of low certainty, but it fits with the NHS's 0.5 to 1 kg for most people. See what it means for your own weight:

For someone weighing 120 kg, 1% is 1.2 kg. Again, in these articles we keep an upper limit of 1 kg a week for everyone. You will find more on the pace, and on why we measure it as a percentage, in the article on how many kilos you can safely lose in a month.

When to talk to a doctor

If after the first week you keep losing more than 1.5 kg a week, or if you feel dizzy, have a racing heart or feel very tired, talk to a doctor. The same applies if you have a history of gallstones and are thinking about any rapid weight-loss diet. And if thinking about food and weight puts you under a lot of pressure, talk to someone you trust or to your doctor.

The MesogeaFit app, coming soon, will turn the pace you choose into calories and grams for each meal, and never beyond 1 kg a week. Notify me when it launches.