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].
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
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.
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.
Sources checked on 30 September 2026.
Log your food and training with Voltra. Free to download; the yearly plan starts with a 7-day free trial. Works fully in English and Arabic.
