Slow metabolism? The biggest diet myth explained scientifically

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How a diet can affect your metabolism

Everyone knows someone (maybe even him or herself) who has tried one diet after the other and, after initial success, quickly gains back all the weight that they lost when dieting. Many diets are completely ineffective at best or at worst do more harm than good. But why is this exactly? And can extreme diets actually harm your metabolism? We shall answer all this and more, but first here is a little history about the origin of dieting.

The history of dieting

Let’s start at the beginning: the term “diet” comes from the Latin word “diaeta”, which means both “diet” in the modern sense and “way of life”. The English word denotes a nutritional approach that is tailored to needs – most often it means the needs of a person wanting to lose weight. Sounds reasonable. What is not reasonable are radical or starvation diets e.g. the “just eat half” diet, cabbage soup diet, the one-day diet, or Hollywood Star Diet.

This is how you recognize a good diet

Before starting a diet, you should ask yourself: “Is this diet tailored to my needs?”. Eat half, no matter what the food is? Only cabbage soup? Eat chewable tablets every other day? These are just a few examples of the amazingly creative and questionable diets found in an article reviewing 55 diets by Focus magazine. Apart from such mainstream diets, there are many diet missionaries with blogs, vlogs and other forms of social media who think they have found the holy grail of nutrition. The yardstick by which every nutritional form should be based, however, is reproducibility: will the diet work for the majority of people? Or is there at least one specific target group for which this diet will work? Here, the chaff quickly separates from the wheat. The individuality of every person should always be reflected in their diet.

Identifying a good diet is not always easy. A good general rule is to not start any diet that is too radical. Extreme diets are more likely to harm your body than help it. A good diet puts your body in a calorie deficit, but is also balanced and healthy, and meets your nutritional needs. This will ensure that your body is supplied with every nutrient, boost your metabolism and help you lose weight in a sustainable, healthy way—without a yo-yo effect. (Tip: With the Upfit BMI calculator you can now easily calculate your optimal nutrient requirement in addition to your BMI.)

These are bad diets

Identifying a bad diet is much easier than finding a good one. For example, “only eat this” diets are generally bad. Diets that only let you eat one food or are very restrictive with what you are allowed to eat can lead to nutrient deficiencies. In addition to the three macronutrients— carbohydrates, proteins, and fats—there is a multitude of micronutrients—vitamins, minerals, trace elements—that our bodies need regularly to maintain body and organ functions. A long-term deficiency can have dramatic effects, from sleep disorders to depression to osteoporosis and cardiovascular diseases. ¹ Instead of stimulating your metabolism, they will harm it in the long term. So we have a first answer to the initial question: can a diet damage my metabolism? The answer is: yes—if the diet creates a nutrient deficiency.

Always avoid such diets. An unbalanced diet is never healthy and recovery from malnutrition is slow and requires much effort. Long-term nutrient deficiency can cause permanent damage. Therefore, always pay attention to your health, activity level, and sense of well-being while you are on a diet. If you notice that one of these three areas begins to decline— e.g. you are constantly getting sick, feeling drained, underperforming or in a permanently bad mood—you should take a break from this diet. But that doesn’t mean that you should binge on desserts, chips or other junk food, but rather nourish yourself with healthy, whole foods.

Boost your metabolism - the goal of a good diet

There is a lot of talk about boosting metabolism, but what does that actually mean? Basically, this is a very general phrase. The term “metabolism” refers to all of the body’s chemical processes to process certain nutrients. When someone says they have a good metabolism, they are often referring to their ability to maintain or lose weight easily. Metabolic processes take place at all times. While genetics are an important factor, the right foods also play a decisive role in how fast your metabolism is.

