Crash Diets: A Review ★☆☆☆☆

By R.Davies, PhD・Diet Atlas
Published June 10, 2026 | 3 min read

Crash diets, rapid weight loss diets, or very low-calorie diets are extreme approaches to dieting. Crash dieters typically consume fewer than 800 calories a day, although some rapid weight loss diets may allow slightly higher intakes. These diets have existed in many forms, often promising quick fixes.

Well-known crash diets include juice cleanses, cabbage soup diets, grapefruit diets, meal replacement diets, and several ‘detox’ diets. While there are many different types of crash diets, they all tend to create a large calorie deficit by severely limiting food intake. 

This is usually done by restricting the overall intake of all foods, certain food groups, or relying solely on single foods or drinks. Many crash diets are promoted through sensational (but inaccurate) marketing claims, and they tend to go in and out of fashion, just with new names and minor alterations (as a rebrand).

They tend to use sciencey-sounding claims like ‘reset metabolism’, ‘flush toxins’ or trigger special fat-burning ‘metabolic states’, and their appeal lies in promises of immediate, visible results without long-term commitment. This can be attractive to people who want to seek quick solutions to ‘lose weight’ before events, holidays, or when feeling desperate about their body image.

Pros

Crash diets' primary benefit is rapid initial weight loss, which provides immediate gratification and motivation for some people. The dramatic changes on the scales happen in days and may create a sense of accomplishment and renewed commitment to their body weight goals. For people about to undergo medical procedures, emergency surgery, or specific weight-controlled (sports) events, supervised rapid weight loss may be needed.

Short-term improvements in some metabolic health markers, including better blood sugar and blood pressure control, will likely occur alongside initial weight loss, and can provide relief for people with severe metabolic and obesity related issues. 

The ‘simplicity’ of many crash diets, consuming only specific foods or following rigid meal plans, eliminates decision-making about food choices, which some people find easier than more flexible diets that require ongoing judgments, decisions and self-control.

Cons

Severe calorie restriction with limited food variety creates substantial risks for nutrient deficiencies of essential macronutrients, vitamins, minerals, and dietary fibre, which can compromise your health (long-term). While the initial weight loss occurs quickly, maintaining it proves difficult, with most people regaining lost weight, sometimes exceeding their starting weight.

This ‘weight cycling’ or ‘yo-yo dieting' may worsen health outcomes, create psychological distress, and potentially increase long-term weight gain more than maintaining a stable weight. Repeated attempts at crash dieting are also associated with eating disorders, negative body image, unhealthy relationships with food and weight, and other mental health issues.

Physical side effects of crash dieting include: fatigue, weakness, dizziness, headaches, irritability, constipation, hair loss, menstrual cycle irregularity, and, in severe cases, gallstones, dehydration, and heart problems. 

Loss of muscle comes with rapid weight loss, which reduces your metabolic rate, metabolic health and makes subsequent weight loss (or maintaining weight) more difficult. Practical adherence to a crash diet is poor, which is really a benefit in disguise. The restrictive nature of the diets, the monotonous food choices, hunger, social compatibility, and psychological burden make crash diets unsustainable in the medium to long term.

Bottom Line

Rapid weight loss comes at a cost: nutritional deficiency, muscle wasting, deteriorating metabolic health, potentially mental health and eating disorders. Outside of professionally supervised, specific and urgent medical emergencies, crash diets offer little long-term benefits and likely harm your health. Avoid them at all costs.

Sources

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Del Corral P et al. Effect of dietary adherence with or without exercise on weight loss: a mechanistic approach to a global problem. J Clin Endocrinol Metab. 2009 May;94(5):1602-7. PMID: 19258409

Grigg M et al. Disordered eating and unhealthy weight reduction practices among adolescent females. Prev Med. 1996 Nov-Dec;25(6):748-56. PMID: 8936578

Nordmo M et al. The challenge of keeping it off, a descriptive systematic review of high-quality, follow-up studies of obesity treatments. Obes Rev. 2020 Jan;21(1):e12949. PMID: 31675146

Scragg J et al. Feasibility of a Very Low Calorie Diet to Achieve a Sustainable 10% Weight Loss in Patients With Nonalcoholic Fatty Liver Disease. Clin Transl Gastroenterol. 2020 Sep;11(9):e00231. PMID: 33094956

Steven S et al. Very Low-Calorie Diet and 6 Months of Weight Stability in Type 2 Diabetes: Pathophysiological Changes in Responders and Nonresponders. Diabetes Care. 2016 May;39(5):808-15. PMID: 27002059

Tsai AG, Wadden TA. The evolution of very-low-calorie diets: an update and meta-analysis. Obesity (Silver Spring). 2006 Aug;14(8):1283-93. PMID: 16988070

Further Reading

Fad Diets: A Review

Low-Fat Diets: A Review

Mediterranean Diet: A Review

Published: June 10, 2026

Lead Author: R.Davies, PhD, MRes, BSc, CPT, FHEA | Author Bio
Dr Davies is a physiologist specialising in human health, performance and nutrition.

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