The golden health index you have been using is being questioned by the scientific community and has a complicated history

The widely known BMI index was not used to measure a person's health status in its early days. It was not until insurance companies linked weight to life expectancy that the BMI index was promoted as an authoritative health indicator. Since then, the idea that obesity is harmful to health has been deeply rooted in people's minds, and it has also shaped a new era of anxiety.

Written by | Xiaoye

There are thousands of terms in physiology and medicine, most of which are obscure and difficult to understand, but none of them is as popular as the body mass index (BMI). According to the definition of the World Health Organization, BMI is a basic index that measures the relationship between height and weight, and is usually used to determine whether an adult is overweight or obese [1]. In daily life, BMI is especially regarded as the "gold standard" by the weight loss community, and in academia, BMI is also widely used in epidemiological studies related to obesity.

However, in recent years, there have been constant questions about BMI in the academic community, mainly regarding the question of "whether BMI is still applicable to various types of research." In January this year, six obesity care organizations in the United States issued a joint statement, saying that "BMI for screening obesity cannot replace clinical diagnosis, and BMI is not a standard for measuring body fat. Even if we can determine an individual's BMI value, race, age and other social factors may change the risk of disease." [2]

Figure 1. Six American obesity care organizations issued a joint statement. Source: American Obesity Association [2]

In fact, BMI has a long history. From its creation to its introduction into modern medicine and finally its connection with obesity research, the purpose and significance of BMI have undergone a tortuous development for more than two hundred years. Understanding the past of this index, its connotation, advantages and limitations, will help the academic community to determine whether it can continue to serve scientific research, and for us in our daily lives, it will help us to look at this index rationally.

Quetelet’s “Perfection Index”

BMI was born in Europe in the 19th century. At that time, it was not called "body mass index" but Quetelet index, named after its creator Adolphe Quetelet.

Figure 2. Statue of Adolphe Quetelet located at the Palace of Sciences in Brussels, Belgium. Source: brusselsremembers.com

Adolphe Quetelet was born in 1796 in what is now the historic town of Gent, Belgium, the fifth of nine children. As a child, he showed extraordinary talent, especially in mathematics, and was also fascinated by the humanities. In 1815, Quetelet entered the newly established University of Ghent and studied under French mathematician Jean Guillaume Garnier, earning a doctorate in science in four years.

In the early 19th century, the most outstanding scientists in Europe set their sights on the sky. Whether a country could build astronomical telescopes and observatories became an important criterion for measuring the country's scientific status. In 1823, Quetelet successfully persuaded the Dutch government (Belgium had not yet gained independence from the Netherlands) to provide financial subsidies to build an observatory in Brussels, and was appointed director of the observatory. While the observatory was being built, he went to Paris, France to conduct astronomical research. In Paris, he met the famous Joseph Fourier, Siméon Poisson and Pierre Laplace, studied with them, and poured endless enthusiasm into learning the theory of probability.

However, plans cannot keep up with changes. Quetelet, who was abroad, learned that a revolution had broken out in his country and the observatory under construction was occupied by the revolutionary army. Before this, Quetelet had no interest in politics or the complexity of the dynamic changes in human behavior. However, social unrest obviously affected the development of his personal scientific research career and became a turning point in his career. Quetelet originally planned to study the hidden patterns of celestial bodies in space, but the social upheaval made him set a new goal: to use the method of averages used in astronomy to analyze humans and their behavior.

In astronomical research, to measure the speed of a celestial body, ten scientists may each obtain ten different measurement values. Which one is the true value is a big problem. But scientists soon found a simple and effective solution: sum up all the individual measurement values ​​obtained from separate observations to get an average value. Compared with relying on a single observation result, this method can improve the accuracy of the observation result. Quetelent, who firmly believed in this method, applied the same idea to human research, trying to find the average value of human attributes, and thus created the concept of "Average Man". He believed that there is a human prototype with a perfect average of various attributes, and every independent individual in reality is a flawed replica of this perfect prototype. In his view, individuals are equivalent to "errors", while "average man" represents a truly perfect person [3].

