She showed that hunger is biological – and reshaped obesity

A child who has never felt full.

For years, hunger has been constant. Food is always on her mind. She eats and still feels driven to eat more.

Then comes the injection.

Nothing happens at first. But after a few days, something changes. When food is placed in front of her, she turns away.

“Before then, she had been constantly hungry, wanting to eat the whole time,” explains Sadaf Farooqi, Professor of Metabolism and Medicine at the University of Cambridge. “After a few days of treatment, she turned the food away – and was more interested in playing, like any other child.”

That moment – quiet, almost ordinary – captured something science had long misunderstood: hunger is not simply a matter of choice.

“For a long time, people assumed that weight gain was under personal control,” Farooqi says. “What our work has shown is that weight is biologically regulated.”

A lesson learned early

Long before obesity, Sadaf Farooqi had already seen how science can overturn assumptions – and change lives. As a medical student in Birmingham, she took part in research on cot death, then the leading cause of infant mortality in many countries.

One observation stood out. Babies of South Asian families had much lower rates of cot death than white British babies – despite higher rates of other illnesses.

“It was a very unusual observation,” Farooqi recalls.

In addition, doctors had begun to notice something else: many infants who died were found lying face down – the position parents were advised to use.

Farooqi studied infant care practices across communities. The pattern was clear. White British families followed the official advice and placed babies on their front. South Asian families, following tradition, placed them on their backs.

The implication was simple – and radical: sleep position might be contributing to cot death. A few years later, public health advice changed. Parents were told to place babies on their backs – and cot death fell by more than 58% in the first year.

“It was incredible as a young student to think that a piece of research could lead to such a dramatic change in behaviour,” she reflects.

The experience shaped her.

“It really convinced me that if we can understand why things happen, we can change clinical practice – and change lives.”

Choosing the unfashionable question

When Farooqi moved into research, she did not choose an obvious field. In the late 1990s, obesity was rising rapidly – but scientifically, it was barely understood.

“At medical school, we learned nothing about obesity,” she recalls. “There was really nothing known about why people gain weight.”

What was known, or at least widely believed, was simple: people with obesity ate too much and moved too little. Fat tissue itself was considered uninteresting – a passive store of excess calories.

“It was seen as quite boring,” she adds.

That, in part, is what drew her to it.

“There are a lot of prejudices, a lot of views that are not informed by science,” she says. “And new knowledge can really change people’s perceptions – and patients’ lives.”

The hormone that changed everything

Then, just a few years earlier, a discovery had changed the landscape. In 1994, researchers led by Jeffrey Friedman identified leptin – a hormone produced by fat tissue that signals energy status to the brain.

“In mice, the finding was dramatic: animals lacking leptin became severely obese, and giving the hormone reversed the condition.”

For the first time, there was a concrete biological signal linking fat tissue to appetite. Farooqi arrived in Cambridge just as the field was opening.

“People were really excited that there could be a hormone that controls weight,” she recalls.

Her first task was simple: measure leptin in patients – and see what it might reveal. Then the children arrived.

“They were extremely young and already severely affected – not just by their weight but by an overwhelming drive to eat. They behaved as if they were starving, unable to feel full,” Farooqi explains.

It was through these children that the system became visible.

The system that defends your weight

For most people, hunger feels simple: you eat, and at some point, you stop. But the biology behind that moment is anything but simple.

“What we have is a highly sophisticated system that regulates body weight over long periods of time,” says Sadaf Farooqi. “It is constantly integrating signals from across the body and adjusting how much we eat and how we use energy.”

At its centre is the hypothalamus. One signal stands out: leptin, produced by fat tissue.

“It signals energy sufficiency,” Farooqi explains. “When leptin drops, the brain interprets it as starvation.”

The response is powerful: hunger rises and the body becomes more efficient at storing calories.

“It is a coordinated response,” Farooqi adds. “The brain is essentially trying to restore energy balance by driving you to eat.”

This is why weight is tightly regulated – and difficult to change.

“You can override the system for a short period,” she says. “But not indefinitely, because it will push back.”

She compares it to breathing.

“You can hold your breath for a while – but eventually, the system takes over.”

The children who exposed the system

That biological pushback is something many people recognise: they lose weight by eating less, but over time hunger increases and weight returns.

