16th July 2019

Crash diets: Do they work?

We all know that crash diets don’t work. You lose the weight, but then you put it all back on so that you end up heavier than before.

But is that really true?

Almost certainly not. There is in fact a growing body of clinical evidence showing that, if you do it properly, rapid weight loss diets, commonly referred to as crash diets, don’t just work in the short term, but in the long term as well.

Worries about crash diets are largely based on studies done in the 1970s, with diets that were very low calorie and contained poor quality protein.

In recent years a number of key studies have begun to transform medical understanding of obesity, and how to tackle it.

The right sort of rapid weight loss diet can be very motivating, because you are seeing huge change. You will not only see the scale move, but if you have high blood sugar levels of high blood pressure, you will see these swiftly fall, drastically reducing your risk of some of the most pervasive diseases of the modern age, including diabetes, heart disease and cancer.

Here, we debunk the myths around rapid weight loss diets, and set out evidence that shows just how powerful they can be.

Myth #1: Crash diets will put you into Starvation Mode

Fact: If done properly-then a low calorie diet will, at least initially, speed up your metabolism

“Starvation mode” is one of the commonest diet myths of them all. The claim is that if you try to lose weight fast, your body will respond by shutting your metabolism down in a desperate attempt to preserve itself. The origins of this particular myth is the Minnesota Experiment. Conducted during the 1940s, this was a medical study of the effects of long-term starvation on a group of conscientious objectors in the US. They were kept in a state of semi-starvation for 24 weeks and fed a low-nutrient, high-carbohydrate diet of potatoes, bread, swedes, turnips and macaroni.

After a period of prolonged starvation, tests found a marked drop in metabolic rate – the rate at which the body burns energy while resting – among the volunteers1.

The problem with applying the results of the Minnesota Experiment to all rapid weight loss diets, however, is clear. Volunteers in Minnesota were kept on a low-calorie, low protein, low-nutrient regime: by the end of the study, they were seriously deficient in vitamins, minerals and proteins.

By contrast, low calorie diets today, such as those successfully pioneered by the Direct and Droplet trials, are carefully designed to ensure optimal nutrition. They are completely different to the Minnesota study and designed to ensure that you maintain muscle mass.

In fact, far from slowing down your metabolic rate, a low-calorie diet can boost your metabolism, at least in the short term.

In a recent experiment2, for example, researchers took 11 healthy volunteers and asked them to stay in a metabolic chamber (a room in which scientists can precisely measure your metabolic rate) while consuming nothing but water for three days.

By day three, their metabolic rates had risen by 14 per cent. This was probably due to a rise in a hormone called noradrenaline, which is known to burn fat, and which, some studies suggest, can rise together with other alertness-boosting chemicals during the first days of a very low calorie diet.3 4

Intriguingly, Mark Mattson, Professor of Neuroscience at the National Institute of Ageing, has suggested that a short-term rise in the body’s own stimulants may be an evolutionary response to a drop in food supply, helping people to search for resources as they grow more scarce.5

Myth #2: After a crash diet, you’ll put all the weight back on

Fact: A low calorie diet can be the best way to avoid this outcome

The claim that crash diets do not work also often quotes a study published by UCLA in 20076, which found that within five years of completing a diet, between one-third to two-thirds of dieters regain more weight than they have lost. The problem with quoting the UCLA study is that it based its findings on a review of 31 diets designed around standard weight loss advice.

What the study actually says, then, is that people who go on normal diets usually end up putting the weight back on. This matters: if you are going to commit time and effort to losing weight, you should be confident that your method is going to deliver lasting results. And conventional diets, as the 2007 study shows, do not.

Low calorie diets work differently. They allow your body to draw on fat reserves held deep within the abdomen – stores that, under conventional dieting conditions, are burned more slowly. It is this tissue, termed “visceral fat”, that helps to maintain insulin resistance, which is a major driver behind food cravings. As your visceral fat melts away, your cravings abate – and more weight comes off, and stays off.

Crash diets work: The proof

The Direct Trial

In this study, termed “Direct”7, Professors Roy Taylor and Mike Lean took 298 people with Type 2 diabetes and randomly allocated them either to an 800-calorie “rapid weight loss diet”, to be followed for up to 20 weeks, or to standard care. Those on the 800-calorie diet not only lost weight fast, they were, at the end of a year, on average 10kg lighter. Even more impressively nearly half had restored their blood sugars without the use of medication. Those receiving standard advice, by contrast, lost on average just over two pounds, and only four per cent of this group had put their diabetes into remission.

The Droplet Trial

In 2019, obesity expert Professor Susan Jebb, of Oxford University, authored an extensive report8 on the best way in which to approach the escalating obesity crisis.

The report included the results of a second study, the Droplet trial9. Like Direct, Droplet tested an 800-calorie regime, lasting eight weeks. Participants then underwent a behavioural support programme.

After one year, Professor Jebb discovered that the 800-calorie “rapid weight loss diet” group – all of whom were originally obese – had lost, and kept off, an average of 10.7 kg, compared to 3.1 kg in obese individuals on a standard diet.

These results were, in the words Professor Jebb, “phenomenal – extraordinary – like nothing we’ve seen in primary care before.”

Although rapid weight loss is not suitable for everyone, Professor Jebb thinks more doctors should be prescribing 800-calorie diets for people in need of dramatic weight loss.

Short-term weight loss can help motivation

So why is rapid weight loss, when done properly, so much more effective than the slow and steady approach that is routinely recommended? According to a recent Australian study10, the success of rapid weight loss is largely down to the psychological boost people get from seeing serious results, and seeing them quickly.