The trend of the times also helped him build his theory. Quetelet happened to catch up with the first wave of "big data" in human history. Countries began to build huge bureaucratic management systems to count and publish a large amount of data about their citizens, such as the number of births, the number of deaths per month, the number of crimes per year, etc. After returning to the Netherlands, Quetelet participated in a national census conducted by the Dutch government. During the investigation, he obtained a large amount of population-related data to unveil the mystery of the "average human type": he calculated the average height, average weight, average skin color, average age of marriage of couples, average age of human death, average birth rate, average education level, and even the average annual suicide rate... All in order to obtain the average value of human attributes.

Starting in 1831, Quetelet conducted a series of cross-sectional studies on human growth. In 1832, he published an article titled "Research on the weight of man at different ages" in the Proceedings of the Academy of Sciences [4]. In 1835, he compiled all his related research into a book and published his three-volume masterpiece "A Treatise on Man and the development of his aptitudes" [5]. In the second chapter of the second volume, "The development of human weight and its relationship with height development", he compared the height and weight of hundreds of men, women, children and infants in Europe. He also collected various records from hospitals, orphanages and even factories, including 9-year-old child laborers. The results showed that on average, the weight of a person (the person here is not a specific individual) should be proportional to the square of his height. This ratio is Quetelet's Index, which has influenced the next hundred years.

Figure 3. Quetelet used in the research to collect children’s height and weight data, source: STAT

It is worth mentioning that the Quetelet Index reflects the average value of human attributes that Quetelet pursues, that is, the "perfect state" of weight relative to height. Moreover, this index is only for statistical purposes. The calculation results obtained by large-scale measurement of population-level data have nothing to do with personal body fat, body shape or health. Quetelet himself is not interested in issues such as obesity. In addition, Quetelet's data sources are mainly French and Scottish, so the Quetelet Index is designed for European whites, which is also an important limitation of the index.

Quetelet also wrote in his Essay on Man and the Development of His Capacities: "Anything that differs from the average proportions and conditions of the average human being is considered a deformity or disease... If an individual in any social and historical context possesses all the average attributes of the average human type, then he is the representative of greatness, goodness and beauty." [5] In the second half of the 19th century, Francis Galton, the famous father of eugenics, borrowed Quetelet's concept of the average person, but made an important change, which cast a shadow of racism on the Quetelet index (Adolphe Quetelet himself was not a racist scientist). Galton equated the average attributes proposed by Quetelet with "mediocrity" rather than an ideal state, and therefore needed to be surpassed, improved and overcome. [6] By the end of the 19th century and the beginning of the 20th century, eugenics at its peak used the average person as a standard for measuring the health of parents, and as a scientific basis for the systematic sterilization of people with physical disabilities, people with mental disorders such as autism and schizophrenia, LGBTQ+ people, immigrants, poor white people, indigenous people, people of color and ethnic minorities around the world. [7-9]

The “Ideal Index” for the Insurance Industry

The idea that weight is one of the most important health indicators has only become popular in the past 100 years. The insurance industry, especially the life insurance industry, has made a great contribution to the promotion of this idea. In the late 19th century and early 20th century, American life insurance companies began to pay attention to and investigate the mortality risks associated with weight measurements. They drew a weight-height comparison table based on the research method of the Quetelet index, trying to associate weight with health and longevity, so as to evaluate policyholders and provide relevant guidance for insurance purchase and medical selection processes [10].

At the beginning, the comparison tables used by insurance companies had a lot of problems and were extremely unreliable. First, the weight and height data obtained by insurance companies came entirely from the self-measurement reports of life insurance customers within a specific period of time, and they were not random samples; second, there was no unified measurement standard for the data, and some users measured their height and weight while wearing shoes and thick clothes, resulting in numerical deviations; finally, the medical examiners of each insurance company had their own set of selection criteria, and the final evaluation forms developed varied greatly from company to company. However, in 1895, George Shepher, the medical director of MassMutual Life Insurance Company of Connecticut, was commissioned by the American Life Insurance Medical Directors Association (ALIMDA) to spend two years analyzing data from more than 70,000 policyholders in the United States and Canada, and eventually established a set of standardized height-weight comparison tables that were widely adopted by the industry.

Some people may ask: Why is it the insurance company that draws up the weight-height comparison table instead of the medical institution?