“This weight regain is incredibly common,” Farooqi notes. “And it is not simply about willpower.”

When weight drops, leptin levels fall – signalling to the brain that energy stores are depleted.

“You become hungrier, your metabolism adapts, and your body becomes more efficient,” she explains. “It is trying to restore the weight that has been lost.”

From a biological perspective, the system is doing exactly what it evolved to do – but in a world of abundant food, that same response works against us. The theory of a biological system regulating weight was one thing. Proving it for humans was another. That proof came from a handful of children.

“They were extremely young and already very severely affected,” says Sadaf Farooqi. “What was striking was not just their weight – it was their behaviour around food.”

From early childhood, they showed an overwhelming drive to eat. At the time, obesity was still largely understood as the result of overeating – not a specific biological defect. But something did not fit.

“They should have had high levels of leptin,” Farooqi recalls. “Instead, there was none.”

The missing signal

The children had undetectable levels of the hormone.

“That was the first clue that something fundamental was wrong,” she says. “We then looked at the gene that encodes leptin – and found that it was mutated.”

It was rare, but decisive – showing for the first time that a single gene defect could cause human obesity.

“If removing one signal changes behaviour this profoundly, the system is real.”

The children also revealed what happens when that system fails.

“It is not just feeling a bit peckish,” Farooqi explains. “It is a persistent, overwhelming drive to eat.”

“I think seeing those children changed how we understood the problem,” she says. “It made it clear we were dealing with a disorder of a system – not simply behaviour.”

Testing the missing signal

The real question was whether replacing it would change anything.

“At that point, we had strong evidence from animal studies,” explains Sadaf Farooqi. “In mice, if you give leptin back, you can reverse the obesity. But we had no idea if that would translate to humans.”

No one had given leptin to a child like this before.

“We were cautious,” she says. “These children were constantly hungry and gaining weight rapidly.”

So the decision was made to try.

“I still remember giving the first injection,” she adds.

At first, nothing happened – “for the first few days, there was no obvious change.” Then something shifted.

“The earliest sign was behavioural,” Farooqi recalls. “She started to turn away food.”

Before, food had dominated everything. Now, that drive was gone.

“There was a moment when her mother looked at me, and we both realised something had changed,” Farooqi remembers.

Over time, the effects became unmistakable: “hunger normalised, eating behaviour became more typical and weight began to fall,” she says. “This showed that restoring a missing signal restores normal behaviour. You are allowing the system to function as it should.”

But the limits were also clear.

“This treatment works for people who completely lack leptin,” she notes. “Most people with obesity already have high levels, so giving more does not have the same effect. There is likely a degree of resistance in common obesity.”

It was not a universal cure – but it was proof.

From rare cases to common biology

The children were rare – but what they revealed was not.

“For us, the key question was never just about those individual cases,” explains Sadaf Farooqi. “It was about what they could teach us about how weight is regulated in all of us.”

That shift – from rare disease to common biology – became central to her work.

“We realised that by studying extreme phenotypes, you can uncover fundamental mechanisms,” she says.

What emerged was not a single pathway but a network – centred on the leptin–melanocortin system, in which signals in the brain regulate appetite. One component stood out: the melanocortin 4 receptor (MC4R).

“We found that mutations in MC4R are the most common monogenic cause of human obesity,” she notes – and that the degree of disruption directly affects how much people eat.

In other words, the system fails not by burning too little – but by driving too much intake. The implications extend far beyond rare cases.

“If you understand the pathways that regulate appetite, you understand something fundamental about obesity in the wider population,” she explains.

Not every case can be explained by a single gene.

“But in many people, it is a combination of genetic and environmental factors,” Farooqi says. “The rare cases give us a clear window into the system.”

By then, the field had shifted. Obesity was no longer just about calories in and calories out – but about a brain that decides when enough is enough.

Why fat is hard to resist

If genes influence how much we eat, a more uncomfortable question follows: do they also shape what we want to eat? For a long time, food preference was seen as culture or habit – not biology.

“But we began to see patterns that suggested something else,” says Sadaf Farooqi.

Patients with disruptions in the melanocortin pathway chose differently. In controlled experiments, one pattern stood out:

“Individuals with MC4R deficiency showed a strong preference for high-fat foods – and a reduced preference for sucrose.”

It was the first clear evidence that a single gene could influence people’s food preferences.

