Extreme Crash Dieting
Do Crash Diets Work?

Crash diets – that ever popular way to drop some pounds – fast.  And a dieting fashion that seems set to remain as shrinking celebrities promote the message that ‘thin is the new black’.  But do crash diets really work, or do they do more harm than good?

Crash Diets Under The Spotlight

By Dietitian Lyndel Costain BSc RD

What Are Crash Diets? 

We’re talking very restrictive, short term diets with a rigid set of rules which focus on a few foods and promise fast weight loss for little effort.  Crash diets are different from medically monitored very low calorie diets, which are nutritionally complete (just low in calories) and require some monitoring from a health professional.

The so-called ‘British Heart Foundation Diet’ is a classic example.  It’s a three day diet that claims you’ll lose 10lbs and “works on a chemical breakdown and is proven.”  It involves controlled amounts of foods like grapefruit, peanut butter, tuna, eggs, beetroot, crackers and ice cream.  Tempted?   What it doesn’t tell you is that the meal plans provide about 800 calories a day and it has nothing to do with the British Heart Foundation.  Much of the weight you lose will be fluid, so when you eat normally again, you’re back to square one.

Then there’s the Cabbage Soup Diet.  First arriving on desks and dinner tables via the fax or internet.  It’s based on the supposedly “fat burning” cabbage soup, but again it’s just a very restrictive, low calorie diet.  This diet was said to come from the Sacred Heart Memorial Hospital in the USA – a hospital that doesn’t exist.  This is a diet that so incensed the American Heart Association that they ‘declared war’ on crash dieting, which they say “can undermine people’s health, are monotonous so can’t be followed for long, cause physical discomfort, and lead to disappointment when people regain the weight soon after.”   

The maple syrup diet is another example, along with the hCG Diet which will also have you injecting hormones into yourself!

Does Crash Dieting Cause Weight Loss?

Crash diets are low calorie diets, and we lose weight when we consume fewer calories than we burn, no matter how weird or sensible the diet.  But much of the weight lost on a short term crash diet is fluid.  Very restricted diets, especially those low in carbohydrate, cause the body to use up carbohydrate stores, known as glycogen, in the liver and muscle. Glycogen is stored with three times its weight in water which is also lost, and can lead to a 4 to 5lb loss on the scales in days.  But once normal eating resumes, glycogen and fluid are rapidly restored and weight goes back up.  To lose a pound of true body fat you need to eat 3,500 calories less than usual – ‘usual’ may be 2000 to 3000 calories – so it’s not physically possible to lose 5-10lbs of fat in days!

How Does A Crash Diet Affect Health And Metabolism?

A balanced diet includes foods from the four main food groups: fruit and vegetables; breads, potatoes and other cereals; milk and dairy foods; and meat, fish and vegetarian alternatives.

A crash diet is so restricted it’s nutritionally inadequate. Think of the ‘Egg Diet’ where copious amounts of preferably hard boiled eggs (you apparently but incorrectly use more calories digesting them) are eaten, and not much else.  Even the book version of the Cabbage Soup Diet carries warnings not to follow it for too long as it’s not healthy and balanced.  Strict low carb diets lack nutritious foods like wholegrain breads and cereals, fruit, pulses and yogurt, and detox diets which exclude wheat, meat and dairy can be low in iron (to prevent anaemia), zinc (for healthy skin and immunity) and calcium (for strong bones). The saving grace is that these diets are so boring and rigid, most people are unlikely to follow them long enough to cause true nutritional deficiencies.  But following them frequently could affect your bone health, and increase the risk of iron deficiency and anaemia which cause lethargy, poor concentration and irritability. 

In fact, a study in the American Journal of Clinical Nutrition by Dr Marta Van Loan of the US Agricultural Research Service, concluded that healthy women who are concerned about their weight and thus restrain their eating and nutrient intake, have lower bone density than ‘normal eaters’, and may increase their chances of developing osteoporosis.

Whenever we lose weight, most of the weight lost is fat but we also lose some lean muscle tissue.   This loss can be minimized by combining gradual weight loss with regular physical activity.  Crash diets fly in the face of this advice.  But there is no evidence that frequent dieting dramatically changes body composition or permanently lowers metabolic rate. 

Psychological Effects of Crash Dieting

As well as affecting our body, crash dieting affects our mind.  Crash dieters tend to get into a cycle of yo-yo dieting and lose confidence in their ability to lose weight.  Dieters feel they have failed, yet the rigid, extreme diet is the failure as it’s impossible to follow for long.  Repeatedly following crash or rigid diets can also increase the risk of binge eating, and developing “all or nothing” thinking where people feel they have failed if they eat a ‘bad’ food and totally relapse and keep on eating.  

Crash diets can encourage the belief that ‘slim equals success’ or that life will be better if you simply lose weight by any means, even if it’s unhealthy.  Crash diets are short term and uni-dimensional and don’t help people address the real obstacles to losing weight and keeping it off.  I always ask people considering a crash diet to look at their past record of dieting success and failures to help them decide whether it really is worth doing.

Crash Diets and General Well Being

If crash diets were a form of medication they wouldn’t be allowed because of the side effects. A crash diet is essentially semi-starvation which is associated with decreased concentration, food preoccupation and a strong desire to eat, feeling tired, anxious and low – so hardly beneficial for our general well being.

Do Crash Diets Help you Keep the Weight Off?

Crash diets try to lure us with testimonials and anecdotal success stories.  But they lack all important scientific research.  Reviews have assessed a wide range of popular diets and concluded that lower fat, balanced diets are associated with the best long term results.  They are the most nutritious, help other aspects of health such as controlling cholesterol and blood pressure, and help regulate appetite. 

Keeping up new healthy habits was also strongly linked to psychological issues such as: coping with emotional eating; getting regular support; keeping food diaries; confronting problems directly; believing that obesity is a problem that can be managed and using personally developed, flexible eating plans.  Getting support and developing flexible restraint are vital for long term weight loss.  Short term crash diets are all about rigid restraint and undermine healthy weight control messages. They reinforce a mind set of ‘strict dieting’ rather than gradual, sustained lifestyle change which is so important for long term success.

Dos and Don’ts of Healthy Dieting

  • Set realistic goals and aim to lose no more than 1 to 2 pounds week
  • Eat regular meals
  • Choose a variety of healthy foods from the main food groups
  • Develop your low fat cooking skills and eat plenty of vegetables and fruit
  • Keep a food and thoughts diary to help you stay conscious of what you eat
  • Adopt a ‘more or less’ rather than ‘all or nothing’ approach to eating
  • Learn how to cope with feelings, not feed them
  • Be active in your daily life and include a regular activity that you enjoy
  • Get some ongoing support


  • Give up if you feel you’ve had a bad day – the lapse isn’t the problem but your reaction to it could be
  • Get impatient with gradual weight loss – it’s more likely to end in long term success
  • Just use appearance as your motivator to lose weight – health is vital too
  • Be lured by the claims for rapid weight loss offered by fad and crash diets – they are the failures, not you

Dos and Dont’s Crash Dieting

  • Avoid them
  • If you really do want to try one remember there is no quick-fix to weight loss.  They may give a kick start but you must be able to make healthy lifestyle changes to keep losing and/or keep the weight off.
  • Believe they are the answer to all your problems
  • Think they help you lose weight magically – they’re just low calorie diets
  • Hope they will make you slim.  Most only cause a small amount of fat loss

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