VoltraCrash diets: why losing weight too fast backfires
Very-low-calorie diets, under 800 kcal a day, belong in specialist weight-management services for people who need to lose weight fast for a medical reason, such as making surgery safer, and not as a long-term way to manage weight [1]. NICE advises against restrictive, unbalanced diets because they don't work in the long term and can be harmful [1].
Aim instead for 5 to 10% of your starting weight over six months, the first goal both the US NIH and Saudi Arabia's health ministry suggest [4][5].
What the guidelines say
| Diet | When NICE says to consider it [1] |
|---|---|
| Under 800 kcal a day | Only in a specialist service, for people with a clinical need to lose weight quickly |
| 800 to 1,200 kcal a day | Only as part of a specialist programme with long-term support |
Either kind should last no more than 12 weeks and isn't a long-term strategy [1].
The risks
- Weight cycling, regaining the weight, and side effects; very-low-calorie diets also bring a risk of constipation, fatigue and hair loss [1], which the NICE committee says are common in its experience [2].
- Gallstones: in a study of 6,640 people, a 3-month diet of 500 kcal a day, followed by 9 months of maintenance, led to three times as many gallstones needing hospital care over the year as a 1,200 to 1,500 kcal diet [3]. Losing weight very quickly may raise your chances of gallstones [4].
- Eating disorders: NICE notes a common concern that people who prefer very restrictive diets are more likely to be vulnerable to one, and advises an assessment first [1][2].
Egg, dates-and-laban and water diets: how much do they really take off?
The boiled-egg diet, the dates-and-laban diet, the water diet and any plan promising "10 kg in 3 days" share one idea: one or two foods for a few days, and very few calories. We found no published study of any of them by name, so nobody can tell you exactly how much they take off. What comes off in the first days is known:
- Much of it is water. The body stores glycogen, its carbohydrate store, with three to four times its weight in water, and spends it in the first days of a very-low-calorie diet. The scale drops fast, and climbs back fast once carbohydrates return [6].
- Fat comes off slowly. The common rule is a deficit of about 7,700 kcal per kilo of weight lost, roughly right for people with a lot of body fat and too high for leaner people [7]. Ten kilos in three days would need a deficit of about 77,000 kcal, over 25,000 a day; even at half that figure, it is several times what anyone burns in a whole day (calculated).
- Same risks. Any diet under 800 kcal a day, whatever its name, is a very-low-calorie diet with the guidance and risks above, gallstones included [1][3].
On 8 June 2026 Saudi Arabia's health ministry warned against following any scientifically unproven diet, or using one in place of prescribed treatment without specialist supervision [8]. It named the Tayyibat diet, which has its own guide.
What to do instead
The US NIH suggests starting with 5 to 10% of your starting weight over 6 months and avoiding crash diets that promise quick results [4]. Saudi Arabia's health ministry gives the same first goal, and notes that people feel healthier after losing even 5 to 15% of their weight [5]. For a 90 kg person, 5 to 10% is 4.5 to 9 kg (calculated). Our cutting guide covers how to lose it while keeping muscle.
In Voltra
Voltra's calculated calorie target never goes below 1,200 kcal a day, and its fastest goal is about 1 kg a week. If you type in your own target below your resting burn, Voltra warns you when you save it.
Frequently asked
What is an "emergency diet"?
A name for a very low-calorie diet of a few days that promises fast weight loss. Any diet under 800 kcal a day belongs in specialist services with a medical reason, not at home, and losing weight fast raises the risk of gallstones.
Are crash diets safe?
Not at home. Diets under 800 kcal a day are for specialist services with a medical reason, for up to 12 weeks, and NICE advises against restrictive, unbalanced diets.
Does losing weight fast cause gallstones?
It can. A 500 kcal a day diet tripled the risk of gallstones needing hospital care over a year compared with a 1,200 to 1,500 kcal diet.
How fast should I lose weight?
A first goal of 5 to 10% of your starting weight over six months, as both the US NIH and Saudi Arabia's health ministry suggest.
Do crash diets cause hair loss?
Hair loss, constipation and fatigue are common on very-low-calorie diets, according to the NICE committee.
How much weight does the boiled-egg diet take off?
No published study has measured it. On any crash diet, much of the first days' loss is water from glycogen, and it returns when you eat normally again. Fat comes off slowly: about 7,700 kcal of deficit per kilo.
How much does the dates-and-laban diet take off in a week?
There is no proven figure for it by name. On any crash diet, much of the first week's loss is water that returns fast; guidelines suggest a goal of 5 to 10% of your weight over six months instead.
Can I lose 10 kg in 3 days?
Not as fat. Ten kilos of fat needs a deficit of about 77,000 kcal, over 25,000 a day, several times what anyone burns. What comes off that fast is mostly not fat, and much of it is water that returns.
Sources
- NICE (2025). Overweight and obesity management (NG246): physical activity and diet, recommendations 1.16.7–1.16.12. Recommendations 1.16.7–1.16.12.
- NICE (2025). Overweight and obesity management (NG246): rationale and impact, low-energy and very-low-energy diets. Low-energy and very-low-energy diets for adults.
- Johansson K, Sundström J, Marcus C, Hemmingsson E, Neovius M (2014). Risk of symptomatic gallstones and cholecystectomy after a very-low-calorie diet or low-calorie diet in a commercial weight loss program: 1-year matched cohort study. Int J Obes 38(2):279–284. Abstract. 3,320 people in each group, a Swedish commercial programme.
- NIDDK, US National Institutes of Health (2017). Dieting and gallstones. Last reviewed November 2017.
- وزارة الصحة، المملكة العربية السعودية (٢٠١٩). السمنة. In Arabic. Treatment, p. 3.
- Kreitzman SN, Coxon AY, Szaz KF (1992). Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr 56(1 Suppl):292S–293S. Abstract. 11 people after 4 days on a very-low-calorie diet.
- Hall KD (2008). What is the required energy deficit per unit weight loss? Int J Obes 32(3):573–576. Abstract.
- Saudi Press Agency (2026). Health Ministry Warns Against Stopping Medications, Following Scientifically Unproven Diets. 8 June 2026.
Sources checked on 30 September 2026.
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