Why Can’t I Lose Weight? The Causes Worth Testing
- 1 day ago
- 7 min read
If you are eating well, moving regularly and the weight still will not shift, the problem is rarely effort. Weight is regulated by insulin, thyroid, cortisol, sex hormones, sleep and muscle mass, and when one of those is working against you, the same behaviours stop producing the same results. Most of it is measurable.
There is a particular kind of frustration that comes with doing the right things and watching nothing happen.
You have changed how you eat. You are moving more than most people you know. You have cut the obvious things, tracked what you were told to track, and been patient with it. And the number has not moved or it moved briefly and then stopped.
At that point, most people conclude one of two things: that they are not trying hard enough, or that this is simply what happens with age.
Neither is a useful explanation, and in clinic neither is usually the correct one.
Why can’t I lose weight even though I’m doing everything right?
Body weight is not simply a matter of arithmetic. It is actively regulated by hormones and by the way your cells respond to them.
The energy in and energy out model is not wrong, but it is incomplete and oversimplified. It describes the outcome without describing the control system. What determines how much of what you eat is stored rather than used, how readily stored fat is released, how much muscle you hold, and how hungry you feel two hours after a meal, is hormonal signalling.
When that signalling is working well, sensible behaviour produces sensible results. When it is not, you can do everything correctly and see very little change for it.
This is why two people can follow the same approach and get different outcomes. It is not effort. It is the system the effort is acting on.
There are a small number of drivers that account for most of what we see. Each one is identifiable, and most are measurable.
Could insulin resistance be blocking weight loss?
Insulin resistance is one of the most common drivers of stalled weight loss, and one of the least likely to be picked up early.
Insulin’s job is to move glucose out of the bloodstream and into cells. When cells respond less readily to that signal, the body compensates by ‘shouting’ at the cells - producing more insulin to achieve the same effect. Circulating insulin rises, and because insulin is a fat storage signal, the body becomes more inclined to store energy and less inclined to release it.
The practical effect is that fat loss becomes harder even when intake has not changed. Weight tends to settle around the abdomen. Energy becomes less stable across the day.
This can develop over years, and it can be present well before blood glucose looks abnormal.
A standard panel that measures glucose alone can look entirely reassuring while insulin is doing considerable, and eventually unsustainable, work in the background to keep it there. I’ve seen patients with optimal HbA1c results who are also deeply insulin resistant, but still compensating.
At Autonomy we measure markers of insulin resistance such as fasting insulin, oral glucose tolerance ‘stress’ tests that also include insulin, HOMA-IR and real-time glucose responses via continuous glucose monitors.
We look for specific biomarker patterns of insulin resistance that show long before high glucose results are a red flag for prediabetes or Type 2 diabetes.
Can your thyroid cause weight gain?
Yes. Thyroid hormones set the pace of metabolic activity, so when thyroid function drops, energy expenditure drops with it.
Underactive thyroid function typically produces a cluster of changes rather than weight alone: fatigue that sleep does not resolve, feeling cold, dry skin, low mood, slowed digestion. Weight change on its own is rarely the whole picture.
It is also an area where interpretation matters. TSH alone provides a limited view of thyroid function, and a result sitting inside a broad reference range does not always mean thyroid signalling is working well for that individual. Deeper testing of thyroid function is needed for this.
Why is it harder to lose weight after 40?
Largely because of muscle. Muscle is metabolically active tissue, and from around the fourth decade most people lose it steadily unless they are deliberately working to hold it.
Less muscle means a lower resting metabolic rate, so the same diet that once maintained your weight now slowly adds to it. Muscle is also where most glucose is taken up after a meal, so losing it makes blood glucose harder to regulate, which feeds directly back into the insulin picture described above.
This is why weight can begin to change in your forties without anything else changing. The inputs stayed the same. The tissue doing the metabolic work did not.
It is also why scale weight alone is a poor guide to progress. Two people at the same weight can have substantially different amounts of muscle and fat, and the difference matters far more than the number.
Does perimenopause make weight loss harder?
For many women, yes, and usually through several mechanisms at once rather than one.
