Slow metabolism: why it is rarely the reason you are not losing weight

"I eat almost nothing and I still don't lose weight, so I must have a broken, slow metabolism." I hear this sentence over and over, and the same relief hides behind it: if metabolism is to blame, I am not. A slow metabolism is rarely the reason the scale won't go down. As long as you blame it, you work on the wrong problem.

Short answer: Rarely. Resting energy use in people of the same height, weight and muscle mass is mostly very similar. The usual culprit is underestimated intake and overestimated energy use: you eat more than you think, and you burn less than you think.

A slow metabolism is rarely the culprit

If you compare two people of the same height, weight and similar muscle mass, their resting energy expenditure (the calories they burn just by being alive and breathing) is mostly very similar. In one study of 150 adults, fat-free mass explained 63% of the differences in resting energy use between people and fat mass 6%. About a quarter of the differences stayed unexplained (Johnstone, 2005, PMID 16280423). So body composition explains most of it. Differences between people exist, but they are rarely big enough to explain why one person gains weight from "nothing" and another does not.

The usual culprit is somewhere else, and it is much more ordinary: underestimated intake and overestimated energy use. You eat more than you think, and you burn less than you think. This is not an insult. I see it in clients all the time, and it is human. But this is where the mystery hides.

"I eat almost nothing" usually does not mean little

When people who "eat almost nothing and don't lose weight" start logging exactly what they eat, the result in my clients is usually the same: they eat clearly more than they thought. It is a blind spot, and an honest one. And there are surprisingly many of them:

• Underestimated portions. A "spoon" of oil is often three spoons. A "piece" of cheese is half a pack. Without a scale the eye guesses low.
• Finishing the kids' plates and leftovers. A few bites from a plate, the rest of the side dish that would be thrown away. You don't log it, but your body counts it.
• Weekends. Five days of discipline, two days loose. Two loose days can be enough to wipe out most of the weekly deficit.
• Liquid calories. Coffee with syrup, juice, beer, wine, smoothies. They don't fill you up, but they deliver a lot of energy, and in your head they don't count as "food".
• Tasting while cooking. A bite here, a bite there. By the time the food is on the table you have eaten half a portion extra.

Add it up over a whole week and "I eat almost nothing" turns into a perfectly normal, often even higher intake. In most cases, metabolism is not the explanation.

In a small classic study, 10 people with obesity who said they could not lose weight on under 1,200 kcal (calories) a day were measured for 14 days. Their total energy use and resting metabolism came out within 5% of the values predicted from their body composition. But they underreported their intake by 47% on average and overreported their activity by 51% (Lichtman, 1992, NEJM, PMID 1454084).

Before you blame your metabolism, try one honest week of logging everything, including the bite while cooking and the weekend. The "mystery" usually solves itself.

What influences metabolism

Metabolism can be influenced, just not with the miracle tricks from the internet. There are levers, and you can work on all of them:

• More muscle. Muscle uses somewhat more energy than fat, even at rest. The direct effect is modest, but strength training protects muscle during a diet and supports your energy use over the long run.
• More daily movement (NEAT, the energy you burn outside of training). Steps, stairs, standing, fidgeting. It can make a big difference in daily energy use, and it is easy to overlook. I see it with clients all the time.
• Enough protein. Digesting it costs the body more energy than digesting fat or carbs (a higher thermic effect). It also fills you up more.
• Sleep. Too little sleep messes up hunger, cravings and your wish to move. Lack of sleep often pushes you toward eating more.

Together they do more for your daily energy use than any "metabolism booster", even though none of them works overnight.

What about adaptation? It does exist

It does, and it is good to understand it, because this is where the myth has a grain of truth. If you diet for a long time, the body adapts and starts to save some energy. This is called adaptive thermogenesis. Your resting energy use goes down a bit, you move a bit less without noticing, and suddenly the same menu that used to work just holds your weight.

