What Causes Obesity? Experts Say It’s Not a Lack of Willpower

What if the cause of obesity isn’t a lack of willpower or laziness, but the very foods we’re encouraged to eat? Mounting research suggests that weight gain stems from specific types of foods and nutrients that trigger biological processes beyond our conscious control—challenging everything we’ve been told about personal responsibility.

This analysis examines insights from leading nutrition experts, including Chris van Tulleken, Michael Pollan, and Michael Greger, who identify four key culprits behind modern weight gain: ultra-processed foods designed to promote overconsumption, the confusion created by “nutritionism,” highly refined carbohydrates that spike insulin, and our evolutionary preference for calorie-dense foods.

1. Ultra-Processed Foods

Many people believe that obesity is caused by a lack of willpower or a failure to exercise. According to Chris van Tulleken in Ultra-Processed People, these beliefs are unfounded. Rather, he says obesity is caused by a genetic predisposition combined with an environment that triggers overeating. That environment, says van Tulleken, is the one created by ultra-processed foods (UPF) and UPF marketing, both of which are designed to get people to eat as much as possible.

Van Tulleken explains that everyone with obesity has a genetic predisposition to it, but not everyone with a genetic predisposition is obese. The difference is the environment: specifically, the environment of UPF and poverty. Poor, urban areas are often characterized by an abundance of fast food restaurants, fast food advertising, and stores selling mostly UPF but very little fresh, minimally processed food. In addition, research shows that all stress, but especially the chronic stress caused by poverty, causes us to secrete a lot more of the hormone cortisol, which increases appetite and causes us to eat more. 

People with a genetic predisposition to obesity who are surrounded by UPF and UPF marketing are therefore more likely to eat more UPF, regardless of their willpower.

(Shortform note: Those who argue that obesity is caused by lack of willpower take the position that to lose weight, people simply need to make different choices—such as not allowing their stress or the advertising on billboards, social media, or streaming services to cause them to eat more. But as van Tulleken suggests, that argument overlooks the fact that something like “stress eating” is itself a behavior that’s influenced by our genes. And in the neighborhoods van Tulleken describes—“food deserts” without access to stores selling healthy, affordable food—healthy choices simply aren’t an option without expending a great deal of effort, time, and money. Studies show that residents of food deserts are at an increased risk for obesity.)

Van Tulleken also claims that, despite the many studies reaching a contrary finding, obesity is not caused by inactivity. Studies show that people burn the same number of calories per day (about 2,500) whether they live in a rural hunter-gatherer society or an urban, more sedentary one. In other words, we can’t lose weight simply by being more active: Whether we exercise every day or sit at home reading a book, the amount of calories we burn is the same.

This is because when we burn calories through exercise, our body compensates by using less energy on routine bodily functions (for example, our immune, endocrine, or stress systems), so our total energy use stays the same. This allows some of our body’s systems to rest and recuperate. On the other hand, if we sit at a desk all day and are inactive, we use our excess energy on things like being stressed.

Why do so many people believe that exercise results in weight loss and inactivity results in weight gain? Van Tulleken suggests that it’s because the ultra-processed food industry has heavily funded many of the studies that supposedly demonstrate this. For example, Coca-Cola has provided millions in funding to hundreds of research projects, finding that inactivity—not sugary sodas—causes obesity. Van Tulleken points out that industry-sponsored studies of sugar-sweetened beverages and weight gain are five times more likely than independent studies to reach conclusions favorable to the industry.

Van Tulleken argues that we’re eating more than we ever have (specifically, more UPF), and that’s what’s causing obesity—not failure to exercise or lack of willpower.

2. “Nutritionism” and Diet Confusion

In In Defense of Food, Michael Pollan argues that when Americans began looking to medical experts to guide how we eat, we lost touch with the tradition that once guided us to eat well. Pollan explains that the doctrine of “nutritionism,” or looking for the magic nutrient-based solution to health problems, produced conflicting views about what is healthy. 

This is because such research focuses on nutrients rather than the whole-food contexts in which they exist. Because studies have concluded that certain nutrients are problematic instead of certain foods, we’ve had trends such as the low-fat era, the anti-cholesterol era, the anti-carb era, and so on—and oddities like low-fat ice cream.

Pollan stresses that so long as we remain confused about diets and nutrition, we’ll struggle to reclaim our health and to change our lifestyles for the better. To change this, his rule of thumb is to eat real, mostly plant-based food in moderation and to listen to your body to know when you’re full.