As oestrogen declines, fat tends to redistribute centrally rather than accumulate faster overall, visceral fat increases even when total weight is stable. This shift is compounded by loss of lean muscle mass, which lowers resting metabolic rate, and by increasing insulin resistance, independent of changes in fat mass. Sleep is frequently disrupted, and disrupted sleep independently affects appetite signalling (leptin, ghrelin) and blood glucose handling.
The result is that an approach which worked reliably for two decades can stop working, over a period where nothing about your effort has changed.
This is a hormonal transition with measurable metabolic consequences, and the two are worth looking at together rather than separately.
How do sleep and stress affect your weight?
Both act on the same hormonal systems that regulate appetite and storage, which is why they change outcomes even when diet and activity are unchanged.
Short or fragmented sleep alters the balance of the hormones governing hunger and fullness, and reduces insulin sensitivity after even a few poor nights. The effect is not subtle, and it is not a question of willpower, which invariably fails.
Cortisol works on a daily rhythm rather than as a single value. When that rhythm is disrupted, eg. elevated when it should be falling, flat when it should be rising, it promotes central fat storage and makes glucose harder to regulate.
If your sleep has been poor for a long stretch, or your stress load has been sustained rather than occasional, it is reasonable to treat that as part of the metabolic picture rather than as background noise.
What is metabolic syndrome?
Metabolic syndrome is the name given to a cluster of findings that tend to appear together: central weight gain, raised blood pressure, unfavourable cholesterol patterns, and rising blood glucose.
What makes it useful as a concept is that it is a pattern rather than a single result. Each individual finding may sit close enough to normal to pass without comment. Together they describe a direction of travel.
It also tends to develop quietly. Most people do not feel it arriving, and it is often identified only when one of its components crosses a threshold and becomes a diagnosis in its own right.
Do weight-loss medications work, and what happens afterwards?
This class of medication does produce meaningful weight loss for many people. The more useful question is what it is being used alongside, and what is the plan when it stops?
Two things are worth understanding before starting. The first is that weight lost includes lean mass — muscle, and to some extent bone density — as well as fat, unless muscle is actively protected through resistance training and adequate protein intake. This matters, because muscle is your metabolic engine and a crucial part of what regulates weight in the first place. The second is that stopping is not neutral. In trial data, most people who discontinue these medications regain a substantial proportion of the weight within a year, along with reversal of the metabolic benefits that came with losing it.
None of that is an argument against medication. It is an argument for knowing your starting position, measuring what changes, and having a plan for maintenance before you need one.
What should you test if you can’t lose weight?
If weight has stopped responding to what used to work, the useful next step is not more effort. It is finding out which of these systems is involved in your case.
The picture is usually built from a combination of markers rather than one test: how your body handles glucose over a many days rather than at a single fasting moment, insulin as well as glucose, thyroid function interpreted beyond a single value, sex hormones where relevant to your stage of life, cortisol assessed as a rhythm rather than a snapshot, and inflammatory markers such as HsCRP, which is not routinely included on a standard New Zealand blood panel.
Body composition belongs in that picture too. Knowing how much of your weight is muscle and how much is fat tells you something the scale cannot, and it changes what a sensible plan looks like.
The point of testing is not to accumulate numbers. It is to work out which mechanism is operating, so that what you do next is aimed at the right thing.
What is a sensible next step?
Weight that will not move is a signal, not a character flaw. In most cases something measurable is working against the effort, and it is more useful to identify it than to try harder against it.
More than 650 New Zealanders have completed an Autonomy clinical programme since inception. (Source: Autonomy patient records.)
Most people start with a Discovery Consultation. It is a conversation with a doctor about what you have been experiencing, what has already been looked at, and whether a fuller investigation is likely to tell you something useful. Sometimes it leads to testing. Sometimes it leads to a clear explanation and a sensible next step without further investigation.
Either way, the aim is the same: to understand what is actually driving this, before deciding what to do about it.
Dr Ula Heywood
MBChB, FACEM — Co-Founder and Lead Physician, Autonomy
