Your Body Weight Was Programmed Before Birth — Here’s the Proof

You may think obesity is caused by behavior — that people with obesity are lazy, that willpower is everything and genetics have nothing to do with it. But You have no idea what you're talking about. A single dose of a common plastic, given to one pregnant mouse, made her descendants obese for five to six generations before the effect finally faded. No genes were mutated, they inherited a chemical "tag" switched on in the womb, sitting on top of their DNA—and your own body weight works the same way. Its shaped not just by what you eat but by marks layered onto your genes: some written before you were born and handed down from ancestors you never met. But importantly, some you can still rewrite today.

DNA Isn’t the Whole Story

Every cell in your body carries the same DNA. The same three billion letters, copied into your skin cells, your neurons, your fat cells. All of it identical. So here’s the puzzle: if a liver cell and a brain cell have the exact same genetic code, why is one a liver cell and the other a brain cell? Why don’t they look and behave the same?

The answer is that DNA isn’t the whole story. Think of your DNA as the hardware—the fixed machine you’reborn with. Epigenetics is the software: the instructions that determine which genes are switched on, which are switched off, and how loudly each one is allowed to speak, without changing a single letter of the underlying code. That’s the basic definition: stable—and sometimes inheritable—changes in gene activity that happen without altering the DNA sequence itself.

It helps explain why identical twins, who share the exact same genome, can become more different over a lifetime. One may develop a disease the other never does. All of this involves a small set of molecular tools. There are three classic ones—and a fourth, newer layer.

The Four Layers of Epigenetic Marks

The first, and best understood, is DNA methylation. Picture a tiny chemical tag: a methyl group. It gets attached directly to one of the DNA letters—the cytosine—usually where a “C”sits right next to a “G” along the strand. When these tags gather around a gene’s control switch, they can turn that gene down—or shut it off completely.

This is the mark we can measure most reliably because it’s chemically stable. It can remain on your DNA for decades. That’s one reason it appears in studies tracing exposures from early life into adulthood. The second tool is histone modification.

Your DNA isn’t floating loose. It’s wound around spool-like proteins called histones. That’s how more than six feet of DNA can fit inside a microscopic cell. Chemical marks on the tails of those spools help determine how tightly the DNA is wound.

Wind it tightly, and the genes in that stretch are tucked away—silenced. Loosen it, and the cell’s machinery can get in and read them. More than two hundred histone marks have been identified: a whole second layer of annotation alongside DNA methylation. The third tool is non-coding RNA.

For a long time, we thought RNA was just the messenger—the working copy a gene sends out to help build a protein. But cells also make short RNA molecules that don’t code for proteins at all. Instead, some of them help silence other genes. They can intercept messenger copies and mark them for destruction before they’re ever translated.

The fourth—and newest—layer is RNA methylation. It’s the same kind of methyl tag, but placed on the RNA message itself rather than on the DNA. It can affect how stable that message is and how much protein gets made from it. Epigenetics, it turns out, reaches into the messages too—not just the master code.

How the Environment Reaches Your Genes

Now, here’s the part that makes all of this matter. These marks aren’t permanent graffiti. Dedicated enzymes write, read, and erase them. That’s what allows the system to respond to your environment.

One family of enzymes—the writers—lays down methyl tags. A second family—the erasers—removes them. And a third set—the readers—recognizes a mark and translates it into an action, turning a gene up or down. Writers. Readers. Erasers. Add, remove, interpret.

Why does that matter? Because these enzymes don’t work without fuel. They need raw materials, including methyl groups that ultimately come from your diet, as well as metabolic cofactors whose levels can change with what you eat and how much fat you carry. And that’s the hinge of this entire video.

The epigenetic machinery is the physical bridge between the outside world and your genes. Your food. Your exposures. Your mother’s pregnancy. Even your grandparents’ circumstances.

These things don’t rewrite your DNA. They can change the marks sitting on top of it. And those marks influence which genes shout—and which stay silent. So when someone says obesity is “genetic,” they’re only half right.

The genes are the hardware. What we’re talking about for the rest of this video is the software. And software can be rewritten.

The Agouti Mouse

If you remember just one experiment from this whole video, make it this one. Picture two mice. One is round and yellow. The other is slim and brown. Everything about them might make you think they’re two different kinds of animal. They’re not.

