Weight Loss

Why crash diets are almost always a bad idea

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*For context, this article is aimed at the typical body conscious gym goer who is always on a diet but never gets the results they are looking for*

One day highly restrictive 1,200kcal diets will die and so too will juice cleanses, shake diets and any form of starvation diet. Why? Because they suck arse and could damage your health, and not just your physical health but your mental health too.

Don’t get me wrong you WILL lose weight on a very low-Calorie diet, that’s basic physics. It’s all maths. If you put less energy into your body than your body needs to function the energy you have stored in your liver and muscles (Glycogen) will deplete quickly. For every 1g of stored glycogen you also store around 3g of water – this is one reason why very low carb diets produce rapid weight loss, because you are depleting carb and water stores. After the body has depleted itself of Glycogen you have to get your energy from somewhere, so your body starts to catabolise (catabolism is basically your body eating itself) muscle in order to break down and metabolise amino acids for fuel and THEN your adipose tissue will get catabolised last.

During periods of very low energy intake, such as these silly 1,200kcal crash diets that every commercial gym seems to advocate in January rapid weight loss occurs but the majority of that comes from water and muscle, this results in a loss of lean mass and that means that your resting metabolism reduces. Lean mass, for the record, is your muscles and your bone minerals, so eating a low energy diet can also damage your bone density.

Resting Metabolic Rate

Your resting metabolism is body mass + activity. When you lose weight quickly your body mass reduces but, importantly you lose muscle which means that your Resting Metabolic Rate (RMR) is lower. Therefore, when you hit your target weight and then go back to eating the way you used to not only has your resting metabolism reduced, but even with extra activity, so too has your overall maintenance Calorie level. This is part of the reason why weight rebound after crash diets is common. Although the often quoted 95% rate of weight rebound that seems to have become nutritional lore has no statistical basis to it. But, looking at Weight Watchers as an example we can see that most participants regain weight over the 5 years after completion with more than 80% failing to maintain their target weight (1). Slimming clubs are happy about this, by the way, That’s their business model and they rely on repeat business… Yep, that’s disturbingly unethical.

Resistance training is one way to slow the reduction of lean mass which is why weight training is so often recommended for people trying to achieve fat loss. Higher protein intakes are also necessary to stimulate muscle protein synthesis and ameliorate catabolism, but if your calorie intake is really low this is hard to achieve.

Using myself as an example to illustrate RMR I weigh 76kg and a quick Calorie equation tells me that my RMR is 1,825kcals. So, if I were to put myself on a 1,200kcal diet I would be eating 600kcals below my basal rate of energy requirement. If I kept this up for a few days I would definitely lose weight. But if I kept it up for any length of time I would experience some interesting adaptations.

Metabolic Damage?

Metabolic ‘damage’ isn’t a thing, you can’t permanently break your metabolism through dieting. There are medical conditions which can affect this, but there is something known as metabolic adaptation (3). Which is a reduction in BMR, but this is unlikely and would require metabolic testing to confirm.

“Energy restriction is accompanied by changes in circulating hormones, mitochondrial efficiency, and energy expenditure that serve to minimize the energy deficit, attenuate weight loss, and promote weight regain.” – Trexler et al. 2014

But there is also Adaptive Thermogenesis. This is when your energy output reduces to match your energy input. In other words, if I was to continue to exercise at my usual rate (remember my RMR is 1,800kcals but including activity my total daily energy expenditure (TDEE) over 2,700kcals per day) while only consuming 1,200kcals my body would slow down to match the energy available to it. You may not even notice this but, trust me, you will slow down. Does that make sense?

This is a common problem I see with one to one clients and members of Ben Coomber’s Fat Loss for Life program and many online forums which are full of people experiencing this. The question usually goes something like this:

“Help, I have been eating 1,200kcals for 6-months, I try to do 8-10,000 steps per day and do weights 3x per week, HIIT every morning and Park Run on Saturdays. I’m not losing weight and find it really hard not to binge at weekends and constantly obsess about carbohydrates.”

