Boosting your metabolism: what counts and what just makes promises

You eat less than you used to. You have joined a gym. You drink warm lemon water in the morning, because you read somewhere that it gets your metabolism going.

And the scales barely move. At some point comes the sentence that feels like an explanation and a verdict at the same time: my metabolism is just slow.

Many people know this pattern. And I understand why that sentence feels so natural. It takes the pressure off. At the same time, it opens the door to promises: capsules, teas, cold tricks, metabolism regimens.

Here we look at something more sober. What does your energy expenditure consist of? What changes it measurably? When is there an underlying cause? And what is there to the home remedies?

A note on language first. This is not about calorie targets, kilograms or training plans. All figures in this text are study data with their reference values. They describe what was measured and are not targets for you.

What metabolism means for weight loss: the four items of your energy expenditure

When someone says their metabolism is slow, they almost never mean the biochemistry inside the cell. They mean energy expenditure over the day. And that is not a single number, but a sum.

Imagine your expenditure as the costs of running a household. There is the basic rent, which is always due. There are delivery charges when food comes into the house. There are the many small trips around the home. And there is the weekend outing.

How large the individual items are varies considerably from person to person. That is why I do not give percentages here that would apply to you. What can be well documented, however, is which factors make basal metabolic rate differ between people.

The large analysis by Pontzer, which we will come to in the chapter on age, also found deviations of more than plus or minus 20 percent even after adjusting for fat-free mass, fat mass, sex and age, according to the full text. Two people of the same build can therefore use noticeably different amounts of energy.

Just two sentences on the second item, because it is covered in detail elsewhere. Diet-induced thermogenesis is real and higher with a protein-rich diet, but it remains a comparatively small item. How differently the body handles different nutrients is explained in A calorie is not just a calorie.

The quiet item: everyday movement

The flexible part of your expenditure is often not training, but what happens between appointments. In a classic overfeeding study by Levine, a large part of the additional expenditure came from exactly this inconspicuous movement. What that looked like in detail is described in Calorie deficit: why it is not enough.

Here, a follow-up question matters. Is everyday movement just a matter of willpower?

Now comes the point where many people are disappointed. More movement does not raise daily expenditure one to one. The body compensates.

The German S3 guideline on obesity puts it this way for people with overweight and obesity in its therapy chapter: they shall be motivated to increase their physical activity in everyday life and leisure time. This is an expert consensus. This article deliberately says nothing about minutes or step counts, because the right amount depends on your starting point.

That also answers a common search question: can basal metabolic rate be raised without exercise? Honestly, hardly. Daily expenditure, on the other hand, can, through the trips around the home. Why sitting for long periods is about more than calories is explained in Sitting and lack of movement. How fat metabolism changes with gentle endurance training is covered in Understanding Zone 2 training, and what movement sets in motion at the cellular level in Movement as medicine.

And now you know why the question is not how to fire up your metabolism, but which item has changed for you.

Muscle as an engine? What muscle mass, strength training, protein and sleep measurably change

Perhaps you have read this sentence many times: muscles burn calories even while you sleep, one extra kilo of muscle means 100 extra kilocalories a day. That sounds like a simple calculation. Build muscle, and your metabolism takes care of itself.

The calculation has a catch. And yet muscle is one of the most important items in this whole topic. Both belong together.

So why does muscle still count? Because there is a lot of it. In a whole-body MRI study by Janssen and colleagues in 468 people aged 18 to 88, skeletal muscle made up an average of 38.4 percent of body mass in men and 30.6 percent in women. A small value per kilogram, multiplied by a great many kilograms, adds up to a noticeable item.

And muscle carries more than resting expenditure. It is the tissue you move with, and so it carries items three and four. After meals, it takes up a large share of blood sugar. It has a say in whether stairs, carrying and getting up remain easy in older age. Why muscle can therefore be seen as an organ in its own right is explained in Strength training after 40. How insulin resistance can make weight loss even harder is covered in Insulin resistance and weight loss.

And if you are no longer young? A meta-analysis by Peterson and colleagues in people aged 50 and over found a pooled increase in lean mass of 1.1 kg with strength training, with high heterogeneity between studies. Older participants gained less, but they did gain. Starting earlier appears to bring more, but starting later can still bring something.

