The body gets stuck burning the same fuel regardless of circumstance.
At a research conference in Monterrey, Cambridge metabolic scientist Antonio Vidal-Puig challenged one of medicine's most enduring assumptions: that a person's weight, expressed as a ratio of height, can reliably predict their metabolic fate. His work suggests that what truly matters is not how much the body carries, but how fluidly it adapts — its capacity to shift between burning fat and sugar as circumstances demand. In proposing metabolic flexibility as a superior measure of health, Vidal-Puig invites medicine to move from the simplicity of a number toward the complexity of a living system.
- Two patients with identical BMIs can face opposite fates — one obese yet metabolically sound, the other thin but already carrying diabetes, fatty liver, and a history of heart attack.
- The body's failure to switch efficiently between burning fat and sugar — metabolic inflexibility — silently precedes diagnosis, offering no obvious symptoms while fundamental damage accumulates.
- When fat storage reaches its limits, the liver absorbs the overflow as a rescue mechanism, but that rescue has a breaking point: inflammation, fibrosis, and eventually the destruction of the pancreatic cells that produce insulin.
- Vidal-Puig's lab is now genetically mapping metabolic inflexibility across fifteen hundred mouse lines, seeking to identify who is vulnerable before the cascade begins.
- Populations like Mexico show disproportionately severe complications relative to BMI, exposing the metric's failure precisely where it is most needed and sharpening the case for a more functional replacement.
At an obesity research conference hosted by Monterrey's Tec, Cambridge professor Antonio Vidal-Puig opened with a disquieting thought experiment: one patient is clinically obese but carries normal blood sugar, triglycerides, and blood pressure; another is thin, thirty-eight years old, and already has diabetes, a fatty liver, and a heart attack behind him. The question of which one is actually sick exposes the central flaw in how medicine has long assessed metabolic risk.
Vidal-Puig argues that BMI — the height-to-weight ratio doctors have relied on for decades — simply does not measure what matters. What he proposes instead is metabolic flexibility: the body's evolved capacity to shift between burning fat and burning sugar depending on what fuel is available. In a healthy person, this oscillation happens constantly and invisibly throughout the day. When it breaks down, the body gets stuck, unable to burn fat at night or process carbohydrates efficiently during the day — with no obvious symptoms to signal the failure.
His laboratory measures this using indirect calorimetry, tracking oxygen consumption and carbon dioxide output to reveal in real time what fuel the body is burning. The gap between the day's highest and lowest points becomes a metabolic flexibility index. His team is now constructing the first genetic map of metabolic inflexibility, analyzing over fifteen hundred mouse lines to understand why some bodies adapt and others cannot — and to catch that failure before it becomes disease.
The disease cascade, when it comes, follows a predictable path. Fat tissue reaches its storage limit; the liver absorbs the overflow as a protective measure. But the liver's capacity is finite. When it is exhausted, inflammation sets in, fibrosis follows, and fat eventually migrates into muscle and then into the pancreatic beta cells that produce insulin. Once those cells are lost, recovery becomes nearly impossible.
This pattern does not unfold uniformly. In Mexico, metabolic complications appear at rates disproportionate to measured obesity levels, suggesting genetic and environmental factors that BMI cannot capture. Vidal-Puig's warning lands at a moment when drugs like semaglutide are reshaping obesity treatment — effective at lowering weight, but without a full understanding of why, and without guaranteeing that metabolic flexibility is restored. His laboratory's deeper bet is that medicine must stop treating obesity as a number and begin treating it as a system that fails differently in each person — one that can be anticipated, if we learn to look in the right places.
At an international obesity research conference hosted by Monterrey's Tec, a Cambridge metabolic researcher presented a simple but unsettling thought experiment: imagine two patients walking into a clinic. The first is clinically obese by standard measures, yet his blood sugar is normal, his triglycerides are normal, his blood pressure is normal. The second is thin, only thirty-eight years old, and already carries a diagnosis of diabetes, a fatty liver, and a history of heart attack. Which one is actually sick?
Antonio Vidal-Puig, a professor of molecular nutrition and metabolism at Cambridge University, has spent years trying to answer that question. His research suggests that the answer cannot be found on a scale or calculated from height and weight. The Body Mass Index—that number doctors have relied on for decades to sort people into risk categories—is simply not measuring what matters most. "The BMI of today is not a good indicator for metabolic risk," he told the assembled researchers. "You have to look for more functional aspects that go beyond waist circumference."
What Vidal-Puig is proposing instead is something far harder to measure but far more revealing: metabolic flexibility. This is the body's capacity to switch efficiently between burning fat and burning sugar, depending on what fuel is available. In evolutionary terms, it is what allowed humans to survive in different environments, to thrive when food was scarce and when it was abundant. A metabolically flexible person wakes up, eats breakfast, and shifts toward burning carbohydrates. At night, when food is gone, the body switches gears and begins to burn stored fat. The oscillation happens naturally, constantly, throughout the day. In a person whose metabolism has become inflexible, that oscillation disappears. The body gets stuck burning the same fuel regardless of circumstance. At night, when it should be burning fat, it cannot. During the day, when it should be processing carbohydrates, it struggles. No obvious symptoms appear. But something fundamental has broken.
