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ISSUE 066 · DEEP DIVE · MYTHS & MISINFORMATION

Adolphe Quetelet was a Belgian astronomer and statistician who, in the 1830s, set out to describe something he called l'homme moyen — the average man. He worked from measurements of soldiers, mostly Scottish Highlanders and French gendarmes, and noticed that across a population, weight tended to rise with the square of height. Divide one by the other and you got a tidy number for describing a crowd.

He was not trying to diagnose anyone. Quetelet's project was what he called “social physics”: the idea that societies, like planets, follow laws that can be measured. His formula then sat in the statistics books for well over a century.

In 1972 the American physiologist Ancel Keys — the same Ancel Keys whose campaign against dietary fat we covered in June — compared several ways of estimating body fat and found Quetelet's formula at least as good as any. He gave it a new name: the body mass index. He also gave it a warning. It was suited to studying populations, he concluded, not to judging individuals.

Medicine adopted the number and dropped the warning. BMI cannot tell muscle from fat, or fat packed around the organs from fat on the hips. By its arithmetic, a heavily muscled rugby player can be obese. And because the original data came overwhelmingly from white European men, it fits other bodies poorly: the World Health Organization recommends lower cut-offs for Asian populations, who tend to develop diabetes at lower BMIs.

In June 1998, the US National Institutes of Health lowered the line for “overweight” to a BMI of 25. Under the previous federal definitions, a man had needed a BMI of around 28 and a woman around 27. Nobody gained a pound. About 29 million Americans became overweight anyway.

The new line matched international guidelines drafted in part by the International Obesity Task Force, whose two principal funders made weight-loss drugs. The chairman of the NIH committee, Xavier Pi-Sunyer, was at the time consulting for several diet-drug makers and Weight Watchers International, according to the political scientist Eric Oliver. If that pattern sounds familiar, it is the same one we found behind the lines for blood pressure and diabetes.

Then the data got awkward. In 2013, Katherine Flegal, a statistician at the US Centers for Disease Control, pooled 97 studies covering nearly three million people. Those classed as “overweight” had a 6 per cent lower risk of death than those at a “normal” weight. The finding caused uproar.

In 2016, a far larger analysis — 239 studies, more than ten million people — answered back. Looking only at people who had never smoked, had no chronic illness and survived the first five years, it found the lowest risk squarely between 20 and 25, rising with every step above. Smokers and the already sick tend to be thin, the authors argued, and they drag up the death rate of the “normal” group. Critics of that approach reply that excluding so much of the data can introduce biases of its own.

Both sides agree that at the heavy end of the scale, risk climbs steeply. They are fighting over the middle — which is where most people live. In 2023, the American Medical Association formally told doctors that BMI is an “imperfect” measure that has caused “historical harm,” and that it should be used alongside waist size, body composition and metabolic tests rather than on its own.

The other number is a shape. In 1992 the US Department of Agriculture released the Food Guide Pyramid, and for a generation it hung on school walls everywhere: bread, cereal, rice and pasta across the wide base, 6 to 11 servings a day; fats and sweets in the tiny triangle at the top.

It very nearly didn't appear. In April 1991 the department halted publication of the finished design. Two weeks earlier, meat and dairy producers led by the National Cattlemen's Association had protested that it stigmatised their products. The official reason for the delay was that the pyramid needed more testing on children and poorly educated adults — an explanation, the nutrition scholar Marion Nestle notes, that was widely disbelieved. The department spent an extra $855,000 testing alternatives, including a bowl shape favoured by the meat and dairy groups. People still preferred the pyramid. It came out a year and a day later, with the graphic adjusted to stress that two to three daily servings of meat and dairy were still recommended, and with the upper range for meat nudged up.

Then there is the base. Luise Light, a USDA nutritionist who worked on the guide, has said the scientists' advice was two servings of grains a day for women and three for men, and that it came back from higher up the department as 6 to 11. Parts of her account cannot be checked, and she is one witness. But the wide grain base arrived in the same era as the low-fat advice that turned sugar into fat's replacement, and it was quietly retired — swapped for MyPyramid in 2005 and for a plate in 2011.

This January, Health Secretary Robert F. Kennedy Jr. and Agriculture Secretary Brooke Rollins unveiled new dietary guidelines with a new picture: the pyramid, inverted. Meat, cheese, vegetables and fruit now spread across the wide top. Whole grains sit at the narrow point, two to four servings a day. The guidelines call for a “dramatic reduction” in highly processed food and no added sugar at all for children under ten.

Reactions split the way you would expect. Christopher Gardner of Stanford, a former member of the government's own advisory committee, said he was “very disappointed” to see red meat and saturated fat at the top. Dariush Mozaffarian of Tufts called the push against processed food “a very positive move for public health.” And for all the talk of ending the war on saturated fat, the fine print kept the old limit: no more than 10 per cent of daily calories.

Neither number came from where you'd assume. BMI was an astronomer's way of describing crowds, renamed by a physiologist who warned against using it on individuals, and moved by a committee whose chairman consulted for the weight-loss industry. The pyramid was drawn by government scientists, redrawn after the meat lobby complained, and has now been flipped by a new administration. Both contain real information. Neither is a measurement of you.

BMI history: Eknoyan, G., “Adolphe Quetelet (1796–1874) — the average man and indices of obesity,” Nephrology Dialysis Transplantation (2008); Keys, A. et al., “Indices of relative weight and obesity,” Journal of Chronic Diseases (1972); WHO expert consultation on BMI cut-offs for Asian populations, The Lancet (2004).

The 1998 change: National Institutes of Health clinical guidelines on overweight and obesity (June 1998), as reported by CBS News; Mother Jones, “Why BMI Is a Big Fat Scam” (2014), citing Eric Oliver, Fat Politics (2006), on the International Obesity Task Force and the committee chair's consulting.

Mortality: Flegal, K. M. et al., “Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories,” JAMA (2013); Global BMI Mortality Collaboration, “Body-mass index and all-cause mortality,” The Lancet (2016); Mathur, M. B. and VanderWeele, T. J., Epidemiology (2022); American Medical Association policy on BMI (June 2023), as reported by Axios.

The pyramid: Nestle, M., “Food Lobbies, the Food Pyramid, and U.S. Nutrition Policy,” International Journal of Health Services (1993); Light, L., What to Eat (2006), as reviewed in The Irish Times (2006).

The 2026 guidelines: NPR, “RFK Jr.'s new dietary guidelines go all in on meat and dairy” (January 2026), including comments from Christopher Gardner and Dariush Mozaffarian.

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