“If the system is restoring energy balance, it makes sense to prioritise fat,” Farooqi notes.

The behaviour is not irrational – it is adaptive. But in a modern environment, in which high-fat food is abundant, that same drive can become a problem. The findings also point to something deeper: appetite is linked to reward.

“It is not just a cognitive process,” Farooqi emphasises. “It is a biological one.”

Nevertheless, she is cautious.

“Food choice is influenced by many factors – culture, environment and availability. Biology is one part of the picture.”

Even so, the implication is clear: obesity is not just about how much we eat but what we are driven to want.

Turning appetite biology into treatment

As the biology of appetite became clearer, a new question emerged: could it be used to treat obesity?

“For a long time, obesity was not seen as something you could treat medically,” observes Sadaf Farooqi.

But the logic of the science pointed elsewhere.

“If the problem lies in the pathways that regulate hunger and satiety, then those are the pathways you need to target,” she adds.

The first proof came from the leptin-deficient children. But most people with obesity already have high leptin levels.

“So the question became: how do we influence the system in a way that works more broadly?” she says.

The leptin–melanocortin pathway provided clear targets.

One example is MC4R.

“If MC4R function is impaired, people tend to eat more,” she explains. “So activating that receptor can reduce appetite.”

That idea has moved into practice.

“MC4R agonists are now used in specific genetic forms of obesity,” she says.

From targets to therapies

More broadly, a new generation of treatments has emerged.

“They act by reducing hunger and increasing satiety – which is exactly what the biology tells us matters. The effects have been substantial. We are seeing outcomes that would have been difficult to imagine a decade ago,” Farooqi says.

And yet, resistance remains.

“There is still a perception that these treatments are a shortcut,” she observes.

Farooqi disagrees.

“If the underlying issue is a dysregulation of appetite, then correcting that is addressing the cause.”

She compares it with hypertension.

“For many patients, lifestyle changes matter – but are not always sufficient. For some, lifestyle changes are enough. For others, the biological drive is stronger – and treatment is needed. These medications are not a universal solution,” Farooqi cautions. “But they are part of a broader approach.”

They do not work equally well for everyone.

“There is variability in response – which is why understanding mechanisms remains so important.”

This next step is already in view.

“We are beginning to think about subtypes of obesity,” Farooqi adds.

The direction is clear: appetite can be targeted.

When biology replaces blame

Long before treatment becomes an option, something else can change a life: an explanation.

“For many families, that is what has been missing,” explains Sadaf Farooqi. “They know something is different about their child – but they cannot explain it.”

In that absence, other explanations take over.

“People are often not sympathetic – including the public, teachers and healthcare professionals,” she says. “And very often, the parents are blamed.”

For families, that judgement can be relentless. A diagnosis can interrupt that cycle.

“When you can show a biological cause – a gene, a pathway – it changes the conversation,” Farooqi adds.

The shift is often immediate.

“People respond differently. Families feel they are finally being heard.”

In some cases, the consequences are immediate.

“We have seen children at risk of being taken into care because their obesity was interpreted as neglect,” she says.

Parents were blamed for causing the obesity. But when Farooqi’s team could show that a child had a genetic condition, the case changed.

“We say: hang on — no, we found a cause. Stop it. And then the child stays with the parents.”

What an explanation changes

For families, the impact is also internal.

“It often brings relief,” Farooqi explains. “It confirms what they already felt – that this is not something they have caused.”

The effect extends beyond rare conditions.

“Even in more common forms of obesity, understanding the biology can reduce stigma,” she notes.

Nevertheless, she is clear about the limits.

“It does not solve everything,” Farooqi says. “But it changes the starting-point.”

And that shift matters.

“Sometimes, simply understanding – and being able to explain – can change lives in a profound way.”

Why weight affects people differently

By now, the outline of the system is clear: the brain regulates appetite, and genes shape how strongly that system pushes. But a second question follows: why do some people develop serious health problems – whereas others, at similar weights, do not?

“That is a really important distinction,” emphasises Sadaf Farooqi. “The mechanisms that drive weight gain are not exactly the same as those that lead to complications.”

There is considerable variation: some develop disease, others do not. To understand that difference, the focus shifts away from the brain and towards the body.