The German S3 guideline on obesity takes this up in two places. If sarcopenic obesity, meaning a lot of fat with little muscle, is clinically suspected, body composition should be measured with a validated method such as BIA or DXA (recommendation grade B, low evidence). And the background text states that strength training should be recommended in the context of weight reduction, taking possible sarcopenic obesity into account.

Protein when you eat less

People who lose weight generally lose more than just fat. Part of the loss is fat-free mass, and with it resting metabolic rate falls. This is where the question of protein comes in.

I deliberately do not give gram amounts here, because requirements depend on age, kidney function, movement and life stage. How to put your requirements into context is explained in How much protein do you need, and why protein is so closely linked to feeling full in Satiety and protein.

Sleep has a say in what a diet costs

Sleep seems like a side issue in this topic. A small but careful study suggests otherwise.

What can support good sleep in everyday life is described in Putting sleep hygiene into practice and in the Sleep Guide.

GLP-1 injections and muscle, in brief

Whether GLP-1 medications slow the metabolism through muscle loss is covered in detail in GLP-1 injections and muscle loss, and how these medications work in the first place in GLP-1 injections: how GLP-1 works. Here are just three newer findings. These medications are prescription-only, and whether they are an option for you belongs in a medical consultation.

In a substudy of the large SURMOUNT-1 trial with whole-body DXA in 160 participants (Look and colleagues, 2025), the weight lost after 72 weeks consisted of about 75 percent fat and about 25 percent lean mass in both groups, on the medication just as on placebo. The substudy was funded by the manufacturer, which was involved in its design and analysis.

A network meta-analysis by Karakasis and colleagues of 22 randomised trials likewise found about 25 percent lean mass in the weight lost for GLP-1 receptor agonists. Relative lean mass, meaning relative to baseline, remained unchanged. The agents with the strongest effect on weight and fat mass were among the weakest at preserving lean mass.

The review by Neeland and colleagues adds two important footnotes. First, the proportion varies widely between studies, from 40 to 60 percent in some to about 15 percent or less in others. Second, lean mass is not the same as muscle. It also includes organs, bone and water. MRI data suggest that muscle quality may even improve, for example through less fat within the muscle. Older age and severe illness remain risk factors for sarcopenia, a pathological loss of muscle mass and strength.

And now you know why preserving muscle, protein and sleep belong together, regardless of whether you lose weight with or without medication.

My metabolism is broken: what diets do to energy expenditure, and for how long

Perhaps you have stopped counting your diets. Each one worked, for a while. And after each one, the weight came back, sometimes with a surcharge. At some point the thought suggests itself that the body has lost something along the way. That the metabolism has fallen asleep or is broken.

The answer from research is uncomfortable in both directions. Yes, under energy deficiency the body saves more energy than weight loss alone explains. This is called adaptive thermogenesis. No, for most diets there is no evidence that this turns into a permanent defect. Both can be shown with four studies.

During the same competition, a research group led by Johannsen and Hall had already measured that resting metabolic rate had fallen by 504 kcal per day more in week 30 than the loss of mass explained. And this despite fat-free mass being comparatively well preserved: 83 percent of the loss was fat, 17 percent fat-free mass. Preserving muscle is therefore important, but it is no shield against every adaptation.

Why does the body save energy at all? From its point of view, a sustained deficit is a state of scarcity. Signals from adipose tissue such as leptin fall, hunger rises, and several systems throttle expenditure at the same time. How these hormones interact is explained in Leptin and insulin, and why a calorie deficit therefore rarely runs as linearly as it does on paper in Calorie deficit: why it is not enough and The calorie myth.

An old study, read with care

There is one study that is often misused in this context, so I describe it with caution. In 1992, Lichtman and colleagues examined ten people with obesity who said they did not lose weight despite eating little.

That leaves the question many people ask: how can the metabolism be boosted again after a diet? The honest answer is quieter than the advertising. According to the data from Martins, the adaptation becomes smaller once weight remains stable. What can count in this phase, you already know from the last chapter: preserving and using muscle, enough protein, enough sleep, everyday movement. And not going straight into the next restriction.

And now you know why an even stricter diet is rarely the answer to the last diet.

After 40, after 50, menopause: what changes and what does not

Hardly any sentence is as widespread as this one: after 40, your metabolism falls asleep. Some guidebooks even state that basal metabolic rate during menopause is a third lower than at 25. If you are at this age right now and your body is changing, that sounds plausible.

