Why am I not losing weight when I eat so little?

Maybe you know this evening. The app shows just under 1,200 calories. You went to your workout. You skipped the bread, the dessert, the glass of wine.

The next morning you step on the scale. The same number as last week. The same number as last month.

And then comes that sentence, quiet and cruel: I am just too weak.

I hear this sentence often in my practice. And it rarely explains what is going on in your body. Your body is not a calculator that subtracts calories. It is a network of feedback loops. And feedback loops react when you put them under pressure.

In this article, we will go through these feedback loops together. Not as an excuse, but as a map. By the end, you will know which questions and which lab values can help you move forward.

Why am I not losing weight when I eat so little? The short answer

Two numbers show how large these effects can be. In a study in the New England Journal of Medicine, people with excess weight who reported eating under 1,200 calories and not losing weight underestimated their food intake by 47 percent on average (Lichtman 1992). And in exercising women, the active thyroid hormone T3 fell by 16 percent as soon as too little energy was left over after training (Loucks 1994).

Both sound contradictory. They are not. The math can be inaccurate, and your body can still be saving energy. Before we take this apart, let me tell you about a pattern I know in many variations.

Let us look at what can be behind a bottleneck like this. We will go from the outside in: first the balance, then the feedback loops.

First the honest question: does the math add up?

I know this question can hurt. You have held yourself together for weeks, and now it is supposed to be the math? Stay with me for a moment. This is not about blame. It is about measurement accuracy.

The second miscalculation concerns training. A team led by Pontzer measured total energy expenditure in 332 adults from five populations using doubly labeled water. At low activity levels, expenditure rose with every bit of movement. In the upper activity range, the curve flattened, as if the body were defending its total budget (Pontzer 2016).

Imagine your energy budget as a household account. If you spend more on training, your body can quietly save elsewhere. The account stays surprisingly stable, even when you move more.

If you like, do a short reality check: one week of weighing instead of estimating, including oil, drinks and little tastes. Not as a new long-term project, but as a measurement. If counting puts you under pressure or feels compulsive, deliberately leave it out. That, too, is an important finding, and you are allowed to take it seriously.

And remember: the scale measures water, gut contents, muscle and fat all together. If you are new to strength training, your muscles often store more water. A tape measure around your waist can therefore sometimes tell you more than the number in the morning.

And now you know why the balance is the first step, but never the only one. Even when it adds up, your body has a say.

Starvation mode and set point: why your body defends its weight

Does this sound familiar? After every diet, hunger comes back louder. You get cold more easily. And the kilos find their way back, as if they had a permanent address.

Experts speak of a set point. Imagine a thermostat: you can throw the window wide open, but the heating works against it until the set temperature is reached again. The popular term starvation mode often exaggerates here. Eating too little does not make you gain weight. But your body can turn its pace down considerably.

A team led by Sumithran in Melbourne followed 50 people after a ten-week very low calorie diet. A year later, the hunger hormone ghrelin was still elevated, satiety signals such as leptin and peptide YY were still lowered, and hunger was greater than before the diet (Sumithran 2011). So your body does not forget a diet as quickly as your calendar does.

Body heat can play a part too. In a randomized trial from Baton Rouge with 48 participants, energy expenditure fell more than expected during six months of calorie reduction, and core body temperature dropped (Heilbronn 2006). If you are constantly cold while losing weight, you may be feeling this effect firsthand.

How firmly this thermostat is set is the subject of lively debate. In 2011, an international team led by Speakman described several models side by side: an actively defended set point, a passive settling point and mixed models in which genes and environment interact (Speakman 2011). So far, no single model explains everything, and honesty means saying so.

What follows from this is unspectacular, but often more sustainable than any crash diet: a moderate deficit instead of a radical one. Enough protein and strength training, so that your body keeps its muscle as something valuable. And breaks from the deficit, instead of pushing deeper month after month. How this fits with calorie math is explored in the article Calorie deficit: why it is right and still not enough.

And now you know why the same diet is harder the third time than the first. Your body has learned, and it remembers.

The thyroid: can it put on the brakes even when TSH is normal?

Cold hands at your desk. Tiredness after training that lasts until the next day. Skin that has become drier. A mind that thinks more slowly than it used to. Many people who eat little and train a lot describe exactly this mix.

First, the picture. The thyroid mainly produces T4, a kind of reserve. Only T3 is active. A large part of this conversion takes place directly in the tissues, via enzymes called deiodinases. A review by Mullur and colleagues describes how one of these enzymes sits in muscle, fat tissue and the hypothalamus and is needed to adjust heat production (Mullur 2014).