3. Processed Carbs

When it comes to carbs and weight gain, the science is complicated. Not all carbs are created equal. Highly refined carbs are the worst because they have been stripped of any fiber, fat, protein, and micronutrients that you’d find in the whole food. (White wheat flour is the prime example.) Because they’ve been processed into a fine powder, the digestive system rapidly absorbs them. This rapid absorption causes a sharp spike in blood glucose levels, which in turn spikes insulin levels. These spikes underlie the development of chronically high insulin levels.

In addition to causing insulin spikes, processed carbs encourage overeating. Fung asserts that because refined carbs have been removed from the whole-food context from which they originate, our bodies have no satiety signals associated with them. In other words, they lack the protein, fat, and fiber that help you feel full. This is why you can snack for hours without feeling satiated—refined carbs aren’t filling. Meanwhile, they provide little to no real nutrients.

(Shortform note: In Salt Sugar Fat, Michael Moss reports that major food producers—including General Mills, Kraft, Kellogg, and Unilever—have intentionally engineered processed foods to produce the effects Fung describes. By manipulating the chemical structures of salt, sugar, and fat, these corporate giants have optimized foods for taste and profit. Recounting a 1999 meeting of top CEOs from the major players, Moss describes one executive’s belief that consumers “bought what they liked, and they liked what tasted good.” Unfortunately, what “tastes good” has come at the expense of nutrition and public health.)  

Added Sugars

In addition to the problems found in all processed carbs, Fung explains that added sugars, such as table sugar and high fructose corn syrup, will severely damage your health.

These sugars are dangerous because they contain fructose, a form of sugar that originates from fruits and vegetables. While every cell in the body can use glucose for energy, only the liver can process fructose. When you consume a high amount of fructose, such as from soda or candy, that sugar goes right to your liver. As Fung describes, a high, concentrated amount of fructose quickly overloads your liver, which races to convert it into glucose and fat. 

(Shortform note: While fructose is deadly when concentrated, health experts generally agree that you can safely enjoy fruits. Whole fruits contain fiber and water that balance out their sugar content. Since fruit is filling, it’s difficult to eat so much that you suffer negative effects from the sugar. However, fruit juice and smoothies can disrupt this balance: They strip away the fiber and make it possible to consume much more fruit than you would if it were whole. So in general, stick to whole fruit and avoid juices, especially those with added sugars.)

Before long, high fructose consumption causes fatty liver, a condition wherein your liver is overstuffed with fat and sugar. To compensate, your liver becomes insulin-resistant, requiring more insulin per unit to continue taking in fructose. 

As we’ve explained, insulin resistance contributes to chronically high insulin levels: Resistance leads to increased levels, which leads to more resistance, and so on in a self-reinforcing feedback loop that drives obesity. So added sugars, especially fructose, contribute to obesity. Note that this isn’t a problem of excessive calories, but a problem with the nature of the food.

(Shortform note: Fatty liver occurs in two varieties: alcoholic fatty liver disease (AFLD) and non-alcoholic fatty liver disease (NAFLD). The disease progresses through four stages, becoming increasingly severe until it culminates in cirrhosis, a serious condition of liver scarring and failure that can cause jaundice, brain fog, nausea, and weight fluctuations. Studies on rodents have shown that high fructose corn syrup causes fatty liver; however, a 2021 study on pigs, a larger mammal, found that a four-week diet of 60% fructose did not induce fatty liver.) 

4. Calorie Surplus

According to Michael Greger’s research in How Not to Diet, there’s only one factor that determines whether you put on weight: your calorie intake—the number of calories you consume and what form they take.

Factor #1: We’re Hardwired to Crave High-Calorie Foods

Greger argues that the first reason we’re prone to weight gain is that we’re naturally inclined to seek out calorie-dense foods—foods that contain a high number of calories per gram. For example, a gram of butter contains more calories than a gram of flour, which makes the butter more calorie-dense than, therefore preferable to, flour. 

He explains that this is because our bodies evolved within an environment of scarcity—we lived in the wild and only ate what we were able to hunt or gather. Since food wasn’t always available, we developed an instinctual preference for calorie-dense foods. The faster we satisfied our caloric needs, the less effort we had to put into hunting and gathering our food. For example, hunting, butchering, and preparing deer meat would take you a few hours of effort but would result in substantially more calories than you’d collect from foraging berries in that same amount of time. Therefore, you’d be more inclined to hunt and eat deer meat than to forage for berries. 

Additionally, consuming high-calorie foods ensured our survival because it allowed our bodies to store excess calories as fat reserves to fall back on in times of famine.