The largest measurement to date paints a different picture.

Two nuances belong here so that this study is not oversold. First, according to the full text, the statistical breakpoint for adjusted basal metabolic rate came earlier than for total expenditure, at 46.5 years instead of 63.0 years. However, the authors point out that there were few basal metabolic rate measurements between the ages of 45 and 65, which makes this value uncertain. Second, the study did not analyse menopause separately. It describes averages across many people, not your own course.

What actually shifts measurably with age is shown by the MRI study by Janssen and colleagues mentioned earlier. There, the relative proportion of muscle decreased from the third decade of life. A noticeable decrease in absolute muscle mass appeared towards the end of the fifth decade, mainly in the lower half of the body. The legs therefore appear to lose muscle earlier than the upper body. This is a cross-sectional study, not a longitudinal measurement, but the direction fits what many people over 45 notice on stairs and when getting up.

What menopause shifts

Many women experience during perimenopause that their body changes even though little has changed about what they eat. That is not imagination. But it is also not the metabolic collapse that some websites describe.

And hormone therapy? It has its own good reasons during menopause, such as severe symptoms. As protection for muscle, however, it showed no significant effect in a meta-analysis by Javed and colleagues of 12 randomised trials in postmenopausal women: women on hormone therapy lost 0.06 kg less lean mass than controls, with low quality of evidence. This says nothing about whether hormone therapy makes sense for you. That decision belongs in a gynaecological consultation, and an ongoing therapy is not changed on your own.

You can find more on this life stage in Menopause: symptoms and phases, Perimenopause: when does it start, Iron deficiency during menopause and Intermittent fasting for women over 40.

And now you know why the year of birth in your passport says less about your expenditure than what has changed in your body and your everyday life.

When a slow metabolism is a symptom: thyroid, iron, energy deficiency, medication

So far, this has been about items that shift in almost everyone. Now it is about something else. Sometimes the feeling that the body is running on low is a symptom with a cause. Then a tea or a new training plan alone starts at the wrong end.

I know this picture well from my practice. Behind the sentence that someone's metabolism is slow, there are often several things at once: a long history of dieting, little protein, little everyday movement, short sleep, low iron stores or an underactive thyroid that has not yet been recognised. That is a clinical observation, not a study finding. But it is the reason why a thorough medical history comes first, not a supplement.

The thyroid

Thyroid hormones set, so to speak, the base temperature of the metabolism. If they are lacking, everything can slow down: pulse, digestion, thinking, heat production. That is why the thyroid comes first in this topic.

A common cause of an underactive thyroid is autoimmune inflammation of the thyroid, Hashimoto's thyroiditis. Why the immune system attacks the thyroid is explained in Understanding Hashimoto's thyroiditis. Which thyroid values count and why TSH alone does not show everything is covered in Thyroid blood tests: which ones count.

And what if you already have an underactive thyroid or Hashimoto's, or no longer have a thyroid? Then good adjustment of your treatment by your treating practice is the foundation, and everything else in this article applies in addition, not instead. If your values are good and the symptoms still persist, you can find guidance in Symptoms despite L-thyroxine and Normal values, symptoms anyway.

At this point, the guidelines and the functional perspective partly diverge. The guidelines see no effect of L-thyroxine on weight in subclinical hypothyroidism. Functional medicine additionally looks at the symptom picture, free hormones, the conversion of T4 to T3 and cofactors such as selenium, zinc and iron. That is a complementary perspective with a thinner evidence base, not an invitation to take hormones without a diagnosis. More on this in Functional hypothyroidism and Selenium, zinc, iron and vitamin D for the thyroid.

Iron

Feeling cold when others do not, and being tired even though you have slept: many women with iron deficiency know this. There is also a biochemical bridge to the thyroid: the enzyme the thyroid uses to build its hormones, thyroid peroxidase, needs haem and therefore iron. Zimmermann and Köhrle summarised this in a review. The data come mainly from regions with iodine deficiency, so this is mechanistically plausible but not documented in detail in humans in everyday life.

For iron deficiency without anaemia, the situation is more contested, and here both sides belong visibly side by side.