Classic thyroid testing starts with TSH, and that makes sense. It is the messenger from the pituitary gland and shows how loudly the brain is calling for hormone. But it does not directly show how much active T3 arrives in your cells. Think of a heating system: the thermostat in the hallway tells you what was ordered, not how warm it is in the room at the very back.

The Framingham study of 2,407 adults also found this dimmer effect. Body weight rose step by step from the lowest to the highest TSH group, in women from 64.5 to 70.2 kilograms on average. Those whose TSH rose over time were more likely to gain weight (Fox 2008). How sensitively resting metabolism responds was shown by a small study from Montreal with nine people on thyroid hormone: when TSH rose across the dose steps from 0.1 to 10, resting energy expenditure fell by about 15 percent (al-Adsani 1997).

Now an important counterpoint, so that no distorted picture emerges. A slightly elevated TSH in people with excess weight is often more a consequence than a cause. A review by Reinehr describes how such values often settle back after weight loss, and how fT3 in people with excess weight actually tends to sit in the upper normal range (Reinehr 2010). In line with this, BMI in the Danish study was associated with TSH and fT4, not with fT3. So a low fT3 becomes a clue mainly when someone eats little and trains a lot.

Low flame from cutting back: a feedback loop that often goes unnoticed

And now the part that surprises many people. Eating little can itself turn down the thyroid.

In competitive sport, this pattern has a name. With REDs, the International Olympic Committee describes a syndrome in which too little energy relative to training can impair health and performance, in women and in men (Mountjoy 2023). The principle behind it can also play a role in recreational sport, especially when training and dieting are ramped up at the same time.

The CALERIE study suggests that the effect lasts longer than a few days. A team led by Redman followed 53 adults for two years, and 34 of them ate about 15 percent less. Their energy expenditure was 80 to 120 kilocalories per day below what their weight loss explained, and the thyroid axis worked in a measurably dampened way (Redman 2018).

The immune system has a say too. In a study from Leiden with 66 women, TSH and T3 fell over three months after weight loss through diet or stomach surgery. The temporary rise in reverse T3, an inactive form of the hormone, was linked to the inflammatory messengers IL-6 and IL-8 (Lips 2013). A review from Amsterdam describes how such messengers can alter the genes of thyroid hormone metabolism, and interprets low thyroid values during illness as a possible energy-saving program (de Vries 2015).

Here you see the logic of KPNI (Clinical Psychoneuroimmunology) on a small scale. Metabolism signals scarcity. The hormonal system dims T3. The immune system can amplify the signal. And the nervous system dials back the sympathetic drive, as the Rosenbaum study showed. No organ works alone.

In anthroposophic medicine, warmth is seen as an expression of living metabolic strength. Someone who is constantly cold is read there as a person who lacks strength for building up. This is a view from clinical tradition, not a study. Here it meets physiology.

What you can have checked

The articles TSH alone is not enough and Functional hypothyroidism go deeper. And one sentence on safety: thyroid hormones are not weight loss drugs. Without a proven deficiency, they can strain the heart and bones. They are prescription-only and belong in a doctor's hands.

And now you know why an unremarkable TSH and a body running on a low flame can both be true at the same time.

Iron and ferritin: why your iron stores have a say in weight loss

You drag yourself to training even though you slept. The stairs feel steeper than they used to. And someone says: that is just stress. Maybe. But maybe it is also a store that is running empty.

A look into the thyroid's workshop shows why this might be. The enzyme thyroid peroxidase handles the first steps of hormone production, and it contains heme, an iron-containing building block. In an animal study from Zurich, its activity in rats fell by up to 56 percent with iron deficiency. Notably, food scarcity alone also lowered its activity (Hess 2002). In humans, this mechanism is plausible but not yet proven in detail.

A meta-analysis from Catania of ten cross-sectional studies arrived at a similar picture. With iron deficiency, fT4 and fT3 were lower, and in non-pregnant women there was no difference in TSH. Iron deficiency was also more often accompanied by thyroid antibodies (Garofalo 2023).

Here I take a clear position. If you are exhausted and not losing weight, a ferritin just above the lab's lower limit rarely means, in my view, that you are well supplied. Many values between 30 and 80 µg/L are formally normal, but functionally often borderline. As a target range, I consider above 100 µg/L sensible, provided inflammation and iron overload have been ruled out. This is my clinical assessment, and experts debate it. You can find the arguments for and against in the article Ferritin above 100 as a target.

One detail from the Spanish study fits with this: the mean values of fT4 and fT3 already fell with each lower level starting from the group above 100 µg/L, not only below the classic deficiency cutoff.