Profits are a major driving factor. As profit-making institutions, insurance companies can obtain the highest profits by improving the medical selection process of future policyholders, so they are very motivated. In addition, the institutionalization of the US medical system was difficult at the beginning. There were many aspects that could not be reached within the medical community, and there were also legitimacy issues outside. As a result, the right to speak on standard setting was handed over to the more mature insurance industry[8].

By the early 1940s, the largest life insurance company in the United States, Metropolitan Life Insurance Company, combined medical experience with more accurate survey data to continuously improve the weight-height comparison table, and eventually launched an "ideal weight" table that did not include age factors. It also introduced a set of arbitrary and subjective body shape classifications: small, medium, and large[11].

In this way, the ideal weight table, which was originally a selection standard for medical procedures, has now been transformed into an "ideal height-weight recommendation guide" for the public, greatly deepening the public's awareness of the risks of obesity. Insurance companies set up welfare departments, launched marketing, and printed brochures to strengthen people's desire for health and longevity and mobilize the public to support disease prevention and treatment. In addition, the insurance company's "ideal weight comparison table" also appeared on the doctor's desk to quickly assess the patient's weight and health relationship, and also set the tone for "what is a healthy weight." The entire trend reached its peak in the 1950s and 1960s.

Figure 4. Ideal height-weight comparison chart developed by Metropolitan Life Insurance from the 1960s to the 1980s, source: obesitycare.com

To this day, the BMI index, which measures the degree of obesity, is still one of the reference standards for insurance companies when underwriting. Overweight people are often in a sub-healthy state and have higher health risks than people with normal weight, which means that they are more likely to receive insurance compensation. Therefore, overweight people such as those with severe obesity may need to pay 20% to 25% more premium to be insured. In addition, severely obese people with heart disease, high blood pressure, diabetes, etc. may even be denied insurance[12].

The "BMI Index" in the scientific community

In the 1940s, William Sheldon first proposed the term "body mass index" in his book The Varieties of Human Physique[10]. However, the calculation formula used by Sheldon was different from that of Quetelet. The former gave the ratio of height (m) / weight (kg) cubed, that is, h/w3.

The term “body mass index” (BMI) first appeared in scientific literature in 1959. A paper published in the journal Psychological Reports[13] applied BMI to dog body size, and the ratio formula had nothing to do with the Quetelet index. The formula given by the author was weight (kg)/height (m) cubed, that is, w/h3.

It was not until 1972 that American doctor Ancel Keys picked up Quetelet's legacy and promoted the Quetelet index in the medical community, renamed it BMI, and linked it to research on body fat and obesity, creating the "BMI index" that is now known to everyone.

Figure 5. Time magazine January 1961 cover, Ancel Keys, source: content.time.com

Ancel Keys was born in Colorado Springs, Colorado, in 1904. When he was a teenager, his family moved to Berkeley, California. His family was not wealthy. In addition to going to school, Keys worked part-time to earn money: he worked as a handyman in a lumberyard, transported dangerous gunpowder in a gold mine, and went to an Arizona bat cave to pick up bat feces. In 1922, Keys was admitted to the University of California, Berkeley. During his first summer vacation, he went to sea to work as a ship oiler. After spending three months at sea, he returned to school to continue his studies. However, he was very talented and completed his undergraduate studies in just two years. After graduation, Keys, who had been working part-time, soon became bored with the boring and repetitive work every day, so he returned to his alma mater to study biological topics that interested him, and received his first doctorate in biology in 1930. [14]

Keys then flew to Copenhagen with a two-year scholarship to study under the Nobel Prize winner in Physiology or Medicine, August Krogh, and then to Cambridge to follow the physiologist Joseph Barcroft, and began to be interested in how the human body adapts to extreme conditions and began to study. In 1936, Keys obtained a second doctorate in physiology.

Keys' early work and study experience made him very clear about his future research direction, and his research career also changed the academic community's views on many aspects of human physiology and health, such as the impact of hunger on human physiology and psychology, and the influencing factors of coronary heart disease. During World War II, his most famous scientific research achievement was the invention of the K-ration, which solved the problem of food and clothing for soldiers on the front line. [15]

After World War II, the American people no longer worried about not having enough to eat, but rather about eating too much, so heart disease became the leading cause of death in the United States. To solve this problem, the government poured a lot of research funds into research, and people began to realize that overweight and obesity, combined with high blood pressure and high cholesterol, increase the risk of cardiovascular disease. Insurance companies, the US Public Health Service (US Public Health) and the American Medical Association joined forces to call on residents across the country to take action to prevent overweight and obesity (Figure 4).

