There is nothing inherently wrong with crash diets. As always, however, context is key.
They are a tool. Much like a chainsaw. And like chainsaws, they are useful in certain situations but dangerous in the wrong hands.
They allow you to diet for a much shorter period of time and then get back to eating in a more sustainable way. Note that I didn’t say “go back to eating normally”. Eating ‘normally’ in our obesogenic modern world is usually what got people into a pickle in the first place.
In essence, a crash diet is a severe calorie deficit or a very restrictive way of eating for a short period of time to achieve results, fast.
Crash diets can include any types of fasting, very-low-calorie diets (VLCD) or protein-sparing modified fasting (PSMF).
People also often use ketogenic, very low carb or even vegan diets for the same reason.
What makes it a crash diet is the fact that people tend to flip into it overnight, go in hard relying on willpower alone and then end up at huge risk of rebound.
The definition is less about the precise method employed and more to do with the context. If people are aiming for rapid fat loss and not expecting to sustain it in the long term then it becomes a crash diet. This includes the kind of fad diets you find in magazines.
Please note that this does not apply to people following diets (such as the ketogenic or vegan diet) and are sustaining it long term as a lifestyle choice.
What tends to happen when people attempt to crash diet, is that they may or may not reach their goal (it depends if willpower holds out or not) and then they rebound HARD because they have no idea how to sustain their weight loss.
To be clear – the end goal is not for this health thing to be an eternal battle of willpower. Eventually, the new lifestyle should be EFFORTLESS (more on this at the end). If you’ve read Atomic Habits by James Clear you’ll know what I’m talking about.
So, in what context can a crash diet be useful and should you do one?
Crash diets can help you drop weight quicker, which of course EVERYONE wants. However, without adequate knowledge and a get-out plan (see below) you are destined to fail.
Should you do one? Probably not. Especially if you’ve attempted them in the past and failed or rebounded.
However, in the right context, they can be a useful tool. Remember, the goal is not necessarily for the diet to be sustainable, we just want the FAT LOSS to be sustainable and crash diets make this tough because 1) We never learnt how to eat to sustain the fat loss and 2) the cravings and hunger usually build up over time and then we can’t control ourselves.
So what is the solution?
I will say again, the best option is to not crash diet. Full stop.
However, if you WERE to incorporate a crash diet then my advice would be to start with a moderate calorie deficit and healthy lifestyle change (exercise etc) then add periods of more severe deficit to accelerate the fat loss before returning to your baseline (moderate deficit) and repeating the process.
The key is to learn how to sustain the lifestyle change and build the mindset and identity required to do this. Essentially integrate the new you and pre-empt the skills you’ll need when you reach your goal.
For example, do 2-4 weeks of moderate fat loss and lifestyle improvement followed by 2 weeks of ‘crash diet’ then back to 2-4 weeks of moderate fat loss or even maintenance. The exact time frames will depend on the individual.
The biggest mistake is that people push too hard and once they have a taste of rapid fat loss they don’t want to back off. You won’t know what “too hard” is until you try it so I say to err on the side of caution.
The more weight you have to lose and the less stress and other commitments you have, the longer you will be able to tolerate the severe deficit. I won’t go into too much detail here as it’s a whole book in itself.
And how do you ‘get out’ of it?
Essentially, the ‘moderate calorie deficit’ is the foundation of the ‘sustainable lifestyle’ and forms the basis of the ‘get-out plan’. It gives you a bridge to long-term lifestyle change at the end of the fat loss phase.
In bodybuilding, this is called a reverse diet and it is a psychological crutch where you remain in ‘diet mode’ for a period even though you’ve reached your goals.
It allows you to bring calories up, establish your new maintenance calorie level and start to remove the negative habits, emotions and behaviours associated with dieting in a gradual way (e.g. cravings, hunger, tracking food etc.)
So that’s how I would run it.
Are crash diets for everyone?
Hell no.
Are they for some people?
Usually not.
Do I use them often on myself or my clients?
Rarely.