Important for interpretation: ferritin can rise with inflammation, like a smoke detector that also beeps when you cook. That is why a CRP belongs with it, and often transferrin saturation too. More on this in the articles Silent inflammation: reading hs-CRP correctly and Iron deficiency despite normal ferritin.

If your stores are empty and tablets are not enough or are not tolerated, a properly performed iron infusion can be an option. Properly means: a clear indication, overload ruled out, a modern preparation and good monitoring. What matters is covered in the safety checklist for iron infusions.

And now you know why ferritin belongs on the list when you eat little and still do not lose weight.

Insulin: when the storage switch gets stuck

Two hours after lunch, the curtain falls. Tired, unfocused, and then that pull toward the drawer with the chocolate. Sound familiar?

Insulin is the key that lets sugar into your cells. At the same time, it is a storage hormone. As long as a lot of insulin is circulating, fat tissue releases its reserves less easily. With insulin resistance, the locks have become dull, and the body responds with even more insulin.

The HOMA index is calculated from fasting insulin and fasting blood sugar. It is a compass, not a verdict. A single value therefore belongs in context with HbA1c, triglycerides, waist circumference and your everyday life. Cutoffs differ from lab to lab.

An exciting question is whether your insulin status determines which diet suits you better. Two large randomized trials come to different answers here.

What follows from this: it is established that both diets can work when the quality is right. It remains open whether a high insulin level predicts which direction suits you better. That is why I use insulin values as a clue, not as dogma.

A glucose sensor can show how strongly your blood sugar reacts to individual meals. For your first sensor, I recommend having it applied with guidance, so that you read the curves correctly and do not interpret every spike as an alarm. More on this in the article What a CGM sensor shows. You will find the basics of insulin resistance in the article Insulin resistance and weight loss.

Directions, not recipes: start meals with protein and vegetables, eat highly processed carbohydrates less often, walk for a few minutes after eating. And use strength training as an ally, because muscles are your body's largest consumer of sugar. The articles Strength training for insulin resistance and Movement after eating show how this works together.

And now you know why a normal fasting blood sugar does not automatically mean a relaxed insulin system.

Can stress slow down weight loss? Cortisol, dieting and sleep

You wake up at four in the morning and do the math. What did I eat yesterday? Am I allowed to have breakfast today? Your mind keeps counting, even when all you want is to sleep.

Staying fair is part of this. A meta-analysis of 13 studies with 357 participants found the clearest rise in cortisol with fasting. With very low calorie and moderate diets, the rise was not significant, and it seemed to fade again after a few weeks (Nakamura 2016). So cortisol is not a permanent enemy of every diet, but it is a player, especially with harsh restriction and in the early phase.

Over the long term, an association also shows up in hair, which stores cortisol over weeks like a calendar. In the English ELSA study of 2,527 people aged 54 to 87, higher hair cortisol was associated with more weight, a higher BMI and a larger waist circumference, and with more persistent obesity over four years (Jackson 2017). The associations were statistically clear, but small.

And then there is sleep, the quietest player of all.

A second randomized trial with 36 adults confirmed the direction in everyday life. Even about one hour less sleep on five nights a week led to a smaller fat share in the weight lost, although the scale showed the same amount. Catch-up sleep at the weekend did not fully make up for this (Wang 2018).

Through the KPNI lens, this forms a coherent picture. The nervous system rates hunger, constant counting and short nights as a threat. The stress axis responds with cortisol. Metabolism listens and holds on to its reserves. This is an interpretation that fits the data well, but its details are still being researched.

Treat sleep as part of the plan, not as a reward. A fixed wake-up time, light in the morning and a calm evening can make a big difference. And if counting stresses you out: pause the counting and keep the structure, meaning regular meals with protein and vegetables. You will find more background in the articles Sleep and weight loss and Cortisol and the HPA axis.

And now you know why your nervous system has a seat at the table in every diet.

The cause tree: from question to findings

Maybe you feel overwhelmed right now. So many possibilities. Where do you start? That is exactly what this tree is for.

It does not replace an examination. It sorts things. Each branch is a question, and each question has clues in everyday life and values in the lab.

The lab values as a next step

Reference ranges differ between labs. Interpretation belongs in a doctor's hands, together with your symptoms and your history. Values without context can reassure where they should not, and worry you where there is no reason to.

And now you know which values can answer your questions, and where a single value is not enough.

What is established, what is plausible, what remains open

An article about causes needs an honest boundary. Here I separate what studies show well, what has been observed in humans and is mechanistically plausible, what remains open and what I observe in my practice.

Three steps you can take this week

Not a list of twenty points. Three levers that can often make the difference in this situation.

If you want to do more than read and get started right away: below this article you will find the option to book an appointment.

Frequently asked questions: why am I not losing weight?

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