In the 1950s, the famous “Seven Countries Study” conducted by Keys’ team found two major healthy diets for future generations: the Mediterranean diet and the Japanese diet.[16] On the one hand, Keys acknowledged that obesity seriously harms health, but on the other hand, he did not agree with the insurance company’s statement that “obesity means weight that exceeds the standard weight corresponding to height.”

Figure 6. A 1950s cartoon warning people to be careful about being overweight. Source: youtube.com

In the 1970s, the medical community was eager to find an effective and easy-to-use weight measurement method, and Keys was also involved. In 1971, he and his team published a paper on "Relative Weight and Obesity Indicators" in the Journal of Chronic Diseases[17]. The paper analyzed the height, weight and other related data of 7,424 healthy men from five countries and discussed the popular individual fat measurement methods at the time.

These methods vary. Some use subcutaneous fat calipers to measure subcutaneous fat-body density and the thickness of subcutaneous fat; some weigh underwater to measure body density. In addition to these two popular measurement methods in the United States, there is also the height/weight ratio used by insurance companies and the Quetelet index that Keys dug out from the archives. In the end, Keys chose the Quetelet index as his first choice. He said, "Although the Quetelet index is not perfect, it performs as well as other relative body mass indices and can be used as an indicator of relative obesity." In addition, the BMI calculation method is simple and fast and can be applied to all populations of all ages.

Quetelet was the first person to calculate the w/h2 ratio, but he never promoted this calculation method. It was Keys' research that really promoted "BMI = w/h2" as a universal method for measuring body shape or subcutaneous fat to the public. In 1985, the National Institutes of Health (NIH) revised the definition of obesity[18] and linked it to the BMI value of individual patients. Thirteen years later, the NIH redefined "overweight" and "obesity" and lowered the threshold of medically recognized fat content. This move was ridiculed by the American media as "making all Americans gain weight overnight without gaining weight."

One of the biggest advantages of the BMI is that it is easy to calculate: any doctor only needs to have a BMI chart in his office, no other tools are needed. The World Health Organization also recommends that the public can use BMI to determine their weight in order to maintain a good and healthy lifestyle [19]. As a result, BMI has set off a new public health panic: the "obesity epidemic". By the turn of the millennium, simple BMI calculations had become a necessary part of seeking medical advice and an important reference indicator when taking out life insurance. Today, people with a BMI of more than 30 kg/m² are considered obese, while those with a BMI between 25 and 29.9 are considered overweight.

Figure 7. World Health Organization's BMI nutritional status classification table, source: World Health Organization

In addition to physiological research on overweight and obesity, BMI is also one of the most commonly used tools in the medical field, especially in studying the risk of various diseases, such as disease prophylaxis research on modern civilization diseases, which mainly involves cardiovascular disease and obesity. Some studies use BMI values ​​to determine the obesity level in developed countries in Europe and the United States, helping people realize that the obesity trend is increasing and preventive measures need to be taken. Other studies have described the relationship between BMI values ​​during adolescent development and the risk of stroke in adulthood, concluding that if young people's BMI increases every year, they will face a high risk of stroke in the future [20].

Another area where BMI is widely used is diabetes. Studies have shown that high BMI values ​​significantly affect the incidence of diabetes, while maintaining an appropriate BMI can prevent the disease and curb the progression of the disease. Nutrigenomics focuses on the effects of nutrients on gene expression and metabolic changes, and also frequently uses BMI, mainly focusing on the relationship between gene polymorphism and BMI, which are closely related to "rich diseases" such as type 2 diabetes and obesity [20].

Limitations, Questions and Countermeasures

As science develops, we gradually realize the complexity of the human body. In recent years, researchers have begun to question whether the simple calculation of BMI can continue to be used in increasingly complex research.