Many diets do not put you at risk of nutrient deficiency because they are less restrictive to your food choices. How do they work and do they also pose a danger to your metabolism? Although diets differ in details, most of them involve creating a calorie deficit and thus rely on simple math: They give your body fewer calories than it currently needs so that it gets the missing calories from your body’s reserves. In the best case, your body gets these reserves from its subcutaneous fat stores, because the ultimate goal of any diet for weight loss is to reduce body fat—mostly with a focus on the stomach and legs. Sounds simple and logical: I simply give my body less energy (calories) and it breaks down its fat deposits.

Dangerous diets that can damage your metabolism

Any diet that lays out rules for you or prescribes a certain amount of food without knowing your individual details —your height, weight, age, gender, activity levels —is not ideal and is potentially dangerous to your health. It may be that you end up eating too much and so do not lose weight, or that you eat too little and create a nutrient deficiency that poses health risks for your body. Losing weight is not a sprint, but a marathon. A diet with too high a calorie deficit—i.e. one that withholds large amounts of energy and building materials from your body—will slow down your metabolism over time and cancel out any short term weight loss. In addition to putting you in a bad mood, your athletic performance and motivation to move will suffer.²if your body fat levels become too low, you can quickly become unhealthy.

Unless you have iron discipline and working out comes as second nature to you(such as for bodybuilders before a competition), extreme diets will negatively affect your performance and make you lethargic and weak³.This will, in turn, sabotage your weight loss goals by minimizing your body’s conscious and unconscious movements (called NEAT – Non-exercise activity thermogenesis). This activity alone (unconsciously!) accounts for about 10% of your total daily energy expenditure. If this activity disappears, you will reduce your targeted calorie deficit that was making you lose weight. As you will burn fewer calories than planned and thus your weight loss will plateau. From a purely weight loss perspective, it doesn’t matter whether your calorie deficit is generated by fewer calories in your diet or by an increased calorie burn through exercise.⁴ However, exercise has the advantage of ensuring that a higher proportion of the reduced weight comes from breaking down fat.⁵ Our free workout plans can help you to complete your workouts regularly and increase your endurance at the same time.

How to speed up your metabolism

A little trick to boost your motivation to move (and thus fight your inner couch potato) is to listen to motivating music before working out or to watch short sport or fitness videos related to the workout that you are about to do. Regular exercise is the best way to improve your general motivation to move (tonic dopamine level in the striatum) and thus helps to increase your overall daily activity level. Take advantage of every opportunity to move that comes up, even if it’s just a flight of stairs that you sprint up with gusto because you’re feeling the urge to do so. So if you reduce calories to lose weight, make sure you motivate yourself to exercise enough, because your body will try to lower your activity level. This effect can be countered in the short term by increasing your calorie intake.

So how do you boost your metabolism? You’re going to get the most results in this area with the right diet and an active lifestyle. You can easily calculate how many calories you should be eating to reach your goal with our calorie calculator.

Consequences of malnutrition

Not only your activity level but also your body composition and health can suffer from an insufficient amount of calories. A diet without sufficient amounts of protein—just like a diet without exercise—will cause your body to lose muscle mass.⁶ To ensure that your body is supplied with all the nutrients it needs (particularly even when you create a calorie deficit )make sure you are eating a balanced, wholesome diet.

What happens to your metabolism in calorie deficit

Why so? Let us take a look at the basic diet model from above: The body gets fewer calories than it needs and gets the missing energy (because food is ultimately energy plus building materials) from the body’s own stores. Your body, however, adjusts quickly to the new conditions: it adapts to the lack of calories by burning less energy. In the short term, as described above, your movement behavior is adjusted via your neurotransmitters. Without realizing it, you start moving less and burning fewer calories, and your motivation to move also begins to suffer from the calorie restriction.

