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How can crash diets affect your metabolic flexibility?

If you've been tempted to go on a crash diet to help you lose weight, you may want to think twice. Although crash diets can help with rapid weight loss in the short term, the mechanism of metabolic flexibility could result in long-term consequences on your waistline.

Your metabolism

You've likely heard that your metabolism can be 'fast' or 'slow' at burning calories from food. By reducing its function to this simplified understanding, your metabolism isn't getting the credit it deserves.

Metabolism is the complex process by which your body converts what you consume into energy. Through chemical (metabolic) reactions, calories in food and drink are combined with oxygen to create and release the energy you need to live.

This is far more expansive than its role in weight loss or weight gain. This energy fuels everything your body does that keeps you alive, including breathing, regulating body temperature, circulating blood, growing and repairing cells, and digesting food.

What is metabolic flexibility?

Your weight changes when there is an imbalance between energy intake and energy expenditure. For example, when you consume more calories per day than you can burn, you gain body mass (known as positive energy balance) and conversely when you consume less than you can burn you lose body mass (negative energy balance).

Metabolic flexibility describes how your metabolism can adapt to changes in energy demand. For example, in times of:

1) Excess calorie intake (overeating).
2) Restricted calorie intake (undereating).
3) High energy output (during moderate to intense physical activity).

The ability of your body's metabolism to be flexible means that it can use whatever fuel source is available to it, whether it's fuel from the carbohydrates or protein in your food or the fat already stored in your body.


Metabolic flexibility is an essential biological survival mechanism. Metabolic flexibility finds the energy to perform the essential basic functions you need to live - like breathing or walking - in times of low or no food consumption.

Benefits of this metabolic flexibility include:

  • Sustained energy.
  • More stable blood sugar levels.
  • Fewer cravings.
  • Adaptable fat-burning.
  • Improved muscle-building.

Crash diets and slower metabolism

Crash diets aim to cause rapid weight loss by ensuring that your body is taking in far less energy than it's using. As well as being potentially dangerous to physical and mental health, this is an over-simplified and flawed concept. In part, this is because extreme low-calorie diets don't account for metabolic flexibility.

Slower resting metabolic rate

One important component of your metabolic flexibility is your resting metabolic rate (RMR), also known as resting energy expenditure. This is the rate at which your body burns energy (calories) while it's not active (such as during sleep or while watching TV). It accounts for 60-75% of total energy expenditure when you're at rest.

Weight loss - and particularly significant, rapid weight loss - can cause your RMR to decrease as your body deliberately tries to preserve energy stores. This means you burn fewer calories during rest, resulting in what is commonly termed a 'slower metabolism'.

"Research has shown that resting metabolic rate (RMR) is suppressed after weight loss and that this effect could last for years," explains Rachel Ball, registered British Dietetic Association (BDA) freelance dietitian.

Because of metabolic flexibility, there is evidence of moderate reductions in RMR following weight loss, accounting for around 10-15% of a decline in 24hour energy expenditure. In one study, participants who lost 10-20% of their body weight following a crash diet saw an RMR decrease by 3-4 kilocalories per kilogram (kcal/kg) of fat-free mass per day.

This substantial loss of fat-free mass (FFM) primarily refers to muscle mass lost through a restrictive diet. This can significantly slow your RMR as muscle tissue is thought to be metabolically more active than fat tissue, contributing to around 20% of your total calories burned per day compared to just 5% for fat tissue.

Essentially, a lower RMR means that you are less efficient at burning excess body mass. This is particularly true when you don't attempt to preserve and build muscle through strength exercises.

Unsustainable weight loss

As a result, you could end up finding it harder to maintain your ideal weight once your crash diet ends: "Weight loss caused through unsustainable dietary changes will only last as long as you stay on the diet, and many crash diets are simply not designed to be followed long-term," says Ball.

"So after the weight loss, there will likely be weight regain. You end up back where you started, or maybe a little heavier."

Once you've lost a significant amount of weight, your body continues to utilise metabolic flexibility to protect itself against the possible negative effects of calorie restriction. For example, by burning fewer calories your body attempts to prevent stores of unused calories (triglyceride stores) from getting too low. This is a risk for certain basic functions like reproduction.

"If the weight is regained, the RMR may remain at this suppressed state. For example, if an overweight person had an RMR of 2000 kcal/day and they lose 10 kg, their RMR may reduce to 1900 kcal/day. However, if they regained the 10 kg, the RMR stays at 1900 kcal/day."

Crash diets versus healthy weight loss

It should be noted that not all research around weight loss and metabolism shows changes in RMR. This may reflect differences between studies in factors such as the degree of weight loss and time spent undertaking physical activity during and following weight loss.

This said, as crash diets involve consuming far fewer calories than your body requires, this form of extreme dieting is likely to trigger greater changes in RMR than healthy dieting. The low-calorie intake prompts the mechanisms involved in metabolic flexibility to protect your body by preserving as much energy as possible.

Making the distinction between crash diets and healthy weight loss is important. Healthy weight loss involves losing excess weight to reduce your risk of health complications related to being overweight or obese. While metabolic flexibility will take place with any dietary change, the results will be much more sustainable in the long term.

As Ball advises: "A quick fix is always tempting and if a diet has an end date, it may be even more appealing. However, a truly healthy lifestyle won't need - and shouldn't have - an end date."

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