Crash diets promise rapid weight loss, but they often do more harm than good. These diets are not sustainable and can lead to various health problems. While there's no official medical definition of a crash diet, it's generally understood as a weight loss approach undertaken on an urgent, short-term basis to achieve very rapid and significant results.
What is a Crash Diet?
A crash diet involves a severe limitation of food intake or food selection intended to help someone lose a lot of weight very quickly. Marketing teams are too clever to call any diet plan a crash diet. Instead, they promote it with vastly unrealistic promises and positive language. Red flags include phrases such as ‘liquid diet’, ‘cleanse’, or ‘detox’. Our bodies don’t need help to get rid of toxins; they’re armed with a liver and kidneys to do that for us!
The Problem with Rapid Weight Loss
Although you lose weight quickly on a crash diet, you cannot lose body fat quickly. The body values stored fat and surrenders it reluctantly because body fat helps ensure survival. When there is too little food available to the body, it becomes more frugal in expending energy by resetting the metabolic rate to a lower level.
How Crash Diets Impact Your Metabolic Rate
On a crash diet, you reduce caloric intake, and the greater the reduction, the quicker the drop in body weight. However, when there is too little food available to the body, it becomes more frugal in expending energy by resetting the metabolic rate to a lower level. In other words, you burn fewer calories per minute when at rest.
The situation gets worse and more challenging for the body as it sheds muscle mass. When food and energy intake is too low, the body searches for a source of glucose because it is the main fuel for the brain, and it finds an unlikely source in the muscles. The hormone cortisol is sent out to the muscles, breaking them down to their constituent proteins which are further broken down to amino acids. The structure of some amino acids is similar to glucose, and these are taken to the liver and converted to glucose. Increased availability of glucose solves the problem but at the expense of losing muscle mass.
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Muscle mass makes up approximately 30-40% of the total body mass in a typical adult. That’s a huge amount and it accounts for most of the body’s metabolic rate because it takes a lot of energy to feed your muscles, even when they are at rest. Less muscle mass means less energy expenditure and a lower metabolic rate. This is the dirty little secret behind losing weight quickly. You lose a bunch of muscle mass, which means you shed pounds quickly because one pound of muscle yields only 700 usable calories, whereas one pound of fat yields a whopping 3,500. As a result, you can lose muscle five times faster than you lose fat.
The Rebound Effect
When you abandon a crash diet and eat as usual again, the body typically goes back to where it was, rebuilding lost muscle mass and increasing metabolic rate to normal levels. However, it takes time, and during the lull in metabolic rate combined with eating as usual, you likely will gain body fat. Therefore, as you bring back muscle mass plus additional body fat, you end up heavier and fatter than you were before going on the crash diet.
Contestants from the TV program "The Biggest Loser" were studied scientifically to determine the effects of a huge weight loss in a short period of time. It was reported that the metabolic rate of contestants fell dramatically as their body weight plummeted. A research study published in the journal "Obesity" took it a step further and examined how long it takes after the diet is over for the metabolic rate to rebound and return to normal.
On average, subjects in this research study lost nearly 40% of their body weight. This equated to an average loss of 128 pounds (from 327 pounds to 199). Accompanying the weight loss, subjects experienced a drop in their metabolic rate of 23%. Now, fast forward six years. What happened? Subjects were struggling mightily to keep the weight off, and most found themselves back to their original weight or close to it, and some were even heavier. That’s bad news, but worse, the metabolic rate was measured again and was still well below where it was prior to losing the weight. In other words, they were burning fewer calories per day, which makes managing their weight more difficult.
Health Risks Associated with Crash Diets
Crash diets can cause transient deterioration in heart function. Rapid weight-loss (3 pounds or more per week in this case) can significantly increase the risk of forming gallstones. Up to 25 percent of individuals who use diets that are very low in calories (800 or less) develop gallstones. Experts think this is from changes in fat metabolism that happen with very rapid weight-loss.
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When someone loses a lot of weight, some part of that weight is always likely to be lean body mass (muscle and bone) rather than fat. Without intervention, a VLCD can produce a greater than 25 percent loss in lean body mass. This is important for multiple reasons including the fact that loss of bone tissue can contribute to later-life frailty and loss of muscle harms both metabolism and strength.
We get our nutrition (protein, fats, vitamins, minerals etc.) from the food we eat. When you dramatically reduce what you eat, it can be very hard to meet your nutritional requirements. Additionally some crash diets may severely restrict the types of food people eat (no carbohydrate, no fat, no dairy foods, and so on) making it more challenging to get complete nutrition.
When you drastically reduce calories, even if you need to lose weight, your body thinks you are starving. As a protective mechanism, your body slows your metabolism - but this makes it harder to keep losing weight. It also means that when you start adding more calories, you can regain weight very quickly.
Total Diet Replacement (TDR) as an Alternative
Total diet replacement (TDR) is a nutritional tool to support weight loss that is often misunderstood to be a crash diet. While it does help the user lose a significant amount of weight quickly, TDR is different to most crash diets because it is an incredibly powerful, safe, method of achieving sustainable, long-term weight loss-and we’ve got the science to prove it.