These mice are identical at the level of their DNA. Letter for letter, their genetic code is the same. So how can the same code produce a fat, yellow mouse and a lean, brown one? It comes down to a single switch.

They carry a gene called Agouti. Right beside it is a short stretch of DNA that works like a dimmer. Cover it with methyl tags, and the gene goes quiet. Remove the tags, and the gene goes into overdrive.

Tags on: the gene is kept in check, the coat turns brown, and the mouse stays lean. Tags off: the gene runs wild. It produces a signal that turns the coat yellow and disrupts the brain’s “I’m full” message. The mouse never feels satisfied. It keeps eating, gains weight, and develops diabetes.

Same gene. Same DNA. The only thing that changed is whether the tags were on or off. Now here’s the part that should feel close to home. What determines where those tags go? What the mother ate while she was pregnant.

Give a pregnant mouse plenty of the building blocks for these tags—folic acid, vitamin B12, choline, and betaine—and her pups come out brown and lean. Give her too little of those same building blocks, and her pups come out yellow and fat. Identical genome. Different diet. Different offspring.

And think back to the plastic chemical from the opening. Give that pregnant mouse BPA, and the result shifts: more yellow pups, fewer tags, more obesity. But give her the methyl-building-block nutrients at the same time as the BPA, and the damage is erased. The pups come out normal.

That’s the whole argument of this video, playing out inside one cage. The environment—what the mother eats and the chemicals she’s exposed to—doesn’t touch a single DNA letter. It changes the tags sitting on top. And those tags help determine what happens to the animal.

No mutation. No edit to the code itself. Just a different set of instructions read from the exact same page, producing two very different creatures. Keep that picture in mind: the fat, yellow mouse and the lean, brown one.

Because from here on, every story in this video is that same story, scaled all the way up to you. So we have these chemical tags sitting on top of our DNA, switching genes on and off. The obvious question is: where do they come from? What determines whether a tag gets written, erased, or moved?

Here’s the key idea for this segment: the environment doesn’t reach into your cells and rewrite the DNA letters. The sequence you were born with stays the same. What the environment changes is the layer of chemical marks sitting on top of those letters. And it does this through four main routes. Let’s take them one at a time.

Route One: The Raw Materials

Think of DNA methylation as a job that depends on a supply chain. To place a methyl tag on a gene, the cell needs methyl groups. It gets them from a molecule called SAM—the universal methyl donor. SAM is like a delivery truck, dropping off methyl tags wherever they’re needed.

Where does SAM come from? Your diet. Folate, vitamin B12, vitamin B6, methionine, choline, and betaine all feed a cycle that keeps the SAM trucks rolling. Get plenty of them, and the supply line is well stocked. Run low on folate or B12, and the supply line can run short. When that happens, the cell may not be able to place as many methyl tags. Genes that should be switched off can drift back on.

This isn’t just a theory. It’s the Agouti mouse. Remember: give the mother more methyl donors, and her pups shift toward brown and lean. The diet was stocking the supply line that writes the tags. And the same machinery is relevant to the Dutch Hunger Winter: when a pregnant woman is deprived of these nutrients at a critical time, the methylation pattern laid down in her child can still be detected decades later.

So the first way the environment can change your marks is almost embarrassingly simple: it affects whether the factory has the parts. Route two: sabotage. The second route is harsher. Some things in the environment don’t just deprive the machinery of what it needs—they interfere with it directly.

Route Two: Direct Interference

Air pollution is one example. When you breathe in fine particles, your cells produce reactive oxygen species—highly reactive molecules that can damage cellular components. They can attack methyl tags already on your DNA and interfere with enzymes that help put tags back. Pollution can also affect the production of methyl donors, choking the supply line from route one at the same time.

The result, as this account describes it, is a genome losing tags faster than it can replace them. Genes that were silenced may start to become active.

And air pollution isn’t the only exposure associated with changes in DNA methylation. Studies have found patterns linked to heavy metals such as arsenic, cadmium, lead, and mercury; to bisphenol A, the plastic chemical from our opening; and to tobacco smoke. Smoking has been linked to methylation changes at thousands of sites, and some can persist for years after someone stops smoking.

So route two is direct interference: the environment can affect the tags themselves or disrupt the machinery that writes them.

