The 40% value was determined by Robert Lustig, I believe, as an approximation based on this study:

Prevalence of Optimal Metabolic Health in American Adults: National Health and Nutrition Examination Survey 2009–2016: https://doi.org/10.1089/met.2018.0105. (Open access).

The study is based on data from 2009 - 2016 and is a survey of 8,721 people. The researchers defined five factors for metabolic health and surveyed people who were not on medications. They found that only 12.2% of American adults are metabolically healthy. In addition, the researchers found that less than one-third of normal weight adults (defined by BMI) were metabolically healthy.

It isn't saying that you can be overweight or obese and not worry about metabolic health (although there is some research describing a small percentage of people who are overweight yet do not present metabolic syndrome), but it is suggesting that metabolic syndrome can appear in "normal weight" individuals and that obesity is not the only risk factor for developing chronic diseases such as dementia (although it does exacerbate it). It's the upstream problem of metabolic syndrome caused by chronically elevated insulin levels - related to diet.

There is also this study exploring normal-weight individuals who are metabolically unhealthy and risk factors for type 2 diabetes: https://doi.org/10.1016/j.metabol.2015.03.009. There's also many papers regarding the TOFI phenotype (thin-outside-fat-inside).

Also a nice open access review here on metabolic syndrome which includes some points I have raised here: https://doi.org/10.1007/s11906-018-0812-z

There is a wealth of studies linking western-style diets with all sorts of chronic diseases (including autoimmune disease). Just focusing on obesity alone doesn't cover the whole picture.

What science is pointing to recently is that at the route of “later-in-life” diseases such as hypertension, dementia, heart disease etc., is a typical western-style diet, i.e., high in ultra-processed foods and sugar/carbohydrates.

I’ve heard notable researchers say that obesity is just the symptom of what is happening, which is metabolic disease caused by sugar. These diseases are also just the downstream affects of chronic insulin irregularity that has shown itself in different tissues/organs, such as the brain (dementia), the heart (heart disease), specific cancers.

Of course there are genetic risk factors and other environmental causes but all of these diseases are increasing in prevalence. This is highlighted by the fact that 40% of the U.S population that isn’t obese or overweight, has metabolic dysfunction, putting them in danger of developing at least one of the major diseases.

Hardly any medical professional is actually talking about this though