In a Calorie Deficit With PCOS (Now PMOS) and Not Losing Weight? What's Actually Going On

You have been tracking for six weeks. The app says 1,500 calories. You have been honest about the weekends. And the scale has moved about a kilogram, in the wrong direction, twice.

So you searched, and the internet handed you a tidy explanation: PCOS — now PMOS, after the 2026 Lancet consensus renamed it Polyendocrine Metabolic Ovarian Syndrome — slows your metabolism, so calories work differently for you. Some pages go further and tell you that counting calories is pointless with PMOS and you should stop.

We read the evidence behind that claim. It does not hold up the way it is usually told, and the real answer is both more interesting and more useful.

First, the question you actually asked: is your resting metabolism slower?

This is the assumption underneath almost every page on this topic, and it has now been tested directly.

A 2026 systematic review and meta-analysis pooled thirteen studies that measured resting energy expenditure — the calories your body burns at rest — in women with PMOS against women without it. The pooled difference was about 30 calories a day, with an interval running from −47 to +113. In other words, the difference was not distinguishable from zero, and 30 calories is roughly one bite of toast. The authors' conclusion is that resting energy expenditure does not meaningfully differ between the two groups.

Two honest caveats. This is a preprint, which means it has not yet been through peer review, and we will update this page when it is. And it is a group-level average: it says the typical woman with PMOS does not have a slower resting furnace, not that no individual variation exists.

But it does mean the most repeated explanation for your stalled scale is probably not the explanation. That is worth knowing before you spend another three months trying to fix a metabolism that is working normally.

So why isn't it working?

Two things are true at once here, and most pages only tell you one of them.

The first is unglamorous: the deficit is usually smaller than it looks. Not because you are lying to yourself — because measurement is genuinely hard. Portion sizes drift upward over weeks. Cooking oil, milk in coffee, and the handful of something while cooking are the classic uncounted items. Meanwhile the "calories burned" figure your watch reports is an estimate built on population averages, and it is the least reliable number in the whole equation.

Then there is water. Body weight on any given morning includes glycogen and the water bound to it, plus sodium shifts and, if you are still cycling, fluid changes across the month. This is standard physiology rather than anything specific to PMOS, but it matters here: several weeks of real fat loss can be entirely masked on the scale by a few hundred grams of retained fluid. If your weight is flat but your waistband is not, that is the most likely reading.

The second is the part that is specific to you, and it is where the honest answer diverges from the dismissive one.

The finding almost nobody quotes: the same programme produces very different results

In 2024, researchers at Karolinska Institutet reported on 55 women with PMOS and a BMI of 27 or above who had been randomised either to a four-month behavioural modification programme or to standard care.

The average weight change after the behavioural programme was −2.2%. Statistically significant, and considerably less than most people expect from four months of structured effort.

The spread underneath that average is the real story. In the behavioural group, 17% of the women lost more than 5% of their body weight — and 21% gained weight. Same programme, same duration, same support. In the control group the figures were 15% and 42%.

Sit with that for a moment, because it reframes the question. Roughly one in five women doing the supervised thing properly still saw the number go up. If that is you, you are not an anomaly and you are not failing at something everyone else finds straightforward. You are inside a well-documented distribution that consumer weight-loss advice almost never mentions.

What predicted which group a woman landed in was not willpower. In that study, baseline gene expression in subcutaneous fat tissue, together with waist-to-hip ratio, explained about 31% of the variation in weight change. Most of the rest remains unexplained. Anyone telling you they know exactly why your body is responding the way it is, is telling you more than the evidence supports.

What PMOS actually changes — and it isn't the arithmetic

Putting those two findings together gives a picture that is neither "calories don't count for you" nor "you must be miscounting."

Energy balance still governs the outcome. What PMOS changes is how hard that balance is to reach and hold. Insulin resistance sits at the centre of the condition, and higher circulating insulin makes stored fat more reluctant to be released while doing nothing helpful for hunger. Appetite regulation, sleep, and the fatigue that so often travels with PMOS all push in the same direction — not on the arithmetic, but on your capacity to sustain the deficit for the months it takes.

That distinction matters practically. It means the lever is rarely "eat even less." Cutting deeper tends to increase hunger and reduce adherence, which is how a stalled 1,500 becomes an unplanned 2,200 by Sunday evening. It also means the driver behind your PMOS is worth identifying, because an insulin-driven pattern, an androgen-driven pattern, and a cycle where the weight is really being driven by something else entirely do not respond to the same first move.

Two look-alikes are worth ruling out rather than assuming. Thyroid underactivity and iron deficiency both cause fatigue and weight change and both are common in the same women — we cover how to tell them apart on our fatigue, thyroid and anaemia page. And if your BMI sits in the normal range while everything else looks like PMOS, lean PMOS behaves differently again.

What the guideline recommends, and what it doesn't

The 2023 International Evidence-based Guideline for the assessment and management of PCOS is clear on one point that saves a lot of wasted effort: no particular diet composition has been shown to be superior for PMOS. Low carbohydrate, low fat, Mediterranean — trials comparing them have not produced a winner. The energy deficit is what does the work, so the best composition is the one you can actually keep to.

The guideline also affirms the familiar 5–10% figure as the point at which meaningful metabolic and reproductive benefit tends to appear. Read alongside the Karolinska numbers, that is a demanding target: it is roughly double what the average supervised participant achieved in four months. Which is an argument for a longer horizon, not a harder deficit.

One more thing worth saying plainly. If the scale has not moved in twelve weeks of genuine consistency, the useful next step is not another restriction. It is finding out which driver is actually in play, and whether anything else is contributing.

That is what our free 5-minute assessment is built for. It reads your pattern across cycle, weight, skin and energy and shows you which driver looks dominant. It asks for no bloodwork and it is not a diagnosis — it is a starting point that tells you where to look first.

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This article is for informational purposes only and does not constitute medical advice. If your weight has changed rapidly or unexpectedly, or you have symptoms that concern you, speak to a doctor.

Sources

• Kirwan R, Peele L, Nuckols G, Kohlhoff G, Cabré H, Olenick A, Steele J. Resting energy expenditure of women with and without polycystic ovary syndrome: a systematic review and meta-analysis. medRxiv [Preprint] 2026 Jan 6. doi:10.64898/2025.12.03.25341536. PMID 41409676; PMCID PMC12706607. (Preprint — not yet peer-reviewed.)
• Hellberg A, Salamon D, Ujvari D, Rydén M, Hirschberg AL. Weight Changes Are Linked to Adipose Tissue Genes in Overweight Women with Polycystic Ovary Syndrome. International Journal of Molecular Sciences 2024 Oct 28;25(21):11566. doi:10.3390/ijms252111566. PMID 39519120; PMCID PMC11547111.
• Teede HJ, et al. International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome, 2023. Monash University / international consortium.
• Glycogen- and sodium-related shifts in scale weight are established general physiology and are not tied here to a single study.