The second victim of this strategy is our fatless body tissues (including skeletal muscles). Overall, your fat-free body mass is the largest energy consumer (i.e. calorie burner) in your body and is an essential factor for energy consumption at rest – the largest proportion of the calories you burn every day. However, unlike your organs, the nervous system, and the brain, this energy consumer is absolutely unnecessary for our body. If you do not exercise during a diet, your body will lose some muscle mass with every pound that you lose. ⁷ Less muscle mass means that you burn fewer calories and thus lose less weight. Muscles weigh more than fat. So your pant size will not decrease proportionally to the weight you lose. Conversely, there are many exercisers who build up considerable muscle mass through regular strength training and —despite ultimately weighing more on the scales— get a significantly slimmer, more athletic figure than before.

How to maintain your muscle mass

Both a regular strength training program and a protein-based diet will help protect your muscle mass, which is usually reduced when you lose weight. Not only will you look better by doing this, but it also is good for your posture and will also save you having to get fit again afterward finishing your diet phase. For every ten kilograms of weight loss, there are about three kilograms of pure muscle mass loss. With a proper fitness plan and a diet rich in protein, the proportion can be reduced to about one and a half kilograms.

Choose the right way to measure body changes

Therefore, make sure to not only use the scale to gauge how much fat you have lost but also to get yourself some measuring tape – changes in body measurements are often more accurate for evaluating changes in your body size. We recommend that you step on the scale once a week and at the same time to measure your abdominal and chest circumference. These measurements provide you with an understanding of your physical changes that is much more realistic than the number on the scale could ever provide. To summarize: when dieting, you want to maintain your muscle mass and reduce your body fat, so make sure you get enough exercise and protein.

Kurzfristige Effekte des Stoffwechsels

Bei den im zweiten Abschnitt beschriebenen Veränderungen des Stoffwechsels handelt es sich weitestgehend um kurzfristige, reversible Effekte auf den Hormon- und Baustoffwechsel. Auch hier gibt es ein paar Dinge zu beachten, die Auswirkungen sind allerdings leichter zu umgehen und bei Auftreten leichter kompensierbar.

Did your metabolism slow down?

The terms “fast” and “slow” metabolism are often heard in conversation but not actually thoroughly researched. These terms came into being with the observation that people who lose weight very quickly (particularly formerly obese people) have a significantly reduced energy metabolism after the phase of intense weight loss. If you consider the above explanations, it quickly becomes clear that a reduced metabolism caused by rapid weight loss is normal: less voluntary and involuntary movement, less fat-free mass than energy consumers, less weight to be carried and less digestive work due to less food—all of this reduces our energy consumption. Lower energy metabolism due to weight loss is therefore quite normal so that the amount of food required after a diet to maintain weight will be less than before the diet. But that shouldn’t reduce your motivation. With adequate training and the right nutrition, you can activate your metabolism.

Radical diets can harm your metabolism

Extreme, radical diets lead to reductions in your metabolism. A study that subsequently observed some participants in the “The Biggest Loser” TV program, known for their radical diets, over a period of six years, found that the participants’ metabolism was far less active while their body was at rest than it should be. Energy consumption while at rest accounts for about 60% of your daily energy requirement and, as just described, changes with weight loss. However, participants’ average energy consumption of at rest was reduced by an average of about around 300 kcal more than expected per day more than expected. Neither a continuation of a regular intense fitness program and a low-calorie diet after the show, nor a return to old diets or a further complete change could ever bring the metabolism back to normal.

In the six years after the end of the program, the participants had on average regained 40kg of the 58kg they had lost—without energy consumption while at rest returning to the original state. Instead, the consumption while at rest decreased on average, so that the difference between the expected and actual consumption while at rest increased to a whopping 500 kcal on average. It became increasingly difficult for the former participants to maintain their weight, and almost all of them have now returned to their original weight. It seems that their bodies did everything they could to regain the lost weight and, when this goal was achieved, to encourage further weight gain. The former participants would have to eat far less and exercise more than an average person with the same physical dimensions to maintain let alone reduce their weight.⁸

Other studies, which also dealt with radical weight loss in obese people, observed that the metabolism of the test subjects fell far below the expected level and then did not return to normal. So there is certainly evidence that an extreme diet (regardless of whether you exercise) can permanently change your metabolism in such a way that your body uses less energy than would be expected based on the corresponding parameters.⁹ So far there is no good explanation for this fact, nature clearly likes to have a backup plan if living conditions change unfavorably.

