PCOS and weight: what losing even a little actually does

If you have been told to lose weight for your PCOS, you have probably also been told a number — usually 5%. It is a well-meant simplification of something more useful.

First, the name

In May 2026 an international consensus process renamed polycystic ovary syndrome to Polyendocrine Metabolic Ovarian Syndrome (PMOS) ^[3]. The reasoning is that the old name put the emphasis on ovarian cysts, which are neither universal nor the central problem, and buried the metabolic part, which usually is.

There is a transition period, and “PCOS” remains the term most people, and most search engines, still use. This article uses both.

What it has to do with weight

Insulin resistance sits near the centre of the condition — which is why “metabolic” is now in the name. Higher circulating insulin drives the ovaries to produce more androgens, which disrupts ovulation and produces the acne, excess hair growth and irregular or absent periods that bring most people to a GP.

Excess weight worsens insulin resistance. So weight and PMOS feed each other, which is frustrating, but also means the loop can be interrupted.

Not everyone with PMOS has a high BMI. Plenty of people with the condition are a healthy weight and still have marked insulin resistance. Weight is one lever, not the definition.

The number worth knowing

A UK multicentre trial, BAMBINI, studied 75 women with PMOS, obesity and absent ovulation. A post-hoc analysis produced the most useful figure in this area ^[2]:

• Each 1% reduction in body weight was associated with a 5.6% increase in the odds of ovulatory recovery
• There was no lower threshold — any degree of weight loss was associated with a higher likelihood of ovulating
• Recovery rates were highest in the 10–15% weight-loss band

That is a better message than “lose 5%”. It means the first two kilograms are already doing something, which matters when the target feels distant.

For context, in that trial ovulation returned in 72% of the surgical arm, which achieved 28.5% weight loss, against 31% in the medical arm, which lost under 2%.

The test you are likely to be offered and should decline

Private clinics and supplement sellers frequently offer fasting insulin or HOMA-IR testing for PCOS. International guidance is explicit that clinically available insulin assays are of limited clinical relevance and should not be used in routine care ^[1].

If you want a metabolic picture, HbA1c, lipids and blood pressure are the tests that change management.

How common is it, really

Community prevalence using diagnostic criteria is around 10 to 13%, and the Endocrine Society now quotes roughly 1 in 8 women. But UK GP-recorded prevalence was only 3.5% in 2020 ^[5].

The gap is under-diagnosis, not disagreement. If you have irregular cycles, unexplained acne or hair changes and have never been assessed, that is worth pursuing.

What actually helps

A sustainable energy deficit. International guidance is clear that no single diet composition is superior for PMOS ^[1]. Low carb is not required. What matters is a deficit you can maintain.

Resistance training. Muscle is the main site of glucose disposal, which is the exact thing that is not working well.

Metformin, sometimes. Guidance suggests considering it at BMI 25 and above for metabolic and anthropometric outcomes, usually starting low and building up to a maximum of 2.5 g daily. In the UK, metformin for PCOS is an off-label use — it is not licensed for this ^[1].

Letrozole, if you are trying to conceive. Now recommended ahead of clomifene for ovulation induction. Also off-label in the UK.

Weight-loss medicines, by the ordinary route. No GLP-1 medicine is licensed in the UK for PMOS. A woman with PMOS may be eligible under the usual weight-management criteria — that is a prescriber’s assessment. If you are prescribed tirzepatide and use oral contraception, note the specific advice to add a barrier method for four weeks after starting and after each dose increase.

When to speak to a clinician

Seek assessment if you:

• have fewer than nine periods a year, or none, and are not pregnant or on contraception that stops them
• have acne, excess facial or body hair, or scalp hair thinning alongside irregular cycles
• have been trying to conceive for a year, or six months if you are over 35
• have PMOS and have not had HbA1c, blood pressure and cholesterol checked
• have been offered insulin resistance testing and want to know whether it is worth doing

PMOS also carries a raised long-term risk of type 2 diabetes, so periodic metabolic screening is part of standard care rather than an extra.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.