You cannot tell how healthy a woman is by looking at her

Of everything a body tells a stranger, its health is what it keeps best hidden. Size is on display from across the street. Blood pressure isn’t. Neither is blood sugar, or cholesterol, or the state of a liver, or how far a woman could walk before she needed to stop, or how she slept last night, or whether she has eaten properly this month. Almost everything that makes up health is invisible, which has never stopped anyone reading it off a silhouette.

That reading is a guess: the one visible fact, presented as the obvious truth about everything else. The useful thing about a guess is that it can be tested.

What happens when you actually measure

Using a nationally representative sample of more than 40,000 American adults, one team of researchers compared people’s body mass index with a set of real markers of cardiometabolic health: blood pressure, triglycerides, cholesterol, blood sugar, insulin resistance and C-reactive protein, a marker of inflammation. Nearly half of the people classed as overweight, and 29 per cent of those classed as obese, were metabolically healthy on those measures. More than 30 per cent of people in the “normal” weight range were not.

That is not a small margin of error. It means that if you sorted a crowd into healthy and unhealthy by appearance alone, you would get a very large number of people wrong in both directions.

Medicine is saying the same thing

Medicine itself has been revising how it uses BMI. In 2023, the American Medical Association adopted a policy recognising that BMI, while correlated with body fat across whole populations, loses predictive power when applied to an individual, and recommending that it be used alongside other measures rather than on its own.

Then, in January 2025, a global commission published in The Lancet Diabetes & Endocrinology went further. It recommended that excess body fat should be confirmed by something more than BMI — a waist measurement, for example, or a direct measurement of body fat — and drew a line between “clinical obesity”, in which excess fat is actively harming the body’s organs or a person’s ability to carry out daily activities, and “preclinical obesity”, in which there is excess fat and a higher future risk, but no current illness caused by it.

You do not need to follow every detail of that debate to see its direction. Doctors are being told, by their own experts, that a body’s size is a starting point for asking questions, not an answer to them.

What this does not mean

It would be easy to take all of that and conclude that weight tells you nothing about health. That isn’t true either, and I would rather be accurate than comforting.

One of the largest long-term studies of women’s health followed more than 90,000 American nurses for up to thirty years. Women with obesity who were metabolically healthy at the start still had a higher risk of cardiovascular disease than metabolically healthy women of normal weight. And most women who began the study metabolically healthy — of every size — had become metabolically unhealthy within twenty years, including 84 per cent of those with obesity and 68 per cent of those of normal weight.

So body weight is one real factor among many, and health is not a fixed state that anyone earns once. Which is exactly why it has to be measured, repeatedly, for everyone — rather than inferred from appearance and then left alone.

The thin woman’s version of the mistake

The same guess that labels a larger woman unhealthy labels a slimmer one healthy, and that error does its own damage. It is one reason a woman in the “normal” range can go years without anyone checking her blood pressure or blood sugar. It is one reason eating disorders go unnoticed: the Academy for Eating Disorders is clear that eating disorders affect people of all body shapes and weights, and that you cannot tell who has one by looking. And it is why a woman who has lost a great deal of weight through illness gets complimented rather than asked how she is — the strange habit of congratulating weight loss before anyone knows its cause.

Appearance-based health judgements fail everybody. They simply fail larger women more loudly.

What actually tells you

If you want to know how healthy a woman is, the honest answer is that you usually don’t need to know, because it isn’t your business. If the woman is you, the answer lies in things that can be measured and felt: blood pressure and blood tests at the intervals your doctor recommends; how your body handles the stairs, a walk, a day’s work; how you sleep; how your mood has been. None of those can be read in a mirror, and all of them can change, in either direction, at any size.

That is also the kind of health worth working towards: the blood pressure, the stamina, the sleep — the things a number on a scale was only ever standing in for. It is easier to pursue without hating your body, and with a doctor who looks past your weight when you need one to.

Look at a woman and you will learn her size, and perhaps her taste in coats. That is all. Everything else, you would have to ask about — or, better, leave to her and the people she trusts to measure it properly.

Sources

• Tomiyama AJ, Hunger JM, Nguyen-Cuu J, Wells C. Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity, 2016.
• American Medical Association. AMA adopts new policy clarifying role of BMI as a measure in medicine, 2023.
• Rubino F, et al. Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology, 2025.
• Eckel N, Li Y, Kuxhaus O, et al. Transition from metabolic healthy to unhealthy phenotypes and association with cardiovascular disease risk across BMI categories in 90 257 women (the Nurses’ Health Study). The Lancet Diabetes & Endocrinology, 2018.
• Academy for Eating Disorders. Nine Truths About Eating Disorders.