Here’s why; forcing your body to perform at such a high level while not providing it with the energy it needs to do this will negatively affect hormone balance, as quoted above. Your body will slow down, meaning that you will fidget less and move less in general. You’ll find yourself taking the lift instead of the stairs, sitting instead of standing, you’ll even blink less and speak slower or more quietly! You might not even notice this, it’s usually a subconscious decision. Not only this but you’ll be knackered all the time and your joints will probably hurt because your body is under recovered. This means that you’ll skip the gym more and more frequently and when you do train, because you are so under nourished and so low on energy your workouts will suck. They’ll feel hard, your rate of perceived exertion will be greatly increased so you will feel like you are exercising with more intensity than you actually are. If you are lifting weights, you’ll find the numbers on the bar going down. If you do endurance exercise, you’ll find it much harder to go further for longer and you won’t be getting any KOMs or QOMs on Strava. Your sleep is probably affected, your mood will definitely be affected and, your self-discipline will be affected, and your levels of motivation will go way down. The reason you crave carbs and binge at weekends is because you’re fucking shattered and your body needs energy.Weight-loss-patternIf this was me (in the example I have given so far) my TDEE  should be 2,700 but while eating 1,200 my non-exercise activity and exercise output will drop down until my actual TDEE is much closer to that 1,200kcal intake. Any weight loss would slow down and eventually stall because the size of the energy deficit is now much smaller, and then where do I go from there? Drop my kcals even lower?

Slow versus fast weight loss

As I’ve already mentioned large deficits do work and you WILL lose weight quickly. But they aren’t long-term sustainable. A 2017 study looked into this to see which would be the most effective (2). A rapid weight loss “crash diet” versus a slow weight loss diet. The rapid group lost weight over 5 weeks while the slow group lost weight over 15 weeks. Both groups lost about the same amount of weight and saw similar reductions in Anthropometric readings, but the slow group maintained better body composition, with the fast group losing more of their body weight from lean mass.

Couple this with the fact that a very low energy intake also means a very low nutrient intake and it’s not uncommon to develop nutritional deficiencies which can have, in some cases, quite serious health implications.

The benefit to seeing rapid weight loss is that you can feel motivated by these early results and be encouraged to keep going (4). A morbidly obese person has an awful lot of excess energy stored up in all that adipose tissue which means their body is basically built for starvation. This means that obese people may well benefit from a month of very low-Calorie dieting followed by a period of slightly higher kcal dieting to avoid some of the negatives associated with rapid weight loss. This means a structured plan, education around energy balance, nutrition, food environment, food choices, time management and a whole plethora of other practical stuff.

Indeed the now famous Newcastle Diet study (5) put obese patients (BMIs ranged from 32 to 45) with type 2 diabetes on a 600kcal per day liquid diet. They lost significant amounts of weight and improved health markers, including reversal of T2D. But, this is very obese, very sick people who were undergoing clinical supervision. This approach isn’t suitable to the typical. otherwise healthy gym going person who wants to get back into their jeans.

So, from a health standpoint (unless you have a BMI of around 40) slow and sustainable is better but requires more effort and more lifestyle changes, but that’s the whole point isn’t it? In order to become a better version of you, you have to cultivate better habits.

How to reverse the decline

If you have found yourself in the loop of dieting too low and are now feeling trapped by your diet what should you do? You need to change your mindset, you can’t diet forever and at some point, you need an exit strategy, you need to learn how to eat to maintenance so that you can lead a functional life, have more flexibility around food but be able to do so without feeling anxious or guilty. Living your life being a slave to your own restrictive mindset and never really being able to enjoy your food or your workouts could mean that you are on the verge of developing an eating disorder.

The simple answer is to bring your kcals back up to maintenance. You will gain a little weight initially, but this will mostly be a combination of extra food matter in your bowels combined with more Glycogen and water in your cells. Scale weight doesn’t always equal body fat!

But, of course, this can be a scary proposition, especially if you are petrified of gaining weight again. So, the alternative is to do it slowly. It still requires a mindset shift because you need to shift from an attitude of lack and self-restriction to one of health and happiness.

If you have been dieting low and got to very low levels of body fat, as with physique competitors, then your body will prioritise fat storage as you gain weight, but that’s probably not a bad thing, it just means that you’re going from abnormal to normal (6).

Increase your kcals by about 200-400 (or any number that makes mathematical sense in your situation) per day if you are less than 1,000 kcals below maintenance, over a number of weeks and by about 4-500 per day if it’s more. But, ideally the quicker the better and you could bring your Calories up to maintenance over just 2 weeks.