“For complications, the key issue is how adipose tissue behaves,” Farooqi explains. “Not just how much fat you have, but how that tissue functions and interacts with other organs.”

“For a long time, fat tissue was seen as passive,” she explains. “We now know it is an active endocrine organ.”

It releases signals that influence metabolism, inflammation and vascular function – constantly communicating with the liver, pancreas and muscle. When that system adapts well, risk can remain relatively low.

“When adipose tissue can expand and store energy without disruption, the system stays balanced,” she notes.

When it cannot, the picture changes. “Fat begins to accumulate in other organs, inflammatory pathways are activated and metabolic control is disrupted,” Farooqi says.

That is when complications emerge – importantly, this layer of biology is partly independent of appetite.

“You can have a strong drive to eat and gain weight,” she notes, “but whether that leads to disease depends on a different set of processes.”

The distinction matters. “It means that we need to understand not just obesity but who is at risk of complications,” she says.

“We do not fully understand why some individuals are protected,” Farooqi adds.

Obesity is not just about weight – but how the body responds.

Rebuilding obesity in human tissue

If obesity arises from signals moving between organs, studying each organ in isolation is not enough.

“Most models simplify the system,” explains Sadaf Farooqi. “You lose the interaction – and that is where much of the biology happens.”

Animal models capture some of that complexity but not all.

“There are clear differences between species,” she notes. “Not everything translates to humans.”

The challenge is to study human biology without losing these connections.

“That is what we are trying to address,” Farooqi says – “building systems that let us see how tissues communicate.”

Connecting human tissues in the laboratory

To capture these interactions, Farooqi and her team are developing multi-organ systems in which human tissues are connected through microfluidic channels so that they can exchange signals much as they would in the body.

The aim is not to recreate the whole body but to capture key aspects – enough to ask meaningful questions. These systems enable researchers to study how genes disrupt communication – and how treatments modify these interactions. The model becomes a testing ground for interventions.

“You can introduce a stimulus – a nutrient or a drug – and observe how tissues respond,” Farooqi says.

But the limits are clear.

“These systems are still simplifications,” she cautions. “They do not capture behaviour, the nervous system or long-term adaptation.”

Nevertheless, they fill a critical gap.

“They bridge very reductionist cell models and whole-body studies in people.”

That shift matters: to understand complications, it is not enough to study parts in isolation but also how they interact under stress.

“It is about thinking in networks,” Farooqi says.

Why some people never gain weight

If one end of the spectrum shows how the system can fail, the other poses a different question: why do some people remain thin – even in environments in which weight gain is common?

“We all know individuals who seem to be protected,” says Sadaf Farooqi.

For a long time, this was explained as discipline or metabolism.

“But those explanations do not fully account for what we see,” she points out.

Instead, thinness – like obesity – appears to be partly biological.

“We now have strong evidence that it can be heritable,” Farooqi notes.

To explore this, her group studies individuals who are naturally thin – not because they restrict intake but because their biology differs. The goal is to identify pathways that protect against weight gain. In some cases, this may reflect appetite.

“These individuals may have a lower drive to eat,” she explains.

In others, it may involve how energy is processed and stored. The picture is still incomplete.

“We do not yet have the same level of detail as we do for obesity,” Farooqi says.

Instead of asking why weight increases, the question becomes why it does not.

But she avoids oversimplification.

“Thinness is not always benign,” Farooqi cautions. “And biology is only part of the picture.”

What matters is how individuals respond to the same environment. Variation in body weight is not simply behaviour. It reflects how biology is tuned.

The moment everything changed

Years after the first injection, the moment still stands out: a child at a table, food in front of her – and, for the first time, no urge to eat.

“It was a simple observation,” recalls Farooqi. “But it told us something profound.”

What changed was not behaviour – but the signal driving it.

“When you restore that signal, the system works as it should,” she explains.

That insight shaped everything that followed. From rare genetic conditions to common obesity – and now to treatment. But its impact reaches beyond medicine.

“For many people, the most important thing is understanding,” Farooqi says.

Over the past two decades, her work has shown that hunger is a regulated signal – and when that regulation is disrupted, the consequences extend beyond weight.

“There is still more to understand,” she concludes. “But we now know that the biology matters.”

The question is no longer whether obesity is a matter of willpower – but how we understand and respond to the biology that drives it – just as understanding once changed how we protected the most vulnerable people.