What is crash dieting? What happens to your body and why it’s a bad idea

Many of us are crash dieting without really knowing it. Here, two nutritionists explain why and how to stop

Pile of bread and toast crumbs on green background to represent crash dieting
(Image credit: Getty Images)

Crash dieting is something that everyone wants to avoid but if you’ve ever tried to lose weight quickly, then chances are, you’ve probably tried one of the many crash diets out there. 

Otherwise known as total diet replacement programs (TDRs), this type of dieting involves cutting back your calorie intake to an extremely low level to achieve a vast calorie deficit in a short period. It’s famously unsustainable, even for a few days, and can often come with unexpected consequences to your long-term health. 

But what does crash dieting actually mean? Plans that teach you how to lose weight in a week or lose a stone in a month have never been more popular, so here, we speak to two nutritionists about crash dieting, whether it works, and the impact it has on your body. 

What is crash dieting?

Crash dieting is where an individual drastically changes their eating habits for a very short period of time, like before an event, explains nutritionist Jenna Hope (opens in new tab). “It’s typically associated with a weight loss goal and often involves a significant reduction in calorie intake or drastic food restriction.”

In some cases, to achieve this radical reduction in energy, diets require participants to forgo solid foods for liquid meals and juices as these naturally contain far fewer calories. 

In practice, crash dieting looks like plans such as the Fast 800 diet, which encourages participants to cut their intake to just 800 calories per day in the beginning, down from the 2000 calories most women are encouraged to have. The juice cleanse, which promotes swapping food for a range of juice alternatives, the 7-day detox plan, and the military diet are all other examples of popular crash diets that supposedly enable people to lose weight quickly.  

Blender full of kale, cucumber and apple to make a green juice, one part of crash dieting

(Image credit: Getty Images)

Does crash dieting work for weight loss?

If you follow a crash diet religiously, you will probably lose weight simply because you’re consuming very little food, says sports nutritionist Rob Hobson (opens in new tab). You’re in a calorie deficit after all, which is the pillar of weight loss. Without this ratio of more calories burned to calories consumed, weight loss is just not possible. It can be achieved, however, through much healthier approaches than crash dieting. 

 "You're better off taking a slower approach over a longer period if you want to lose weight, as you're less likely to put the weight back on," says Hope.

The slower the weight loss, she adds, the more likely exercise and healthy eating are to become part of your lifestyle. This, research by the Medical University of South Carolina (opens in new tab) explains, means you’ll be able to lose weight more easily in the future if that’s your goal and avoid the cycle of dieting but not losing weight. You’ll also drastically reduce your risk of serious conditions such as cardiovascular disease and type 2 diabetes, which in turn lowers your risk of early mortality. 

Crash dieting fosters an unhealthy attitude to food that can lead to serious issues such as disordered eating as well, Rob says. "But there are actually a number of reasons why this approach to weight loss is likely to set you up for failure."

Crash dieting side effects

1. Hunger

This one’s obvious, but it’s the key factor that sets crash dieting up for failure from the beginning. “You may be able to deal with this in the initial stages of your diet but as time goes by, the incessant hunger may lead to relapses. In some cases, this may trigger episodes of binging which is characteristic of yo-yo dieting,” Hobson says. 

“This type of dieting may also impact your mood as you deal with hunger, restriction, and possibly nutrient deficiencies,” he adds, all of which are also more likely to lead to binging tendencies, a review by the University of North Carolina (opens in new tab) points out. 

2. Constipation

“A low energy intake may also lead to low-fiber intake, which in turn could contribute to changes in bowel movement and an increased risk of constipation,” explains Hope. 

2. Nutritional deficiencies

 “Consuming a low-energy, restrictive diet can pose a risk of nutritional deficiencies as there’s less chance you’ll be getting all the nutrients you need,” Hope says.

This could lead to signs of a nutrient deficiency like low iron levels, and a lack of B vitamins and magnesium, adds Hobson. "Drastically reducing your food intake alongside cutting out certain food groups may lead to nutrient deficiencies which can impact your health in many ways," he says. "Low levels of iron can lead to tiredness and fatigue and cause low mood, while a lack of B vitamins and magnesium can also impact your mood and sleep. Low intakes of calcium can also be detrimental to bone density."

The culmination of these together, both experts agree, makes it very difficult to achieve any health goals, weight loss-orientated or otherwise. "The actual impact of such deficiencies is dependent on the time spent extreme dieting but effects may be felt even in the short term," Hobson says. 

Woman counting calories on phone with bowl of fruit

(Image credit: Getty Images)

3. Slower metabolism

When it comes to losing weight, a slow metabolism is the last thing you want. As a study by the National Health and Medical Sciences Institute (opens in new tab) explains, the metabolism is responsible for a number of the body’s processes, including how quickly you burn calories during rest and exercise. 

“When the body feels threatened by a low energy intake, it can conserve energy by dialing back on high energy processes. As such, this can contribute to slowing down the metabolism,” explains Hope. 

But it won’t only affect your ability to lose fat, as when your energy intake is too low, the body will begin to break down muscle mass for energy. “This can also contribute to reducing your metabolism as muscle mass has a greater energy requirement than fat mass,” she says. 

5. Disrupted hormone levels

One of crash dieting’s most worrying long-term impacts is the increase of hunger hormones, agrees the experts. “Crash dieting is unsustainable and wreaks havoc on our hormones,” says Hope. 

How this happens, Hobson explains, is simple. “You can successfully lose weight for a while but at some point, this will stop as your body fights to prevent starvation,” he says. “Research from Christian-Albrechts University (opens in new tab) has shown we have a weight set point, which is the weight your body is programmed to be by the influence of several factors, such as genetics, hormones, behavior, and environment.”

One way the body fights this starvation is by decreasing the levels of leptin (the satiety hormone) and increasing ghrelin (the hunger hormone levels, he says. “This will make it harder to keep weight off in the future for some people.” 

Perhaps even more worryingly though, evidence suggests this isn't just a short-term side effect. As a study by the University of Melbourne (opens in new tab) describes, the severe restriction can seriously disrupt your hormone levels so that you feel permanently hungrier than you did before the diet - yes, really. 

The study looked at 50 overweight or obese people who had been consuming between 500 and 550 calories every day for 10 weeks. Participants lost weight over that period but they regained half the weight they lost, and at the end of the trial, all participants had significantly less leptin and peptide YY hormones, which are involved in creating satiety cues, and more ghrelin and gastric inhibitory polypeptide, which can trigger feelings of hunger. This meant that participants felt hungrier than they did before, even a year after the study ended. 

So whatever the event or situation, crash dieting is just not worth the possible short-term discomfort and future health concerns, especially when it's entirely possible to lose weight healthily over a longer period of time. If you want help with how to lose weight, speak to your doctor. 

Grace Walsh
Health Editor

A digital health journalist with over five years experience writing and editing for UK publications, Grace has covered the world of health and wellbeing extensively for Cosmopolitan, The i Paper and more.


She started her career writing about the complexities of sex and relationships, before combining personal hobbies with professional and writing about fitness. Everything from the best protein powder to sleep technology, the latest health trend to nutrition essentials, Grace has a huge spectrum of interests in the wellness sphere. Having reported on the coronavirus pandemic since the very first swab, she now also counts public health among them.