Route Three: Hijacking the Control Panel

This route is more indirect. The chemical doesn’t simply damage the system; it can mimic a signal the body already responds to. Your cells are covered in receptors—molecular switches that respond to signals such as hormones. One is called PPAR-gamma, a key regulator of fat-cell development. When activated, it helps turn on a program of genes involved in fat storage.

Some environmental chemicals, known as obesogens, may affect this system. Examples include bisphenol A, phthalates from some plastics, tributyltin, and dioxins. They can interact with pathways involved in fat-cell development and metabolism. Researchers are investigating how such exposures may influence gene regulation and epigenetic marks.

The idea is that certain chemicals can act on the body’s control systems, potentially shifting how cells develop and store fat. That’sone way environmental exposures may matter beyond a person’s behavior alone.

Route Four: Stress

The fourth route may be less obvious, because it isn’t something you eat or breathe. It’s what you experience. Chronic stress activates the HPA axis—the hormonal circuit that helps regulate cortisol. Sustained stress signaling has been associated with changes in methylation on genes involved in stress responses and metabolism. The same cellular machinery and the same kinds of tags are involved, but the signal comes from within the body rather than from a chemical exposure.

So there are four routes: affect the supply line, interfere with the enzymes, influence the control panel, or send sustained stress signals. Different pathways, but each can affect the chemical marks involved in gene regulation. Now, the showpiece. I want to walk through an example that helps illustrate how these pieces may fit together.

How a Plastic Chemical Can Flip a Switch

Let’s return to BPA and the FTO region, which has been studied in relation to obesity. In one proposed mechanism, losing a methyl tag at a particular site near this region exposes a binding site for a protein called CTCF. CTCF helps organize the genome by bringing distant stretches of DNA together into loops.

With the tag gone and the binding site exposed, CTCF may bind there and help form a loop. That loop can bring the FTO region into contact with two other genes, IRX3 and IRX5, which are involved in energy balance and metabolism.

The proposed chain is striking: an environmental chemical doesn’t change the DNA letters themselves. Instead, a change in a chemical tag may affect how the genome folds in three dimensions, influencing how genes are regulated. Studies have tested parts of this mechanism, including the role of the CTCF binding site.

That’s a powerful way to think about how environmental signals may reach beyond DNA sequence and affect the genome’s organization. It also shows why body weight can’t be reduced to willpower alone. So far, we’ve talked about epigenetic marks as if they were just switches—on or off, methylated or not. But here’s another important part: those switches rely on the cell’s metabolism.

Food, Gut Bacteria, and Your Epigenome

Enzymes that write or erase epigenetic marks need particular molecules to do their jobs. The enzymes that add methyl tags need a methyl donor called SAM, which is produced through pathways that use nutrients from food, including folate, B vitamins, and amino acids. Other enzymes use molecules such as acetyl-CoA, NAD, and alpha-ketoglutarate. These also play important roles in cellular metabolism.

What does that mean? The machinery involved in gene regulation is connected to the cell’s fuel supply. When metabolism changes, the availability of these molecules can change too. Obesity can alter metabolism, and researchers are studying how those changes may affect gene regulation in turn. That creates the potential for a feedback loop: genes influence metabolism, and metabolism can influence which genes are active.

Now here’s another part of the story: some of these signals come from the bacteria in your gut. When you eat fiber—from vegetables, beans, and whole grains—gut bacteria can ferment it and produce short-chain fatty acids. One of these is butyrate. Butyrate can affect enzymes that regulate chromatin, the structure that helps control how accessible DNA is to the cell’s machinery. By influencing those enzymes, butyrate may affect which genes are active.

So the pathway is: food feeds gut bacteria, the bacteria produce a chemical, and that chemical can influence gene regulation in your cells. It’s one way diet, microbes, and genes may communicate. A low-fiber diet can change this source of signals, though the effects depend on the person and the broader context of their diet and health.

So we have food supplying molecules used by the machinery, and gut bacteria producing signals that can influence it. That brings us to one of the biggest questions in this field.

The Missing Heritability

Obesity often runs in families. Researchers estimate that inherited factors account for a substantial share of the differences in body weight between people—often reported in the range of 40 to 70 percent. That’s a large number. For years, scientists have searched for the genetic variants involved by scanning the DNA of millions of people.

