Ask a Nutritionist: What Happens to Your Body When You Crash Diet?

crash dieting


Rhetoric regarding "squeezing into a dress" ahead of a wedding or "dropping a size" before a beach vacation was par for the course for as long as we can remember. Frankly, it still is. As such, you've probably considered a crash diet on more than one occasion. And despite more recent movements championing health-first nutrition and body positivity (or neutrality), an alarming amount of people are still crash dieting. If you want to look and feel healthier and stronger, eating nutrient-rich foods and moving your body regularly are key—but agonizing over every last calorie you consume and severely restricting your intake is neither fun nor safe for your body or mind. To explain exactly what happens to your body while you're crash dieting, I called on nutritionist Filip Koidis.

Meet the Expert

Filip Koidis is a London-based clinical nutritionist and dietitian and the founder of W1 Nutritionist.

"Crash diets and the philosophy that comes with them often leads to distorted eating behaviors (binge-eating, overeating), unpleasant emotional states (guilt and eating-related stress), as well as poor metabolic and body composition effects," says Koidis.

Koidis says crash diets are rightfully called this because you are bound to 'crash' emotionally. When you try to outsmart your body, it outsmarts you back, so when you start eating less, your body adapts and simply starts to use less energy. Koidis says the following processes start to come into effect:

  • Your body reduces its 'thermic effect' (energy your body uses to digest food), as you are eating less.
  • Your resting metabolic rate decreases as you weigh less.
  • Non-exercise activity thermogenesis decreases.
  • You absorb the full spectrum of the calories you consume, whereas in a normal state you wouldn't absorb as many.
  • Your hunger signals start to accumulate and push food cravings through the roof.

We asked Koidis to map out what happens to the body immediately after starting a crash diet as well as what would happen if you kept it going over a matter of weeks or even months.

lemon water

After 12 Hours

The first 12 to 18 hours on a crash diet, your body is preparing to get into "starvation" mode and is utilizing as much of its stored energy (glycogen) as possible.

After 24 Hours

As your energy stores are reduced, due to the lack of energy intake, cortisol levels rise, which can cause our bodies to hold onto more water and make us feel "bloated" and less lean than we actually are.

Also at this stage, our natural response to keep our core temperature stable (non-exercise activity thermogenesis) decreases, which might manifest as shivers and coldness.

After 48 Hours

At this stage, our body's energy stores are most likely completely depleted, and as the body can't tell the difference between food scarcity and starvation, it slows the metabolism to conserve energy.

Our bodies have an amazing survival instinct, allowing us to reduce our energy expenditure in an attempt to conserve energy, which makes us sluggish and great at storing fat.

After 72 Hours

Your metabolism slows way down, so it's difficult to burn calories. As your metabolism has been decreased and your body is preferentially storing fat (fat has more energy), lean tissue starts being burned off, which is mostly muscle. Furthermore, as your thyroid function is affected, adrenaline secretion is reduced, which makes you feel mentally and physically run-down.

After One Week

According to Koidis, by the first couple of weeks, there is a great chance that at least 50% of your weight loss is lean muscle. From this stage onward, you are also likely to start missing out on important nutrients (protein, B12, and iron) due to the restrictions of crash diets, compromising your immune system, mental ability, and overall health. Food obsession is likely to start affecting your personal and social life, as you can't stop thinking about when your next meal is or what and how much you are allowed to eat. You'll most likely spend your days obsessing about food.

As the days go on, your cravings will go through the roof, as there is a buildup of appetite-stimulating hormones in your system (ghrelin, neuropeptide Y) and binge-eating episodes are more likely to happen.

After One Month

By this point, the best way to describe you would be "hangry." Your appetite hormones will be out of control, and you'll also be in a constant bad mood. You'll find yourself in a vicious cycle of being hungry and moody.

There's a good chance that there have been some "breakout" cases at this point, where you gave in to your body's constant "nagging" for energy and consumed everything you found in front of you.

What Happens Once You've Stopped

As you have been trained in an "all or nothing" mentality for months, you are probably going all out over those "forbidden foods". Having lost most of your muscle mass, your body will be a fat-absorbing machine, as its priority will be to replenish the energy stores. Our body’s fat cells have a great memory, and the original body fat levels can be reached much faster following a crash diet and are, in many cases, surpassed.

The Takeaway

After reading all the ways a crash diet affects your body and mind, it's easy to see that crash dieting is not the answer, and honestly, it's even not worth trying. Want to form a better relationship with food and look and feel healthier? Try following a diet that focuses on intuitive eating and balanced meals instead.

Article Sources
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  2. Johannsen DL, Knuth ND, Huizenga R, Rood JC, Ravussin E, Hall KD. Metabolic slowing with massive weight loss despite preservation of fat-free mass. J Clin Endocrinol Metab. 2012 Jul;97(7):2489-96. doi: 10.1210/jc.2012-1444

  3. Stachowicz, M., Lebiedzińska, A. The effect of diet components on the level of cortisol. Eur Food Res Technol. 2016;242:2001–2009. doi: 10.1007/s00217-016-2772-3.

  4. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. J Int Soc Sports Nutr. 2014 Feb 27;11(1):7. doi: 10.1186/1550-2783-11-7.

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