In part 1 of this series, we went on a crash diet weight loss journey, during which we lost 5kg of body weight. If you haven’t read it yet, you can find it here. Part 2 will focus on your metabolism and show you why crash dieting is bad for your health.

What you’ll learn at a glance throughout the article series:

• we analyze how any crash diet works in part 1
• in part 2 we focus on why the crash diet is potentially bad for your health and your metabolism
• next, in part 3, I’ll show you how to lose weight in a sustainable way and
• finally, part 4 will provide you with all the tools you need to create a successful weight loss protocol for your personal lifestyle

After a really hard week of crash dieting, cardio and lots of restraint we are back at our initial starting point. We eat as we always did, an amount of food that kept as in balance.

A few weeks down the line we realize that we are even gaining extra weight. How’s that possible?

Yo-Yo Effect

While a yo-yo is a really fun gadget, but the moment we enter the realm of nutrition, the yo-yo effect becomes a dreaded enemy. You put yourself through a lot to lose those 5kg. But not only are those 5kg back on the scale, you actually got heavier. That’s the yo-yo effect.

People wonder why this happens, it seems like magic and is unfair. In worse cases, people feel terribly bad about themselves and jump right back on the next crash diet with the goal to make things right. The problem with this habit, though, is that it makes things much worse.

Understanding your metabolism

Our bodies are incredible. It is a very resilient ecosystem, that strives constantly for homeostasis (7). In order to make noticeable changes in our bodies, we need to break out of homeostasis, though (8).

After that our body adapts to these changes. Keep this thought of stimulus and adaptation in mind throughout this crash diet series, it’s important.

Basal Metabolic Rate

We are done with our crash diet, we gained the 5kg back and added maybe another kilo the first few weeks after we went from our crash diet back to our regular maintenance diet.

The big question is: where did this extra weight come from if we ate like before (maintenance diet)?

A big part of the answer is your basal metabolic rate, or BMR for short. Your BMR is the amount of calories that you need to stay alive (9).

In car speak, it is your engine running in neutral. This takes a certain amount of gas just to keep things working.

This makes up for the biggest energy needs of our bodies. Most people have a BMR (depending on age, gender and weight) that ranges from 1200 to 1900 calories. You can see how just staying alive is hard work for our body.

The remaining calories are from your daily activities, being at work, walking, cooking, cleaning, gardening, sitting etc and are called NEAT, non-exercise activity thermogenesis. If you do work out in the gym, play tennis or go jogging this comes on top of your caloric output. The total is called TDEE, total daily energy expenditure.

But why is that important and why should I care?

Remember when I said our bodies strive for balance and adapt to new situations? That’s why you should care.

Going on a crash diet for a couple of days, especially with lots of cardio, puts our body under a lot of stress. Or in other words, it sends a strong signal to our body. This crash diet challenges our body and quickly realizes that there’s a big caloric deficit and apparently no end (Our bodies don’t know that it’s only going to last for a week). Once this realisation happens, our body adapts and the single most important task is to keep you alive – even if not many calories are coming in. We are adapting to the signal (10, 11, 12).

Our body has basically two ways of handling this situation:

1. using available reserves and transform them into energy or
2. conserve as many energy as possible.

Energy reserves

The first point is what people aim for when they say they want to lose weight. This makes a lot of sense, as fat is our main energy reserve and is stored throughout the body in fat cells.

You’d think that once we have a caloric deficit our body just opens the tap on these cells and we have energy flowing into the system. Unfortunately, this process is not that simple.

Luckily for our homeostasis driven body, there are even better ways to get energy: muscle tissue. On a very low-calorie diet your lean body mass will decrease (13). This is better for this process because our body manages to hit two birds with one stone:

1. You get access to energy while keeping your fat reserves as high as possible (who knows what happens), and
2. Less muscle mass (and overall lean body mass), will lead to a lower BMR, which further decreases your need for energy in form of calories.

Energy savings mode

Next, our body will scale down internal processes as much as possible (14, 15). Everything in your body will still work, but not in an optimal state. You might feel overall more tired, get exhausted quicker, might get sick easier, recover slower and show less strong mental abilities.

Your metabolism is “slow”, which means it’s in energy savings mode. You might have decreased your BMR a lot, how much isn’t easy to tell, but several hundred calories are possible.

Rebounding with the yo-yo effect

Done with our crash diet, weight goal achieved, we head back to what we believe is our maintenance diet. The energy equation is in balance and all is well. But it isn’t. We gain weight.

The reason you gain weight is that you live in a caloric surplus, you eat more than you need to stay in balance.

Thanks to the crash diet you reduced your BMR. This means you eat way too much every day, hence the weight and fat gains.

Did you find a diet that looks great but you aren’t sure if it’s a crash diet? Or maybe you want to create your own weight loss diet and don’t want to fall into the crash diet trap. Get my free “Crash Diet Worksheet” with 8 red flags to watch out for!

Your BMR will stay low for a while. How long is hard to say, science isn’t sure, but depending on how hard you approach crash diets and how severely you punish your body, this can take days, weeks or even months to get back to a healthy level. Your body will also try to slow you down overall if you’re in a big deficit (16)

The solution to this, in the eye of many people, is the next crash diet. And the cycle begins anew. That’s the yo-yo effect.

Health Impacts

We’ve covered many things concerning crash diets by now. One important part that’s missing, are the impacts on our health.

Overall, studies (17, 18, 19, 20) have shown that decreasing your body weight has beneficial effects on your health. This is particularly true for obese people and the metabolic syndrome.

Short term goal, long-term habit

As we discussed, we might end up in this vicious cycle of crash diet and yo-yo effect. Staying for long periods of time in a (very) low-calorie diet will possibly lead to a long-term habit with severe consequences:

1. nutrient depletion,
2. slower metabolism,
3. muscle loss,
4. vitamin and mineral deficiencies,
5. GI problems,
6. blood pressure and electrolyte imbalances and

other consequences such as

• eating disorders (21)
• psychotic disturbances (22)

We already dived into the #2, #3 and #5. Let’s look into nutrient depletion for a moment.

Micronutrient depletion

This one is straightforward. If you are on a caloric deficit, it gets very difficult to manage to meet all your daily micronutrient requirements. The higher and longer the deficit, the more challenging this gets.

Our bodies need micronutrients, like vitamins, minerals and electrolytes, to keep a multitude of different processes in our body up and running. Once certain minerals aren’t available in the needed quantities anymore, our system gets compromised. The result can be catastrophic and even lead to death (like the sailors that died of scurvy).

Before this happens you will see changes in your body. Your immune system might be weak, you bruise more easily or you heal slower, you get cardiovascular problems, get more susceptible for infectious diseases (23) and so on. It is a serious issue at any given state of life (24) and nothing that should be underestimated.

By now you know why you should care about how a crash diet works.

Before we move on I want to emphasize one important thing about weight loss: stop obsessing about the number on the scale!

Instead focus on feeling better, improving your measurements and take progress pictures. These methods are much more motivating than a number on the scale. Especially as the number doesn’t mean anything, even if it goes down.

Here’s part 3 of the series: “How To Lose Weight The Right Way“