They’ve found some. FTO is one of the best-known examples. Other genes and regions, including IRS1, HIF3A, PPAR-gamma, LEP, POMC, and TNF, have also been studied in relation to body weight, appetite, fat-cell development, or inflammation.

But identifying genetic variants hasn’t explained all the inherited variation in body weight. The contribution of known variants is much smaller than estimates of overall heritability. This gap is often called “missing heritability.” It doesn’t necessarily mean the explanation lies in epigenetics alone; many genetic and environmental factors may contribute.

Epigenetics is one area researchers are investigating. A family’s shared experiences and environment can affect health, and scientists are studying whether some epigenetic patterns may persist or be passed between generations.

Researchers have also identified methylation patterns associated with body fat. One study of more than 5,000 people reported 187sites where DNA methylation was associated with body-fat measures. Those findings show links between methylation and body fat, though they don’t by themselves establish which caused which.

So some of what looks like inherited risk may involve more than DNA sequence. Epigenetic marks may be part of the picture, alongside genes, environment, and other influences. The question isn’t only which genes you inherited, but also how their activity may be shaped over time.

One Gene, Two Directions: SREBF1

Up to now, we’ve talked about the epigenome in broad strokes: tags being added or removed, and groups of genes being turned up or down. Now let’s zoom in on one gene and follow the process step by step. This gene doesn’t explain obesity by itself, but it’s a well-studied example related to fat production and metabolism. And there’s an important detail that is often oversimplified in popular explanations.

The gene is called SREBF1. Think of it as a regulator of fat production. When it’s active, it produces a protein that enters the nucleus and helps regulate genes involved in making fatty acids and triglycerides from available nutrients. SREBF1 also has roles in the development of fat cells. So changes in its activity may influence the body’s fat-making and fat-storage processes.

Now here’s where the chemical tags come in. On the control panel of this fat-making gene, there are a handful of specific spots where a methyl tag—a tiny chemical flag—can attach. In large human studies, people with more tags at those exact spots tend to have higher triglycerides, lower levels of good cholesterol, more belly fat, and a higher risk of type 2 diabetes.

And there’s a possible explanation: the tags may change how much of the fat-building protein the gene produces. The tag on the switch can affect how strongly that switch is turned on.

But association alone isn’t enough. The real question is: how much of the effect actually runs through this gene? Researchers can try to trace the path—from the chemical tag, to changes in the gene’s activity, to the final outcome. By those estimates, the gene accounts for a meaningful share: around a sixth of the effect on triglycerides, and up to roughly a third of the effect on body weight.

That’s not the whole story. But it is a measurable share, flowing through this one switch. And the fallout splits into two lanes. First, the fat lane: an overactive gene can push fat cells to store triglycerides and disrupt the hormones that help keep metabolism balanced. Then there’s the inflammation lane. This same switch is connected to the immune system, where it can influence immune cells inside fat tissue and contribute to inflammation.

So one chemical tag, on one gene, may be connected to both fat storage and the slow-burning inflammation that can go along with it.

Here’s the twist I promised—and this part matters if we want to be precise. This isn’t a one-way street. It’s a loop. There’s genetic evidence that the tag can influence blood fats and body weight. But there’s also evidence pointing in the other direction: high triglycerides and excess body fat may help drive the tag onto the gene in the first place.

So don’t picture a single arrow. Picture one with arrowheads at both ends. Diet and body fat may help lay down the tag—and, once it’s there, the tag may feed back into the fat-making program. Cause and consequence, following each other around in a circle.

And only part of that relationship is truly causal in either direction. A lot of what we measure may simply be the body recording the state it’s already in. That distinction—what’s driving the disease, and what’s just a readout of it—is what separates real science from hype. We’ll come back to it, because it’s the question that determines whether any of this can actually be reversed.

The Womb: Your First Environment

Here’s something that may unsettle you. The most important environment you ever lived in is one you can’t remember—and can never return to. It’s the one you spent nine months in before you were born.

The conditions in that first environment—what your mother ate and how much, whether she was stressed or smoked, what was happening with her blood sugar—could leave marks on your genes while your organs were still forming. Those marks may still be influencing your weight, cholesterol, and risk of diabetes decades later.