Important for the topic of metabolism: measurably lower energy expenditure has not been shown for iron deficiency without anaemia, but the feeling of exhaustion can improve. And iron is not a harmless dietary supplement to take on suspicion. It should only be taken after a confirmed deficiency and under medical supervision, because too much can cause harm of its own, and a deficiency can have a cause that needs to be clarified. More on the link between iron and energy in Iron deficiency, tiredness and exhaustion and Iron deficiency, thyroid and sleep.

Too little energy

Now comes a paradox that many people do not expect. Eating too little can lower your expenditure. This applies especially when a lot of movement is added.

In its 2023 consensus, the International Olympic Committee describes this state as Relative Energy Deficiency in Sport, REDs for short: a syndrome with consequences for health and performance in female athletes and explicitly also in male athletes when energy intake is too low relative to training expenditure. That the same principle can affect recreational athletes who train a lot and eat little is an obvious extrapolation, not a statement of the consensus itself.

Even without sport, persistently lower energy intake can make the body more economical. In the CALERIE study mentioned earlier, the activity of the thyroid axis fell alongside expenditure in people without obesity. That is adaptation, not thyroid disease. And it is explicitly not a reason for thyroid hormones.

If you train a lot, eat little and constantly feel cold, your periods stop or you are frequently injured, this belongs in a medical assessment. The answer then is not to become even stricter. I will come back to this in the last chapter.

Medication

Some people gain weight after starting a new treatment and wonder whether it is their fault. Sometimes it is the medication.

On this point, the German S3 guideline on obesity recommends with recommendation grade B: for medications with a possible relevant weight gain, this should be taken into account when weighing benefits and harms, patients should be informed about it, and weight management as well as a possible switch where appropriate should be discussed together. In the background text, it names, among others, neuroleptics, lithium, valproic acid, tricyclic antidepressants, beta blockers, insulin, sulfonylureas, glitazones, glucocorticoids and progestins. The 2015 guideline of the Endocrine Society similarly states that the weight profile can be taken into account in the choice of medication where possible.

If you suspect that a medication is affecting your weight or your energy balance, talk to the prescribing doctor. Do not stop it on your own and do not change the dose. This applies to antidepressants, neuroleptics, antiepileptics, beta blockers, insulin, cortisone, hormone preparations and all other prescribed medications. With some of these medications, stopping abruptly can be dangerous, and the underlying condition for which they were prescribed counts at least as much in the weighing up as weight does.

Short sleep also belongs on this list, as the study by Nedeltcheva in the second chapter suggests.

And now you know why an assessment, rather than a supplement, can come first.

Getting your metabolism tested: which values make sense and why you should not interpret them alone

Can metabolism actually be measured? The question is justified, and the answer has two parts.

First: blood values do not measure your expenditure. They look for causes that can change it, such as an underactive thyroid, anaemia or inflammation. Second: resting metabolic rate itself is measured with indirect calorimetry via the respiratory gases, and body composition with methods such as BIA or DXA. For the latter, the German S3 guideline names a clear reason: clinical suspicion of sarcopenic obesity.

Why are there no reference ranges in a table here? Because they differ from laboratory to laboratory and from measurement method to measurement method. The German S3 guideline on obesity itself gives an example: the upper limit of the TSH reference range lies between 2.5 and 5.0 mU/l depending on the laboratory. A table from the internet next to your results can therefore easily lead you in the wrong direction.

And a single value says little without symptoms, medical history, medication and the course over time. Deriving a treatment from it therefore belongs in a conversation with a doctor, not in your own hands.

I did not examine commercial tests that certify you a metabolic type for this article. I therefore neither recommend nor evaluate them. How genes and ancestry can help shape metabolism is covered in Genetics, metabolism and ancestry, and which blood values count when iron deficiency is suspected in Iron deficiency: which blood tests.

And now you know why the most sensible metabolism test begins with a conversation.

Fat burners, green tea, chilli, coffee, cold water, breakfast: the fact check

A glass of cold water in the morning, something spicy at lunch, green tea in the afternoon, plus a capsule from the drugstore. Each of these ideas has a measurable starting point. The question is how big it is.

Water shows how a myth is born. The figure of 30 percent comes from a study of 14 healthy people whose authors merely extrapolated the effect of two litres a day. A more precise re-examination found no significant rise after normal water. Drinking remains important, but it is not a metabolism trick.