One of the limitations that experts have criticized the most is that BMI cannot clearly distinguish whether the excess weight comes from muscle, fat, or bone. As we all know, muscle density is higher than fat. People of the same height but with denser muscle tissue are heavier. According to the BMI calculation method, it is easy to classify this person as obese. As early as 2004, a paper published in the journal Current Sports Medicine Reports, which used athletes as research subjects, pointed out that BMI tends to misclassify athletes [21]. An athlete's weight is often affected by the sport he or she is engaged in, the degree of exercise, and eating habits. For example, basketball players have a BMI between 21.4 and 26.7. Some people may be overweight if they only look at their BMI, but their average body fat percentage is only 10.7%. What is overweight is muscle mass, not fat mass. In other words, BMI cannot measure body fat percentage.

Francisco Lopez-Jimenez, director of the Division of Preventive Cardiology at Mayo Clinic, whose research focuses on obesity and cardiovascular disease, is vocal in his opposition to BMI: “We rely too much on BMI in many cases. It seems to work well at the population level, but it doesn’t work well at the individual level. For example, someone with a BMI of 28 may be healthier in many ways than someone with a BMI of 25 because they may exercise every day and have more muscle mass.”[22]

Secondly, fat stored in different parts of the body has different effects on disease risk assessment, but BMI cannot indicate the distribution of body fat. In 2015, Lopez-Jimene's team published an article in the Annals of Internal Medicines [23], suggesting that both men and women with normal BMI (22 kg/m²) but central obesity (i.e. fat mainly stored around the waist) face a higher risk of death than the control group with the same BMI but no central obesity.

In addition, BMI cannot reflect the disease risks of different populations. As mentioned above, Quetelet's data source was limited to European whites, and his successor Keys' analysis was also mainly based on Europeans and Americans. Other groups also only considered Japanese, so BMI is more applicable to Europeans and Americans. A paper published in the Annals Academy of Medicine Singapore in 2009 pointed out that the BMI cut-off points (i.e. the critical values ​​between normal, overweight, and obese) of Asians are lower than the international BMI cut-off points recommended by the World Health Organization [24]. A study by a team from Harvard Medical School in 2018 also revealed that men and women of different ethnicities have different BMI cut-off points [25]. For example, the BMI of healthy black women is higher than the international unified BMI standard; when measuring diabetes risk, the BMI of black women is closer to 33, while the BMI of white women is an average of 29. Therefore, the article opposes the one-size-fits-all assessment model of BMI and calls for the calibration of BMI values ​​based on biological methods to assess obesity in a more flexible and humane way. [26]

In summary, the simplicity and ease of calculation of BMI values ​​have given it two hundred years of vitality, spanning across different industries and still being popular today; but this has also become its shortcoming, making it difficult to continue to adapt to increasingly sophisticated and complex scientific research and health management. Today, in addition to BMI values, we can also choose more accurate ways to measure body fat according to different needs, using other "health indicators" [27].

The simplest and most economical way to test is the skin fold test. Professional doctors use non-invasive body fat calipers to measure the thickness of skin fat on the back, arms and other parts of the body, and compare them with the body fat assessment standards (mainly based on the standards of young to middle-aged people). The results are accurate and help to judge the level of physical health. In the healthiest state, the body fat mass of women should be 20% to 30% of the total body weight, and that of men should be 12% to 20%.

Bioelectric Impedance Analysis is also a commonly used measurement method. The principle is to let a small amount of current pass through the human body and measure the current speed to determine the body composition. At present, there are household smart electronic scales (body composition scales) on the market that can be connected to mobile devices such as mobile phones and tablets to monitor and record your body data at any time.

Another measurement method that requires higher professional skills and is more accurate is dual-energy X-ray absorptiometry (DEXA), which can not only monitor the body's fat content, but also check the "hidden" visceral fat tissue volume, which is an important reference indicator for disease prevention and treatment. This instrument is generally available in hospitals.

In addition to directly measuring fat content, we can also assess our situation by looking at the distribution of fat. Fat accumulation in different parts of the body has different effects. Measuring waist circumference can assess abdominal fat content. This fat is hidden deep in the body and accumulates around important organs, which increases the risk of obesity-related diseases such as type 2 diabetes, hypertension, and coronary artery disease. This is much more harmful than subcutaneous fat [28].