Scientists have a name for this idea: the developmental origins of health and disease. The basic claim is simple, but powerful: a lot of your adult health may be shaped before you reach adulthood. During a few critical windows early in development, your cells are deciding what to become and how to function. Change the environment around a developing fetus during those windows, and you may change more than the baby’s size at birth. You may also affect the settings it carries through life.

And here’s what makes this more than an abstract idea: researchers can measure some of these marks. A developing fetus receives signals through the placenta—nutrients, hormones, stress chemicals—and those signals may influence where methyl tags are placed on particular genes. Some of those genes help regulate appetite in the brain, how fat cells store and release energy, and how the body grows.

The womb isn’t just building a body. It may also help shape how that body responds to food later in life. So how do we know this happens in people, and isn’t just a theory from the lab? To answer that, we need to go back to one of the darkest winters of the twentieth century.

The Dutch Hunger Winter

The Netherlands, the winter of 1944 into 1945. In the final months of the Second World War, a German blockade cut off food supplies to cities in the western part of the country. Rations collapsed. At the worst point, people were surviving on around five hundred calories a day—about as much as a couple of slices of bread.

Then, when the war ended in May 1945, food returned almost overnight. It was a brief, brutal famine, with a clear beginning and a clear end.

That terrible event created something scientists could never ethically arrange on purpose: a natural experiment. Some babies were in the womb during the famine. And many of them had siblings—conceived before or after it—who weren’t. The siblings had the same parents, much the same genes, and grew up in the same home. One major difference was whether they had been in the womb during those months of starvation.

Researchers tracked these people down decades later and compared siblings who had been exposed to the famine with those who hadn’t. What they found is one of the most important results in this field.

Sixty years later, people exposed to famine around the time of conception still had a different chemical mark on a key growth-and-metabolism gene than their own unexposed brothers and sisters. That gene helps regulate growth and body size. The famine had chemically turned its dial down before these people were born, and the difference could still be detected six decades later.

Think about what that means. A few months of starvation, during one particular window early in life, left a molecular signature that outlasted the war, the recovery, and entire adult lifetimes. And it wasn’t random. The difference showed up in people exposed around conception, pointing to a specific window when these marks may be set.

These weren’t people who were starving as adults. By the time researchers studied them, they’d lived full lives with plenty of food. But the mark was still there. The famine group also went on to have higher rates of obesity, worse cholesterol, and more diabetes.

Siblings Born Before and After Bariatric Surgery

Now here’s the question that turns the story around. The famine showed that too little food in the womb can leave a lasting mark. But what about today’s problem—not too little food, but too much? What happens when the womb is an environment of excess?

For that, we have what may be one of the most elegant human experiments of all: weight-loss surgery.

Picture a woman with severe obesity who has one child, then undergoes bariatric surgery, loses a significant amount of weight, and later has a second child. Now you have two siblings who gestated in very different environments. The first developed in a body affected by obesity, high blood sugar, and inflammation. The second developed after the mother’s metabolic health had changed following surgery.

Same mother. Same father. Different metabolic conditions in the womb. When researchers compare siblings like these, they find differences in methylation marks on genes involved in blood sugar regulation and inflammation. The mother’s body changed, and the marks detected in her next child changed too.

Put the two stories side by side, and the message is striking. The Dutch famine showed that scarcity in the womb may reprogram the child. Studies of siblings born before and after bariatric surgery suggest that excess in the womb may also affect the child’s development—and that improving the mother’s metabolic health can change the conditions for a later pregnancy.

In both cases, the baby isn’t a blank slate. Its development is influenced by conditions it has no say in and will never remember. That brings us to the uncomfortable truth at the center of this segment. Some of the marks on your genes may have been shaped by circumstances that were never yours to choose—by what was or wasn’t available to your mother while you were developing.

You inherited a starting configuration. That isn’t your fault. It was never your choice. But—and this is where we’re headed next—the story doesn’t stop with your mother. Some of these marks may reach further back than that, to people you never met.

Can Effects Pass Through Generations?

Now for the part that sounds like science fiction, but isn’t. Let’s start in a small parish in far northern Sweden, a place called Överkalix. For generations, people there depended on the harvest. Some years, the crops failed and there was famine. In other years, the harvest was abundant and there was plenty to eat.

