The 5-day diet for 5 kg: why it does not last

‘5 days for 5 kg’ diets come in many versions. They are usually based on very little food: one or two foods, soups, juices, or menus presented as a ‘detox’. In October 2026 we searched the PubMed database and found no trial that tests a diet like this. We can, however, look at what happens in the body over a few days on very little food, because that has been measured.

The arithmetic of 5 kg

A kilo of fat corresponds to about 7,700 kcal. Five kilos is therefore about 38,500 kcal, a deficit of 7,700 kcal every day for 5 days (example).

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. Even with no food at all, which nobody recommends, a man with this energy use would have a deficit of about 12,500 kcal over 5 days. That corresponds to less than 2 kg of fat (example). Losing 5 kg of fat in 5 days is not possible.

What you actually lose

Carbohydrates are stored in the muscles and liver as glycogen, and every gram of glycogen holds at least 3 g of water with it. On very little food, and especially with few carbohydrates, glycogen runs down in the first few days and the water leaves with it.

An old study measured exactly what is lost. Six people with obesity followed an 800 kcal diet for 10 days:

Using the study's averages, over 5 days the first diet would give about 2.3 kg, of which about 1.4 kg would be water. The second would give about 1.4 kg, of which about half a kilo would be water (example). The study is small and old, so the figures are of low certainty. It is fairly certain that in the first few days a large part of the loss is water.

Why the weight comes back

As soon as you eat normally again, glycogen fills up again and takes the water with it. The scales go up within a few days, without this being fat. A 1992 paper describes exactly this: the rapid rise in weight when carbohydrates come back after a very strict diet.

The UK's NICE (2025) advises avoiding strict diets that are not balanced, because they do not work in the long term and may cause harm. For diets under 1,200 kcal, which it suggests only within specialist services, it says that weight regain is likely. For those under 800 kcal it also mentions risks such as constipation, tiredness and hair loss.

The ‘detox’ claim

Many of these diets are sold as a ‘detox’. A 2015 critical review looked at the evidence. It found very little clinical evidence, few studies with small samples and weak methods, and no randomised trial of commercial ‘detox’ diets in people. So there is no evidence that the weight lost on diets like these comes from any ‘cleansing’. The simplest explanation is water and eating less.

Is fast weight loss always bad?

Not necessarily. The US guidelines say that intensive programmes with a diet under 800 kcal, full meal replacements and medical supervision gave about 14 to 21 kg over 11 to 14 weeks. 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 difference from a 5-day diet is that these programmes have medical supervision, nutritionally complete meals, a duration of up to 12 weeks according to NICE, and a plan for what you eat afterwards. A diet you put together yourself for 5 days has none of these.

What to do instead

• If you have an event in a few days, whatever you do, the scales will mainly show water. It is not worth going hungry for a number that will come back.
• Start something that lasts. The NHS suggests 0.5 to 1 kg a week, and the US guidelines a deficit of 500 to 750 kcal a day. The first week will show more because of water, and that is normal. The article on the rate of weight loss shows what the pace means for your own weight.
• Choose a way of eating you enjoy. The well-known diets give similar results, and the one that wins is the one you can keep to. You will find what the trials showed for keto and other well-known diets in the article on them.
• Think about maintenance from the start. The US guidelines suggest 200 to 300 minutes of activity a week and weighing yourself every week. In a US registry, people who kept the weight off for years reported about an hour of activity a day, regular weighing and similar eating on weekdays and at weekends. These are people who chose to join the registry, so this shows a link, not a cause.

Who should stay away from diets like these

• People who have or have had an eating disorder. Before a diet under 1,200 kcal, NICE asks for an assessment for a possible eating disorder.
• Pregnant women, for whom NICE says deliberate weight loss is not recommended, and women who are breastfeeding.
• People who take insulin or other medicines for diabetes, without their doctor.
• People with a history of gallstones. In a Swedish programme, the 500 kcal diet had about three times the risk of gallstones needing hospital care compared with a gentler diet.
• Anyone under 18.

The MesogeaFit app, coming soon, will build a plan with Greek food and a pace that does not exceed 1 kg a week. Notify me when it launches.