Researchers took 200 obese volunteers and put half on a very low calorie diet (fewer than 800 calories per day) for 12 weeks. The other half cut their calories by 500 a day (enough to lose a pound a week) for 36 weeks.

Frustrated by slow progress, fewer than half of the steady dieters made it to the end. More than 80 per cent in the rapid weight loss programme, however, stuck to it.

The very low calorie diet was significantly more effective: 81% of those following it managed to lose at least 12.5% of body weight, compared to just 50% on the gradual programme.

Although both groups put some weight back on over three years, those who completed the crash diet did not put on extra weight compared to those who completed the steady programme.

Dietician Katrina Purcell, study leader, said: ‘Achieving a weight loss target is more likely, and drop-out lower, if losing weight is done quickly.’

Backing up the findings of the Australian study, a review in the New England Journal of Medicine11 cited four separate studies that linked more ambitious weight-loss goals to better weight-loss outcomes than those achieved by slow and steady programs.

In sum: across the board, the rapid weight loss diets won.

Losing weight, fast or slow? The one-minute take

Crash diets have endured a bad rap down the years. And yet the evidence shows that when done properly, with correct nutrition, rapid weight loss diets work. Fast reductions in belly fat, cravings and weight – along with increased levels of alertness and energy – make powerful motivators, leading to the spectacular results of the Direct and Droplet trials.

These studies are transforming the way we think about weight loss and crash diets. So, how could a low-calorie diet transform you?

But don’t just take our word for it. Find out more about real-life stories of people who have a whole new life to look forward to – and who haven’t looked back.

The Fast 800 testimonials – click here

Tom Watson article – click here

Jimmy Kimmel article – click here

Low calorie diets may not be suitable for all people, particularly those who are under 18, pregnant or breastfeeding, with a history of eating disorders or currently on medication. Before embarking on a low calorie diet, we recommend seeking advice from your GP.


Müller, M.J., Enderle J. et al (2015)Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisitedThe American Journal of Clinical Nutrition, Volume 102, Issue 4, October 2015, Pages 807–819 https://doi.org/10.3945/ajcn.115.109173, available at <https://academic.oup.com/ajcn/article/102/4/807/4564599>, accessed [19.06.19]

Zauner C et al. (2000), Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrineThe American Journal of Clinical Nutrition, Volume 71, Issue 6, June 2000, Pages 1511–1515, https://doi.org/10.1093/ajcn/71.6.1511available at <https://www.ncbi.nlm.nih.gov/pubmed/10837292> accessed [19.06.19]

Chan, J.L., Mietus J.E., Raciti P.M., Goldberger A.L., Mantzoros C.S (2007), Short-term fasting-induced autonomic activation and changes in catecholamine levels are not mediated by changes in leptin levels in healthy humans, Clin Endocrinol (Oxf) 2007 Jan; 66(1): 49-57 – https://doi.org/10.1111/j.1365- available at < https://www.ncbi.nlm.nih.gov/pubmed/17201801>, accessed [19.06.19]

Jasper, R.C. (2006), The ‘drive for activity’ and “restlessness” in anorexia nervosa: potential pathwaysJ Affect Disord.2006 May;92(1):99 doi: 10.1016/j/jad.2005.12.039 available at https://www.ncbi.nlm.nih.gov/pubmed/16448703accessed [19.06.19]

Mosley, M. and Spencer, M., 2014. ‘The Fast Diet’, London: Short Books, p56

Mann, T., Tomiyama, AJ, Westling, E., Lew, AM, Samuels, B., & Chatman, J. (2007). Medicare’s Search for Effective Obesity Treatments: Diets Are Not the Answer. American Psychologist, 62(3), 220-233. http://dx.doi.org/10.1037/0003-066X.62.3.220 available at https://escholarship.org/uc/item/2811g3r3#author, accessed [19.06.19]

Lean, M.E, Leslie W.S. et al (2018), Primary care-led weight management for remission of type 2 diabetes (DiRECT): an open-label, cluster-randomised trialLancet. 2018 Feb 10;391(10120):541-551. doi: 10.1016/S0140-6736(17)33102-1, available at https://www.ncbi.nlm.nih.gov/pubmed/29221645, accessed [19.06.19]

Susan Jebb (2018), Susan Jebb: Interventions to treat obesity work—so why am I not celebrating? The BMJ Opinion, available at < https://blogs.bmj.com/bmj/2018/09/27/interventions-treat-obesity-work-why-not-celebrating/ >, accessed [19/06/19]

Astbury, N.M., Aveyard P., Nickless A., Hood K., Corfield K., Lowe R., Jebb S.A. (2018), Doctor Referral of Overweight People to Low Energy total diet replacement Treatment (DROPLET): pragmatic randomised controlled trial’, BMJ. 2018 Sep 26;362:k3760. doi: 10.1136/bmj.k3760 available at <https://www.ncbi.nlm.nih.gov/pubmed/30257983>, accessed [19.06.19]

Purcell K, Sumithran P et al. (2014) The effect of rate of weight loss on long-term weight management: a randomised controlled trial Lancet Diabetes Endocrinol, 2014 Dec;2(12):954-62. available at <https://www.ncbi.nlm.nih.gov/pubmed/25459211>, accessed [19.06.19]

Casazza, K., Fontaine, K. et al (2013), Myths, Presumptions and Facts about Obesity N Engl J Med 2013; 368:446-454 doi: 10.1056/NEJMsa1208051 available at <https://www.nejm.org/doi/full/10.1056/NEJMsa1208051>, accessed [19.06.19]

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