But watch the words "a bit". It is a small, normal response and no disaster. It looks like this:

• A limited effect. It can slow progress, but the deficit still works. You may just need to fine-tune it. The extreme case is described in yo-yo dieting.
• Largely reversible. In most people the adaptation fades once they stop dieting and eat at maintenance again. It is a response to the diet, not permanent damage.
• Deficits still work. After a long diet it makes sense to take a break, eat at maintenance and then continue.

So yes, adaptation is real. And it is far from the scary thing marketing makes of it.

When health is the cause

Now the reason I don't want you to read this article as "metabolism is always an excuse". In a minority of people, a medical cause stands behind weight problems, for example an underactive thyroid (hypothyroidism) or other hormonal conditions. This happens and it should be taken seriously.

It does not belong in self-diagnosis as "I have a slow metabolism". It belongs to a doctor, with an examination and blood tests. If you feel that something is seriously off (marked fatigue, feeling cold, hair loss, dry skin, swelling, a very low mood), don't guess and don't read forums. Book an appointment and get it checked. Either it confirms and you treat it properly, or it rules it out and you know the levers from the section above are the answer. Eating very little for a long time can cause similar symptoms, which is one more reason to get checked.

If your low mood feels heavy, or you have thoughts of hurting yourself, don't wait for a checkup. Contact a crisis line or emergency services in your country right away (in the Czech Republic: Linka první psychické pomoci, 116 123).

What to do in practice

Before you put the blame on metabolism, go through this short list. Most of the time the "mystery" is solved before you even get to metabolism:

• Log your intake exactly for a week. Everything: the weekend, the tasting, liquid calories. Honestly, on a scale. This alone is often the biggest aha moment. If you have a history of an eating disorder, or if weighing and logging food makes you anxious, skip the logging week and talk to your doctor first.
• Add steps. Raise daily movement (NEAT). A few thousand extra steps a day do more in the weekly balance than you expect. See how many steps a day.
• Add strength. Lifting protects and builds muscle, and with it your long-term energy use and shape. See strength training in one hour a week.
• Eat enough protein. It fills you up, protects muscle, and costs the body more energy to digest. See how much protein and the protein tool.
• Sleep. Don't underestimate it. Good sleep straightens out hunger and the wish to move. See sleep and fat loss.
• See a doctor when needed. If you have symptoms like marked fatigue, feeling cold, hair loss or swelling, go now, without waiting. If something still doesn't fit after all this, book a checkup with your doctor.

So next time you hear yourself say "it is my slow metabolism", stop. Most of the time the causes are underestimated intake, overestimated energy use and a few levers you haven't pulled yet. Start with the honest week. Be Effective!

Frequently asked questions

Is a slow metabolism the reason I am not losing weight?

Rarely. Resting energy use in people of the same height, weight and muscle mass is mostly very similar. The usual culprit is underestimated intake and overestimated energy use: you eat more than you think.

Why do I "eat almost nothing" and still not lose weight?

Because after exact logging, "almost nothing" is usually clearly more: underestimated portions, liquid calories, finishing leftovers, tasting while cooking, and loose weekends that can wipe out much of the weekly deficit.

When should I see a doctor about my weight?

If you have symptoms like marked fatigue, feeling cold, hair loss or swelling, see a doctor now, without waiting. Otherwise, give it an honest week of logging first. If something still doesn't fit, go for a checkup, not to an internet forum. Blood tests can show whether there is something to treat.

Sources, so you can check:

1) Underreported intake, normal metabolism: Lichtman SW, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. N Engl J Med. 1992;327(27):1893-1898. DOI: 10.1056/NEJM199212313272701 | PMID: 1454084

2) What explains differences in resting metabolism: Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr. 2005;82(5):941-948. DOI: 10.1093/ajcn/82.5.941 | PMID: 16280423

Martin Barna

Online nutrition and fitness coach. Since 2013 I have coached 600+ clients. Evidence-based coaching.

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This article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.