WHAT BODY MASS INDEX «DOESN’T TELL YOU»: Junk Food Is Changing Your Brain!

Photo by Carlos Henrique on Unsplash

Excessive consumption of trans fats, sweets, and ultra-processed foods often leads to weight gain and obesity. While doctors have had mixed success in instilling this fact in the public consciousness, numerous misconceptions have taken root — particularly around the widely used Body Mass Index (BMI). Recent studies show that the harm caused by poor diet may not always be visible. Even short-term intake of unhealthy foods that don’t result in weight gain can significantly impair brain function.

HEIGHT AND WEIGHT ARE NOT UNIVERSAL INDICATORS!

cientists are urging a reevaluation of the Body Mass Index (BMI), which is calculated by dividing a person’s weight by the square of their height. For decades, BMI has served as an international standard for determining healthy weight. It was considered an important measure of body fat — an unacceptably high level of which was seen as a marker for increased risk of metabolic disease and even death.

Over the past 50 years, global obesity rates have tripled, and the World Health Organization (WHO) has officially recognized obesity as a chronic disease. As doctors and patients have fought this condition, they’ve developed a kind of «dependence» on BMI as the sole diagnostic tool.

However, new research shows that height and weight reveal very little about a person’s actual health. As a result, the American Medical Association (AMA) has called for a broader set of weight-related health indicators to be taken into account. The association described BMI as an imperfect measure with a questionable history. And that history, briefly, is as follows…

«THE IDEAL HUMAN» AND EUGENICS

The origins of BMI had little to do with health. Nearly 200 years ago, Belgian astronomer and mathematician Adolphe Quetelet became obsessed with calculating the proportions of the «average man». To this end, he began recording measurements of the human body — primarily those of Western European men.

Quetelet discovered that weight roughly correlated with the square of height. Dividing the two figures to describe weight relative to height became known as the Quetelet Index. His concept of «normality» quickly gained popularity in Europe and even played a role in the early development of eugenics.

The term itself (from the Greek εὐγενής, meaning «well-born») was coined by Francis Galton, a cousin of Charles Darwin, who envisioned eugenics as a kind of new religion. For Galton, it was obvious that Christianity had failed in its centuries-long mission to improve humankind.

This task, he believed, should now fall to eugenicists — to assist natural selection in purging the gene pool of so-called «degenerates». Later, the experiments on humans conducted in Nazi Germany, where efforts to rid the human race of «genetic waste» were carried out on an industrial scale, severely tarnished the reputation of eugenics.

THE REVIVAL OF INTEREST IN THE QUETELET INDEX

For a time, interest in Quetelet’s index faded — until 1972, when American physiologist Ancel Keys found that it was an excellent predictor of body fat percentage. To avoid undesirable associations, the Quetelet Index was renamed the Body Mass Index (BMI). Charts were created linking socially «ideal» body sizes to the lowest risk of death.

However, over time, it became clear that while BMI correlates well with mortality risk at the population level, it is far too imprecise when applied to individuals. As a tool for assessing health risks, it proves useful only with significant reservations. In fact, the risk of death among those in the «overweight» category was found to be similar to that of people with a «healthy weight».

Suddenly, scientists realized that 30% of those classified as obese were actually in good cardiometabolic health. This led them to consider other factors — such as blood pressure, cholesterol levels, blood sugar, and even family history and genetics.

THE RISKS OF CHASING BEAUTY

BMI has lost its credibility as a universal metric among scientists, as it fluctuates depending on age, sex, and racial or ethnic background. For example, Asians are more prone to heart disease at lower BMI levels compared to white Europeans.

Still, regardless of the stance on BMI, excess fat has been recognized as extremely dangerous — particularly visceral fat, which surrounds internal organs rather than subcutaneous fat beneath the skin.

Ultimately, the American Medical Association (AMA) declared BMI an «imperfect measure» that may have done more harm than good. Doctors are particularly concerned about the growing trend of taking popular new medications like Ozempic based solely on BMI readings.

They urge people to consider not only their aesthetic goals but also medical indicators.

IT’S ALL ABOUT INSULIN

Another reason not to rely on BMI: consuming unhealthy food can leave a person visually within the «normal» range while triggering serious internal changes. Most notably, in the brain. This effect has been observed even in healthy individuals.

Why does this happen? After each meal, the pancreas releases insulin to help with metabolism. A portion of this insulin enters the brain and works to suppress appetite. In people with obesity, this response is weakened.

To study insulin’s effects on the brain, researchers recruited 29 young, healthy male volunteers. One group ate a regular diet, while the other was provided with customized packs of ultra-processed foods for five days.

These packs included snack bars, potato pancakes and chips, packaged snacks, processed meats, white bread, and other junk foods high in fat and sugar. Each pack had a caloric value of 1,500 kcal, though in practice, they increased daily intake by about 1,200 kcal.

JUNK FOOD MAKES THE BRAIN «SICK»

To increase insulin levels in the brain, participants in both groups used a special nasal spray, after which scientists visualized cerebral blood flow. They then compared brain activity levels before, during, and after the experiment. After just 5 days, the group consuming junk food showed higher activity in three brain regions associated with responses to food cues and rewards.

These same neural patterns are typical of people with obesity and type 2 diabetes. One week after the extreme diet experiment ended, activity in these areas declined. The study demonstrated that even if traditional signs of obesity are not visible — BMI remains unchanged — your brain can still begin to «suffer».

It’s also important to note that this study involved healthy young men. For individuals who are not as young or healthy, the consumption of processed foods may have even more damaging effects.

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