Hello,
A quick note before we jump into today’s topic: on Friday I shared The Full Picture, a set of modules that takes each PMOS symptom in turn, explains what is driving it, and shows what the evidence says you can do about it. You can start with the free module on how your cycle works. Founders pricing ($29 per module, or $99 for The Full Collection) closes on Thursday, 12 November at 12pm ET.
Last week we spoke about racism and how it shaped our view of body shape. Today we understand where the BMI measure came from and how skewed it is.
The “average man”
Around the same time that body size was becoming a moral marker, a Belgian mathematician called Adolphe Quetelet was trying to describe the “average man”.
Quetelet was a statistician and astronomer who applied probability to human characteristics. In 1832, from his studies of human growth, he observed that weight tends to increase with the square of height. This ratio, weight in kilograms divided by height in metres squared, became known as the Quetelet Index.
His aim was statistical: to describe populations. He was not trying to diagnose individuals, and he was not building a racial hierarchy. But his “average” was based on the European populations he studied, and the idea of an average body quietly became the idea of a normal one.
From insurance tables to BMI
The next chapter comes from insurance companies. Actuaries noticed that their heavier policyholders had higher mortality, which triggered a search for a simple index of relative weight.
In 1972, the American physiologist Ancel Keys and colleagues compared several weight-for-height formulas and concluded that Quetelet’s was the most useful. They renamed it the body mass index. Their analysis was based on 7,424 “healthy” men from 12 groups in five countries: the US (white men), Finland, Italy, Japan and South Africa.
Notice what’s missing. No women were included.
Over the next 20 years, BMI became the standard way of defining overweight and obesity, both in populations and in individual patients.
I am now offering 1:1 online coaching for women with PMOS. You can book via: https://leya.health
What BMI misses
Today, strong evidence shows that the same BMI does not mean the same thing for everyone.
NICE now recommends lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds: overweight from a BMI of 23, and obesity from 27.5.
In 2023, the American Medical Association went further. It adopted a policy recognising the historical harm of BMI, its use for racist exclusion, and the fact that BMI cut-offs are based mainly on data from previous generations of non-Hispanic white populations. It also noted that while BMI correlates with body fat across a population, it loses predictive power at the individual level.
If you measure thousands of people, those with a higher BMI tend, on average, to have more body fat. The two rise together across the group. That’s what “significantly correlated” means. It’s why BMI is useful for public health research, such as tracking how obesity rates change in a country over time.
At the individual level
When you apply that average to one person, it often gets them wrong, because BMI only uses height and weight. It can’t tell:
Muscle from fat. A muscular rugby player can have a BMI in the “obese” range with very little body fat.
Where the fat is stored. Fat around the organs (visceral fat) carries much more metabolic risk than fat on the hips and thighs, but BMI treats them the same.
Differences between people. At the same BMI, women generally carry more fat than men, older adults more than younger adults (as they lose muscle), and different ethnic groups carry different metabolic risk, as in the Caleyachetty data in part 3.
So two women can have exactly the same BMI and very different bodies inside. One may have plenty of muscle and low visceral fat, while the other has little muscle and more fat around her organs.
An analogy
It’s a bit like shoe size and height. Across a crowd, taller people tend to have bigger feet. But if I only know your shoe size, I can’t tell you your height with any confidence. The pattern is real across the group, but it’s a poor guide for any one person.
The PMOS connection
This matters for PMOS because the condition itself looks different across ethnic groups.
A systematic review of 35 studies compared women with the condition from different ethnic backgrounds . South Asian and East Asian women tended to have a lower BMI but more fat around the middle. South Asian, Middle Eastern, Hispanic and Black women had a higher prevalence of insulin resistance than white women. Black women had better cholesterol profiles but higher blood pressure and cardiovascular risk.
Put that next to everything above, and a pattern emerges. If your clinician relies on a standard BMI chart, a South Asian woman with PMOS at a “normal” weight may be told her metabolism is fine, when her risk is actually higher. And a Black woman with PMOS may be given weight-focused advice without anyone checking her blood pressure.
What you can do with this
BMI is not useless. It’s quick, free and still useful for looking at populations. But it’s one number, and it was never designed with you in mind. A few things that can give a fuller picture:
Waist-to-height ratio. NICE recommends keeping your waist to less than half your height, and says this measure works across all ethnicities and sexes (NICE, 2022). You can measure it yourself at home: if your waist measurement divided by your height is above 0.5, it may be worth a conversation with your GP.
Ask whether your BMI thresholds reflect your ethnicity. If you are from one of the groups NICE lists, the lower cut-offs apply to you.
Ask about metabolic markers, not just weight. Blood pressure, cholesterol, HbA1c (average blood sugar) and, where available, insulin levels tell you far more about what is happening inside your body than the scale does.
Even when my BMI was 26.5 my care still focused on my weight…
Next week
We’ve seen how body size became a moral marker, and how a number was attached to it. Next week, we look at how fashion turned that marker into an aesthetic: from the 1920s flapper to Chanel, Dior, Marilyn Monroe and heroin chic, and what feminism had to do with it.
P.S. Founders pricing on The Full Picture closes this Thursday. If you haven’t watched the free module yet, you can find it https://leya.health/course.
See you next Sunday,
Francesca
Eknoyan, G. (2008). Adolphe Quetelet (1796–1874): The average man and indices of obesity. Nephrology Dialysis Transplantation, 23(1), 47–51. https://doi.org/10.1093/ndt/gfm517
Fletcher, I. (2014). Defining an epidemic: The body mass index in British and US obesity research 1960–2000. Sociology of Health & Illness, 36(3), 338–353. https://doi.org/10.1111/1467-9566.12050
Keys, A., Fidanza, F., Karvonen, M. J., Kimura, N., & Taylor, H. L. (1972). Indices of relative weight and obesity. Journal of Chronic Diseases, 25(6), 329–343. https://doi.org/10.1016/0021-9681(72)90027-6
NICE. (2022). Keep the size of your waist to less than half of your height, NICE recommends. https://www.nice.org.uk/news/articles/keep-the-size-of-your-waist-to-less-than-half-of-your-height-nice--recommends
Sendur, S. N., & Yildiz, B. O. (2021). Influence of ethnicity on different aspects of polycystic ovary syndrome: A systematic review. Reproductive BioMedicine Online, 42(4), 799–818. https://doi.org/10.1016/j.rbmo.2020.12.006