Vidal-Puig's laboratory measures this flexibility using indirect calorimetry, a technique that tracks the oxygen a body consumes and the carbon dioxide it produces, revealing in real time what type of fuel is being burned. The difference between the highest and lowest points across a full day becomes what his team calls the metabolic flexibility index. Unlike BMI, this number actually reflects how the metabolism functions. His team is now building the first genetic map of metabolic inflexibility, analyzing more than fifteen hundred mouse lines to identify which genes determine each individual's capacity to process different nutrients. The goal is to understand why some people's bodies can adapt and others cannot, and to catch that failure before it cascades into disease.
The cascade itself follows a predictable path. When the body's fat tissue reaches its storage limit, excess fat begins to accumulate in other organs. The liver acts as a rescue organ, absorbing the overflow to protect the rest of the body. Many people have fat in their liver and suffer no complications. But when that mechanism reaches its own limit, inflammation begins, fibrosis develops, and in advanced cases, cirrhosis or liver cancer emerges. The damage does not stop there. If fat continues to accumulate, it moves into muscle tissue, then finally into the beta cells of the pancreas. Once fat reaches those cells, recovery becomes nearly impossible. The pancreas cannot defend itself. The cells die, and with them, the body's ability to produce insulin.
This understanding changes how we should interpret conditions like fatty liver disease. It is not simply a symptom of obesity. It is a signal that metabolic flexibility has been deteriorating for some time, and that the body is running out of safe places to store excess energy. "The ideal is that the liver helps you," Vidal-Puig explained, "but you should not abuse it too much, because one day it will get angry."
The pattern does not unfold identically across all populations. In Asia, people develop severe metabolic complications at BMI levels considered normal elsewhere. In Mexico, the prevalence of complications is disproportionately high relative to the degree of obesity, suggesting genetic and environmental factors that remain poorly understood. This variation is precisely why replacing BMI with a more functional assessment—one that includes fat distribution, the health of metabolic organs, and markers like metabolic flexibility—could make a significant difference, especially in populations where the old metric fails to predict who will actually become sick.
Vidal-Puig's warning arrives at a moment when drugs like semaglutide are transforming obesity treatment. These medications work. They lower weight. But they work without anyone fully understanding why, and that gap in understanding limits medicine's ability to prevent complications rather than simply treat them after they appear. Losing weight without restoring metabolic flexibility may be insufficient. The real question is not whether the number on the scale goes down, but whether the body recovers its capacity to adapt. That is the bet his laboratory is making: that the medicine of the future will stop treating obesity as a number and start treating it as a complex system that fails differently in each person, but can be anticipated if we know where to look.
Citas Notables
The BMI of today is not a good indicator for metabolic risk. You have to look for more functional aspects that go beyond waist circumference.— Antonio Vidal-Puig
The metabolically healthy obese person is real, but it is an unstable intermediate stage. Do not ignore it: it is an opportunity for prevention.— Antonio Vidal-Puig
La Conversación del Hearth Otra perspectiva de la historia
So if someone has a high BMI but normal blood sugar and cholesterol, are they actually fine?
Not necessarily. They might be metabolically healthy right now, but that's often an unstable middle ground. The real question is whether their body can still switch between burning fat and sugar efficiently. If that flexibility is already gone, complications are coming—they just haven't shown up yet.
How do you even measure something like that? It sounds invisible.
It is, mostly. But we can measure it with indirect calorimetry—tracking oxygen and carbon dioxide to see what fuel the body is burning at different times of day. The gap between the highest and lowest points tells you how flexible the metabolism is.
And this shows up before diabetes or heart disease?
Yes. That's the crucial part. Metabolic flexibility deteriorates first. Everything else—the fatty liver, the high blood sugar, the damaged pancreas—those come later. If we could catch the loss of flexibility early, we could actually prevent the cascade instead of treating it after the organs are already damaged.
Why does it matter that different populations respond differently to obesity?
Because BMI is a one-size-fits-all metric, and it doesn't fit everyone. In Mexico, people develop serious complications at BMI levels that would be considered safe in other places. That suggests there are genetic or environmental factors we don't understand yet. Using the same cutoff for everyone misses the people who are actually at risk.
What about these new weight-loss drugs? Don't they solve the problem?
They lower weight, which is real progress. But they work without us fully understanding the mechanism. If you lose weight without restoring metabolic flexibility, you might still be vulnerable. The drugs treat the symptom, not necessarily the underlying dysfunction.
So what should someone actually do?
The honest answer is we're still figuring that out. But the direction is clear: stop looking at the scale and start looking at function. Can your body adapt? Can it switch fuels? That's what matters.