Sustainable diet changes

Should you be worried about all this with respect to your own diet? No—if you arm yourself with the proper knowledge and plan you have nothing to worry about. The “biggest losers”, like participants in other studies, had a massive calorie deficit of sometimes more than 1000 kcal per day, plus they practiced an average of three hours of exercise per day over a period of three hundred days, and competed with each other every week. As is so often the case in life: consistent, patient hard work brings you to your goal more successfully and healthier than quick-fixes and short-term thinking. Extreme diets with a high calorie deficit should only be used for a very short time and seem to be tolerated better by people with high muscle mass and rather low body fat. Better not to take such extreme measures at all. All you really need is motivation, plus perseverance and patience.

Summary: stimulate your metabolism—but correctly

The human metabolism occurs in as many variations as the physical appearance. Environment, sleep, stress, nutritional history, exercise, and genetics influence what foods we can metabolize, in what quantities we can metabolize them in, and how well and quickly we can do it. The main goal of a diet is to cover all (or as many as possible) of your nutritional requirements. A diet that sets the same rules for all its users, without regard for individual differences, is nonsense. Just like one that limits your food choices to just a few foods. And the latter diets run the risk of causing nutrient deficiencies, which can result in permanent damage.

Regular exercise is important to maintain motivation and muscle mass during and after weight loss. Your body will already subconsciously reduce your movements anyway, so don’t let yourself slack off entirely in this department, even if your workout stagnates somewhat during your weight loss phase. Diets with a high protein content are preferred because they help you maintain your muscle mass. Most importantly, if a diet triggers health problems, permanent negative mood, or physical fatigue, it is time to give your body a break from the diet—and nourish your body with whole, healthful foods (not junk foods).

The success of a diet is better measured in the mirror than on the scale. If you wear two sizes smaller pants due to a reduced waist size but have also built up muscles through training so that the scales indicate an unchanged weight, you will still like the image in the mirror much better. Extreme ambition is not appropriate in fitness or when losing weight. In both cases, you will do more harm to your body than good. Be consistent, but realistic, and give your body time to adjust.


  1. (see European Journal of General Medicine, 2015, Vol. 12, Issue 3, p. 261-266)
  2. (see Best Practice & Research Clinical Endocrinology & Metabolism, 2002, Vol. 16, p. 679-702)
  3. (see Two Days of Calorie Deprivation Induced by Underfeeding and Aerobic Exercise Degrades Mood and Lowers Interstitial Glucose but Does Not Impair Cognitive Function in Young Adults, The journal of nutrition, 2017, Vol. 147, p. 110 – 116)
  4. (see Effect of Calorie Restriction with or without Exercise on Body Composition and Fat Distribution, The Journal of Clinical Endocrinology & Metabolism, 2007, Vol. 92, p. 865-872)
  5. (see Meta Analysis: effect of exercise, with or without dieting, on the body composition of overweight subjects, European journal of clinical nutrition, 1995, Vol. 49, p. 1-10)
  6. (see Effect of a high-protein, energy-restricted diet on body composition, glycemic control, and lipid concentrations in overweight and obese hyperinsulinemic men and women, The American Journal of Clinical Nutrition, 2003, Vol. 78, p. 31-39)
  7. (see Meta Analysis: effect of exercise, with or without dieting, on the body composition of overweight subjects, European journal of clinical nutrition, 1995, Vol. 49, p. 1-10)
  8. (see Persistent metabolic adaptation 6 years after “The Biggest Loser” competition, Obesity – a research journal, 2016, Vol. 24, p. 1612-1619)
  9. (see Physical activity and resting metabolic rate, Proceedings of the nutrition society, 2007, Vol. 62, p. 621-634)

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