In addition to waist circumference, waist-to-hip ratio (WHR) is also very valuable for reference. According to the World Health Organization's standards, the best healthy waist-to-hip ratio for women is ≤0.85, while the best for men is ≤0.9 [29]. In addition, in 2005, a study published in the Lancet analyzed data from 27,000 subjects from 52 countries and found that waist-to-hip ratio was closely related to heart attack and had a better predictive effect than BMI. [30]

Finally, if one wants to check one’s physical condition from the perspective of disease monitoring or prevention, indicators such as waist-to-hip ratio, blood sugar, triglycerides, blood pressure, cholesterol, and heart rate are “windows” that can better reflect one’s health status, as these indicators are more closely related to diseases such as heart disease, diabetes, and stroke [31].

Although there are many criticisms of BMI in academia, it is still widely used today and will obviously not disappear overnight. Understanding the complex history of BMI, we may be able to view it as a "starting point" for us to value our own health, rather than the only "gold standard", broaden the dimensions of health management, and benefit from different health indices.

References

[1] https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight

[2] https://www.prnewswire.com/news-releases/countrys-leading-obesity-care-organizations-develop-consensus-statement-on-obesity-301734250.html

[3] https://www.theatlantic.com/business/archive/2016/02/the-invention-of-the-normal-person/463365/

[4] Quetelet, Ad.. "Nouveaux mémoires de l'Académie Royale des Sciences et Belles-Lettres de Bruxelles 7 (1832)

[5] Quetelet A. , A Treatise on Man and the Development of his Faculties. Originally published in 1842. Reprinted in 1968 by. Burt Franklin, New York

[6] http://eugenicsarchive.ca/discover/connections/5233cb0f5c2ec5000000009c

[7] https://elemental.medium.com/the-bizarre-and-racist-history-of-the-bmi-7d8dc2aa33bb

[8] https://www.genome.gov/about-genomics/fact-sheets/Eugenics-and-Scientific-Racism

[9] https://ihpi.umich.edu/news/forced-sterilization-policies-us-targeted-minorities-and-those-disabilities-and-lasted-21st

[10] https://journals.sagepub.com/doi/pdf/10.1177/1536504217732057

[11] https://www.psychologytoday.com/us/blog/the-gravity-weight/201603/adolphe-quetelet-and-the-evolution-body-mass-index-bmi

[12] http://www.cntaiping.com/service/78277.html

[13] https://journals.sagepub.com/doi/abs/10.2466/pr0.1959.5.h.495

[14] https://www.mayoclinicproceedings.org/article/S0025-6196(19)31088-2/fulltext

[15] https://www.nytimes.com/2004/11/23/obituaries/dr-ancel-keys-100-promoter-of-mediterranean-diet-dies.html

[16] https://www.sevencountriesstudy.com/about-the-study/

[17] https://academic.oup.com/ije/article/43/3/655/2949547?login=false

[18] https://academic.oup.com/ije/article/43/3/655/2949547?login=false

[19] https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle---who-recommendations

[20] https://biomedgrid.com/fulltext/volume4/influence-significance-and-importance-of-body-mass-index-in-scientific-research-and-various-fields-of-science.000816.php

[21] https://journals.lww.com/acsm-csmr/Abstract/2004/08000/Overweight_Athlete__Fact_or_Fiction_.5.aspx

[22] https://www.statnews.com/2023/03/09/bmi-body-mass-index-troubled-history/

[23] https://www.acpjournals.org/doi/10.7326/M14-2525?articleid=2468805

[24] https://annals.edu.sg/pdf/38VolNo1Jan2009/V38N1p66.pdf

[25] https://www.mayoclinicproceedings.org/article/S0025-6196(18)30807-3/fulltext#%20

[26] https://www.insider.com/guides/health/is-bmi-accurate

[27] https://pressbooks-dev.oer.hawaii.edu/humannutrition/chapter/indicators-of-health-body-mass-index-body-fat-content-and-fat-distribution/

[28] https://www.nytimes.com/2021/05/18/style/is-bmi-a-scam.html

[29] https://www.who.int/publications/i/item/9789241501491

[30] https://www.sciencedirect.com/science/article/abs/pii/S0140673605676635

[31] https://health.clevelandclinic.org/is-bmi-accurate/

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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