TDR is a form of low-calorie diet that delivers everything your body needs nutritionally while keeping daily calories to around 800. Often confused with meal replacements, TDR can be used in this function but it's important to note that meals labelled as ‘meal replacements’ are not nutritionally-complete-that is, they do not provide the level of nutrition your body needs to function healthily. TDR diets involve powdered meals-think porridges, soups, and shakes-that are typically around 200 calories in total. Four of these meals a day bring total daily calories to 800. At Habitual, patients follow a TDR diet for 3 months before gradually reintroducing food over the following 3 months.
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One study determined that consuming food as liquid tricks the stomach into thinking it’s more full than it really is-blend your meal and you’ll feel fuller than if you ate it. Another concluded that losing weight through a combination of TDR and weekly support from a counsellor is more effective than attending a series of appointments with a nurse, highlighting the need for access to quality, regular support during weight loss journeys.
One of the most surprising moments for people on TDR is when they realise that they don’t feel hungry. The first few days are certainly an adjustment period for your body and mind as they get used to living off fewer calories and the change in routine but after this, your body, incredibly, adapts. TDR also puts the body into a mild state of ketosis (not diabetic ketoacidosis!), which contributes to this feeling of fullness.
Why TDR is Not a Typical Crash Diet
There’s a common misconception that TDR is unacceptable, unsafe, and leads to rapid weight gain as a result of the speed of weight loss. But, scientific results are in fact showing the opposite. In one study, researchers found that the rate of weight loss doesn’t affect the speed of weight regain-if the weight is going to return, it will take the same amount of time regardless of how quickly it was lost in the first place. Another was the first study to look into the effectiveness and safety of TDR in a large group of adults and found that participants on TDR lost an average of 11kg over the course of a year, while the group following routine care lost just 3kg.
To protect against weight regain, behaviour change and gradual food reintroduction are essential. The latter is because the dramatic reduction of calories during TDR slows down your metabolism, meaning that eating food in the same way as before TDR would leave you susceptible to weight gain. Every research paper we turn to is in agreement-TDR works best if used in conjunction with behaviour change. The time on TDR is the perfect opportunity for a reset, giving your mind and body a break from thinking about food, cooking food, and planning what to eat. By using the time you free up to create new habits, learn about nutrition, grow your confidence, and start changing your relationship with food and exercise, you’ll be ready to build a brand new lifestyle for yourself when the TDR period finishes. The gradual reintroduction of food goes on to support this as you get into the habit of preparing and eating healthier dishes. It’s the behaviour change that keeps the incredible weight loss off once TDR finishes and having seen hundreds of patients go through it, we can confidently say that it works.
TDR and Type 2 Diabetes
The amount of weight that’s possible to lose as a result of TDR has been shown to reverse the disease process underlying type 2 diabetes, putting the condition into remission. This is a life-changing discovery that could change the nature of type 2 diabetes care, improving patient health and saving the NHS vast amounts of money. Not only is TDR more successful in helping people lose weight than other treatments, its capacity to reverse type 2 diabetes reduces the number of people both needing medication and experiencing complications of the condition.
Following the astounding results from the DiRECT trial, the team are now exploring whether TDR can be used to reverse type 2 diabetes in people who don’t need (or shouldn’t!) to lose a substantial amount of weight. In the study, called ReTUNE, the team investigated whether people with type 2 diabetes who are of a healthy weight can enter remission following 2-week cycles of TDR. As they are of a healthy weight, 3 months of TDR wouldn’t be safe-they simply don’t need to use that much weight-but a short burst could be enough to reduce the amount of fat in the liver and pancreas (it’s this internal fat that affects natural blood sugar regulation). ReTUNE has only just finished but the early results look promising-from the first data collection, participants on the TDR programme lost weight and fat from their liver and pancreas, and two-thirds entered remission.
Gradual Weight Loss: A Sustainable Approach
If losing body fat is your goal, lose it slowly, at the rate of only a couple of pounds per month. Keep your metabolic rate high and preserve muscle mass by making only a subtle reduction in caloric intake. Gradual diets have been shown to be more sustainable and have a less negative impact on your metabolic rate compared with crash diets. Gradual diets can also help maintain energy levels enough to exercise, which can help you lose weight. These types of diet also preserve the function of our mitochondria - the calorie-burning powerhouses in our muscles. The ideal diet is one that reduces body weight by around 0.5 to 1kg a week.
Fats and carbohydrates use fewer calories to power digestion, compared with protein. Indeed, high-protein diets increase your metabolic rate 11-14% above normal levels, whereas diets high in carbohydrates or fats can only do this by 4-8%. As such, try to ensure around 30% of your day’s calories are made up of protein when trying to lose weight. High-protein diets also help you feel fuller for longer.
Lifestyle Changes for Long-Term Health
Aim for lifestyle changes that switch your mindset to eating for health not size. This approach includes supporting and acknowledging your relationship with food, being flexible with eating, and accepting and welcoming all foods that support our physical health and emotional health. We can all be healthy in our own way and time, especially with the individualized help of our health care team.