With coffee, the short-term rise is well measured. But caffeine can shorten sleep, and sleep, as you saw in the second chapter, is an item in its own right. More on this in Coffee, cortisol and adenosine. With brown fat, the physiology is fascinating, but the studies are small, and cold applications are not suitable for everyone. With cardiovascular disease, Raynaud's syndrome and during pregnancy, this should be discussed with a doctor beforehand. What cold can set in motion as a stimulus in the body is explained in Cold, the immune system and hormesis.

For breakfast: it does not start your basal metabolic rate, and skipping it is just as little a metabolism trick. If you are hungry in the morning, you are allowed to eat. How often you eat can follow your hunger, your daily routine and what agrees with you. And you can find a detailed assessment of metabolism regimens, detox teas and morning miracle drinks in Detox regimens under critical scrutiny.

Fat burners and metabolism capsules

Capsules to rev up the metabolism are available in pharmacies, drugstores and online. I do not name any here. The question is more fundamental: what do well-conducted studies show about dietary supplements for weight loss overall?

The German S3 guideline on obesity draws a clear conclusion from this. In the prevention chapter, it states as an expert consensus: people shall be advised against dietary supplements for weight control, homeopathic weight loss remedies and other non-evidence-based methods.

On top of that come risks that are not on any label. In US emergency departments, adverse effects of dietary supplements led to 23,005 visits per year according to an estimate by Geller and colleagues. Weight loss or energy products were involved in 71.8 percent of the supplement-related events with palpitations, chest pain or tachycardia. An analysis of the warning database of the US Food and Drug Administration by Tucker and colleagues found the undeclared appetite suppressant sibutramine in 269 of 317 adulterated weight loss products, or 84.9 percent. And the substance 2,4-dinitrophenol, which is sold online as a slimming agent, uncouples energy production in the mitochondria so strongly that, according to a review by Grundlingh and colleagues, there had been 62 published deaths up to that point.

And now you know why there is no capsule here.

When the wish for a faster metabolism becomes a burden

This chapter is short, and it matters to me. For some people, the search for the right metabolism trick is not a hobby but a cycle. Eat even less, train even more, one more rule. And the body can respond with feeling cold, exhaustion and an expenditure that tends to fall rather than rise.

You have seen in this article why that can happen. Too little energy with a lot of movement can turn down the thyroid axis, as the study by Loucks shows and as the consensus of the International Olympic Committee describes for female and male athletes. More restriction is then not the solution for a slow metabolism, but can be part of its cause.

A second observation belongs here: a long dieting history and strict restriction are themselves considered risk factors for disordered eating. A frequently cited basis is a cohort study by Patton and colleagues that followed around 1,700 school students in the Australian state of Victoria, initially aged 14 to 15, for three years. Girls who dieted severely were 18 times more likely to develop an eating disorder (recorded as partial syndrome) than girls who did not diet, and those who dieted moderately were five times more likely. Psychological distress predicted an eating disorder independently of this. The authors named dieting the most important predictor of new eating disorders. These are data from adolescents, and they cannot be transferred one to one to adults.

This does not mean that every diet leads to an eating disorder, and nobody is to blame if it happens. It means that warning signs may be taken seriously before they grow.

What remains in the end

Energy is not a luxury. Energy is the freedom to get up in the morning, take the stairs, play with children or grandchildren and not negotiate with your own body every day. That is what this topic is about, far more than a number on the scales.

If you take three directions away from this article, let them be these:

• Preserve and use your muscles. Not because they burn a lot while you sleep, but because they protect what you could lose in any phase with less energy.
• Make everyday life more active. The trips around the home are the quietest and often the most flexible item of your expenditure.
• Have causes assessed instead of buying capsules. Thyroid, iron status, energy balance, sleep and medication belong in medical hands, not in a shopping basket.

How all these building blocks around weight fit together can be found in the overview Understanding weight holistically and in the Weight Loss Guide.

If you would rather not just read about this but approach it with medical support: below this article you will find the option to book an appointment.

And now you know why your metabolism needs less of a push than a careful look.

Frequently asked questions about metabolism

Where metabolism connects with the rest of the body

Your energy expenditure depends on muscles, hormones, iron, sleep and the history of your diets. Here are the paths that lead on from this article.