Again, using myself as an example (but, as I have already said,  you don’t HAVE to do it slowly. It’s just that habitual dieters are petrified of gaining weight and the slower process may induce less anxiety). Here’s how this slow approach might look:

Week 1: 1,200kcals per day

Week 2: 1,700kcals per day

Week 3: 2,200kcals per day

Week 4: 2,700kcals per day.

You get the picture, right? What will this do for you? It’s not so much that eating more food makes you burn fat, not directly – you still need to be in an energy deficit to achieve this – but as your energy intake starts to get closer to your energy output your body will respond positively. (3,6).

What I would urge you to do ideally if you have been dieting low for a while is to just bring your kcals up to maintenance and then stay there for a month or two. Learn to eat to maintenance, enjoy your training, enjoy the mental clarity and the emotional parity that comes with being adequately nourished and not feeling the need to binge because you are now fuelling your body adequately.

During this time, you will secrete less stress hormones, your appetite hormones Leptin and Ghrelin will balance out. Your mitochondrial efficiency will improve, meaning that your body produces more ATP and increases your thermic output (Calorie burn). Muscle contraction will happen more rapidly, you’ll fidget more, you’ll start taking the stairs two at a time, your workouts will feel much better because your muscles recover quicker and the numbers on the bar will start to increase again. As your body starts to move more the energy deficit you eradicated by having such a low-Calorie intake will increase automatically.

Body Composition

Then, if your goal is still to lose fat, create a smaller deficit of around 10 or 15% or of maintenance moving forward. The smaller deficit is more sustainable and means that you still have the energy to train hard and, more importantly, recover between workouts.

I always encourage clients to focus on performance more than body weight. Setting your goals and your sense of worth solely on what the scales tell you is a slippery slope to mental illness for some people. But, if you set yourself performance goals and eat in accordance with the training load for that goal it’s a lot more motivating. This, of course, means a change in lifestyle but that’s a good thing. Be the best performing version of yourself, aim for optimal health, rather than being stressed out and having a disordered and obsessive attitude to food that constantly makes you feel under pressure. Life is too short to waste it berating yourself. I feel the Bobby McFerrin  song (80s nostalgia alert) is quite apt here; “Don’t Worry be Happy” and happiness is a loaded barbell (or cast iron kettlebell).


  • Crash diets are almost always a bad idea. A very low energy intake means a very low nutrient intake and dropping your kcals too low will have a negative influence on your metabolism and reduce your potential for fat loss. You’ll also feel like shit.
  • With a very low energy intake your energy output will reduce to match this and reduce the size of the deficit. Dieting on as many Calories as possible will not only be better for your physical health but will be better for your mental health and allow you to enjoy your food more while still losing fat.
  • Therefore, I encourage you to eat in a smaller, more sustainable Calorie deficit and focus on physical health and performance rather than scale weight alone. If you’re not sure how to do this or feel you need help with external accountability drop me a line. There is no shame in asking for help or hiring a coach.

Coach Troy

To learn more about personal nutrition sign up for my online program The Nutrition Solution: CLICK HERE


  1. Lowe, M., Kral, T. and Miller-Kovach, K. (2007). Weight-loss maintenance 1, 2 and 5 years after successful completion of a weight-loss programme. British Journal of Nutrition, 99(04).
  2. Ashtary-Larky D, Ghanavati M, Lamuchi-Deli N, et al. Rapid Weight Loss vs. Slow Weight Loss: Which is More Effective on Body Composition and Metabolic Risk Factors? International Journal of Endocrinology and Metabolism. 2017;15(3):e13249. doi:10.5812/ijem.13249.
  3. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition. 2014;11:7. doi:10.1186/1550-2783-11-7.
  4. Nackers LM, Ross KM, Perri MG. The Association Between Rate of Initial Weight Loss and Long-Term Success in Obesity Treatment: Does Slow and Steady Win the Race? International journal of behavioural medicine. 2010;17(3):161-167. doi:10.1007/s12529-010-9092-y.
  5. Rehackova L, Araújo-Soares V, Adamson AJ, Steven S, Taylor R, Sniehotta FF. Acceptability of a very-low-energy diet in Type 2 diabetes: patient experiences and behaviour regulation. Diabet Med. 2017;34(11):1554-1567.
  6. Trexler, E. (2019). The Metabolic Adaptation Manual: Problems, Solutions, & Life After Dieting. [online] Stronger by Science. Available at: https://www.strongerbyscience.com/metabolic-adaptation/ [Accessed 6 Aug. 2019].

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