Because this was a remote community with detailed church records, researchers can trace families across more than a century: who was born, who died, and—crucially—how much food was available to each child in each year. Here’s what those records suggest. Whether a boy had plenty to eat or very little during a narrow window just before puberty appeared to predict how long his grandsons would live. Not his sons—his grandsons.

Boys who had plenty of food during that window tended to have grandsons who died younger and had more diabetes and other diseases. The pattern seemed to skip a generation and show up in grandchildren who hadn’t even been conceived yet.

The pattern was also sex-specific in a striking way. A grandfather’s food supply was linked to the health of his grandsons down the male line. A grandmother’s food supply was linked to her granddaughters. It’s almost as if a message had been written into the eggs or sperm and sent two generations into the future, with different outcomes for boys and girls.

These are historical records, so they can’t prove the mechanism on their own. But animal studies offer clues about how something like this might work—and point, in particular, to the father. For a long time, we assumed that anything passed down from aparent’s experience had to come from the mother, through the womb. The father contributes just one cell, so what could sperm carry besides DNA?

It turns out, potentially quite a lot. Sperm contain tiny RNA molecules—small fragments that can influence how genes are regulated. The levels of these fragments can change with the father’s diet. Feed a male mouse a junk-food diet and the RNA cargo in his sperm changes. Breed him, and his offspring can show metabolic problems, including trouble regulating blood sugar and a greater tendency toward obesity—even if they eat a normal diet throughout their lives.

And there’s a striking experimental finding: researchers can take some of the RNA fragments from the sperm of a metabolically unhealthy father, inject them into a healthy embryo, and reproduce aspects of the disease. No harmful DNA variant is required. The information can be carried in the RNA—an example of how a father’s diet may influence his sperm, and how that information may affect his offspring.

Then there are chemical exposures. Remember the plastic chemical from the beginning of this video. In mouse studies, exposure to bisphenol A during pregnancy has been linked to effects on later generations—not just the mother’s pups, but also their offspring, and in some experiments for as many as six generations.

Those later generations weren’t exposed to the chemical themselves. Researchers are investigating how the effects might be carried forward, including through changes in the germline—the cells that give rise to eggs or sperm. It’s as if an initial exposure leaves a message that can be copied forward. And here’s the twist that matters most if you’ve ever lost weight and watched it come back: your fat may have a memory.

Fat Tissue Memory

When researchers studied human fat tissue before and after major weight loss—including weight loss after bariatric surgery—they found that some molecular signs associated with obesity were still present two years later. The scale had changed, and body weight had fallen, but the cells still carried parts of the molecular signature associated with obesity.

That memory may not be passive. It could help prime fat cells to expand again when more calories become available. This may be one reason keeping weight off can be harder than losing it in the first place. You’re not simply fighting a lack of willpower. Your body may also be responding to lasting biological changes from its earlier state.

So step back and look at what we’ve seen: a famine linked to effects in grandchildren; a father’s diet reflected in his sperm; a chemical exposure associated with effects across several generations in mice; and a body that may retain molecular traces of having been heavier. Your DNA is not your destiny. But the marks layered on top of it may reach further—and last longer—than most people realize.

Cause or Footprint?

Now for the part many videos like this leave out—the part that helps you decide how much of the story to believe. We’ve spent an hour talking about chemical marks on DNA that track with obesity. Marks that can change with diet, chemical exposures, and even what your grandmother ate. But a mark that shows up alongside obesity isn’t necessarily a mark that causes it.

Streetlights come on as night falls, but they don’t cause the sunset. So what’s happening here? Do these marks help trigger weight gain? Or does weight gain leave the marks behind, like footprints?

For most of them, the evidence points to footprints. Researchers have found more than 200 places in the genome where methylation is associated with body weight. But when they tested which way the influence ran, the arrow usually pointed from weight to methylation—not the other way around. In the largest blood study of its kind, essentially one site looked as if it might be driving weight upward. Just one. Overall, excess body fat appeared to change the epigenome on a much larger scale than epigenetic marks changed body fat.

How can scientists tell a footprint from a possible trigger? One approach uses the genetic variants you inherit at conception. You’re born with those variants, before you’ve eaten a meal or gained a pound. If a variant influences methylation and is also linked to body weight, that can offer a clue that the methylation might come first.

Researchers can test both directions: Does the mark influence weight? And does weight influence the mark? When they do, the “weight changes the mark” arrow has lit up again and again, while the “mark changes weight” arrow has usually been much weaker. Usually—but not always.

A small number of marks have stood out. One is in SREBF1,a gene involved in building fat. Another is near NFATC2IP. Methylation at that site has been linked not only to body weight, but also to future weight gain—sometimes before the scale changes. And when researchers looked directly at human fat tissue instead of blood, they found around ten additional sites that may play a causal role, based on the same kind of genetic evidence.

So the triggers may be real. They just seem to be rare: a handful of specific marks among hundreds associated with weight.

And that shift—from blood samples to fat tissue—reveals one of the biggest limitations in this field. Most studies measure methylation in blood. That’s practical: you can draw blood. Taking a sample of someone’s fat tissue is a much bigger ask. But blood isn’t the only tissue involved in obesity. Fat tissue itself matters, and the patterns found in blood don’t always match what’s happening there.

When researchers compare the two directly, only a small fraction of the marks may agree. So a mark that looks important in a blood sample may not tell us much about what’s happening in fat tissue. And a mark that matters in fat may not show up in blood at all. In a way, we’ve been trying to read the story in one book when much of it is written in another.

So where does that leave us? Not with the hype, but with a more solid conclusion: obesity appears to change epigenetic marks more often than those marks change obesity. Many of the marks are consequences—real and measurable, but downstream. A smaller number may help drive weight gain, especially when they’re supported by evidence from the right tissue. Those are the marks researchers will want to understand better—and perhaps, one day, learn how to change.

Can These Marks Be Changed?

That brings us to the question that matters: if the environment can influence these marks—in the womb, through earlier generations, or through the food and chemicals around us—can they change again? Can we write over them? Some of them can change, at least in part. And that’s one of the more hopeful findings in this research.

Start with food and movement. In one study, women with severe obesity followed a reduced-calorie diet for six weeks. Researchers found changes in methylation at more than 16,000 sites across the genome. Exercise has also been linked to changes in marks on genes involved in how muscles use fuel.

After substantial weight loss through bariatric surgery, some obesity-associated marks—including marks in genes related to methylation, leptin, and appetite—have shifted toward patterns seen more often in people without obesity. These findings suggest that some marks respond to changes in diet, activity, and body weight. They aren’t necessarily written in stone.

But—and this may help explain why keeping weight off can be so difficult—they may not all return to their earlier patterns. Studies of fat tissue suggest that some molecular signs associated with obesity can remain even after major weight loss. Some marks shift; others may persist. Researchers are still working out what those lasting changes mean and whether they contribute to weight regain. They may be part of the biology involved, but they don’t explain everyone’s experience—and they don’t make regain inevitable.

Can we target these processes directly? Researchers are beginning to investigate that, too. Medicines such as metformin, glitazones, GLP-1 drugs, and SGLT2 inhibitors have been associated with changes in epigenetic activity in tissues such as the liver, muscle, and fat. Experimental drugs that target enzymes involved in epigenetic regulation are also being studied, including in cancer research.

That doesn’t mean these medicines work by rewriting someone’s metabolic destiny. They’re used for specific medical purposes, and their effects on epigenetic marks are still being studied. But this research suggests that the machinery controlling those marks could be a potential target for future treatments.

There’s another possibility: these marks may become a kind of report card. Researchers can measure methylation patterns, and some patterns may carry information about a person’s weight or metabolic health—sometimes before changes show up on the scale. One long-term goal is precision nutrition: instead of giving everyone the same advice, researchers hope to better match recommendations to a person’s biology. That idea is still being studied; methylation tests can’t yet tell someone exactly what diet is right for them.

The Bottom Line

So let’s come back to where we started. Obesity isn’t simply about behavior, willpower, or laziness. Body weight is shaped by many things, including biology, environment, and the conditions in which a person develops. Epigenetic marks are part of that picture. Some are associated with diet, chemical exposures, and early-life conditions. Some may influence health; others may simply record what the body has already experienced.

Your genes matter, but they don’t tell the whole story. The environment can influence how genes are regulated, and some of those patterns can change over time. Researchers are still working to find out which marks matter most, how long their effects last, and whether changing them can improve health.

That’s a more complicated story than “it’s just willpower”—but it’s also a more honest one. And it leaves room for hope, without promising more than the science can support.