Our environment has evolved over time, and high-calorie foods are now readily available: Instead of eating only what we can hunt or gather, we go to the store and choose what we want to eat. However, Greger argues, our bodies have not evolved, so we still crave high-calorie foods. This means that we’re constantly tempted to gorge on more calories than we need. Each time we do, our bodies store these excess calories as body fat. But, because we’re no longer afflicted with food shortages, our bodies never get the opportunity to use these fat reserves—resulting in a continual accumulation of fat.

Some of Us Are More Genetically Predisposed to Store Fat Than Others

Many authors mirror Greger’s claim that, thanks to evolution, we’re hardwired to crave high-calorie foods and store excess calories as fat. Some notable proponents of this theory include James Clear (Atomic Habits), Glenn Livingston (Never Binge Again), Neil Shubin (Your Inner Fish), and John Walker (The Hacker’s Diet).

However, while this theory clarifies why we crave high-calorie foods, it doesn’t explain why some individuals can consume excess calories without putting on weight while others store all excess calories as fat. Researchers explain that not all of our ancestors adapted to the environment in the same way—some developed the ability to store more fat reserves than others and passed on this tendency to their offspring. This tendency continues to pass from generation to generation. As a result, some of us are genetically predisposed to store fat while others aren’t. 

In light of this research, pharmaceutical companies are examining the genetic similarities among obese individuals. This knowledge may help them develop new drug strategies to prevent or treat weight gain and counter the rising rates of obesity.

Factor #2: Processed Foods and Animal Products Are Calorie-Dense and Low in Nutrition

Greger argues that the second reason we’re prone to weight gain is that the food industry takes advantage of our instinctual preference for calorie-dense foods. It does this by encouraging us to eat processed foods and animal-derived products such as meat, fish, and dairy, which are cheap to produce and distribute. 

These food groups directly contribute to weight gain because they lack nutrition and are high in calories. Greger explains that the food industry strips crops of all nutritional value and fiber by either running them through processing mills or feeding them to livestock to create animal-derived products (such as meat, dairy, and eggs). It then adds high-calorie, low-nutrition components such as sugar or salt, oil, preservatives, artificial colorings, and flavorings to create fatty, starchy, sugary, or salty products. 

According to Greger, eating these foods creates four negative consequences that lead to weight gain:

1. They decrease your metabolism. 
2. They increase your appetite.
3. They stimulate inflammation in your body.
4. They increase the number and size of fat cells in your body.

5. Seasonal Weight Gain

Your body wants you to gain weight in the winter—and it’s not just because of those holiday cookies. While most of us blame our winter weight gain on festive indulgences and hibernation-like tendencies, scientists have uncovered an evolutionary story behind those extra pounds. Our ancient ancestors’ survival depended on their ability to pack on weight before winter’s food shortages, and our bodies still carry this prehistoric programming. 

Why Do We Gain Weight Over the Winter?

Studies have consistently found a pattern of weight gain during the winter months and attributed it to common factors like decreased physical activity and dietary shifts. While the extent of weight gain may vary, research indicates that on average, people tend to gain one to two pounds over the winter, with some studies reporting an average gain of around 0.8 pounds between Thanksgiving and New Year’s.

At the core of winter weight gain lies an evolutionary drive rooted in our ancestral past. Our prehistoric ancestors faced periods of food scarcity during the harsh winter, and the ability to store extra fat reserves gave them a crucial survival advantage. Researchers at the University of Exeter found that our evolutionary drive to avoid starvation is stronger than our desire to prevent overeating, which might push us instinctively toward increased food intake during winter. This innate urge helped to ensure people’s survival. 

Additionally, research from the University of Alberta suggests that the body seems to respond to reduced sunlight exposure during winter by increasing its fat storage. The study found that fat cells beneath the skin tend to shrink and release stored fat when exposed to the blue light wavelengths from the sun. This mechanism could have helped our ancestors conserve energy during the darker winter months.

What Choices Contribute Most to Winter Weight Gain?

Assuming that we evolved to gain weight over winter to ensure our survival during the coldest months, questions remain about exactly how our bodies put on those extra pounds.

One of the primary mechanisms that leads to winter weight gain is a decrease in physical activity. As temperatures drop and daylight hours decrease, people tend to spend more time indoors, away from sunlight. Indoors, they’re more sedentary. Additionally, dietary shifts play a significant role, since we often indulge in richer, more calorie-dense comfort foods and holiday treats over the winter. 

Learn More About Weight Gain

Want to dive deeper into the science around what causes obesity? Here are the full Shortform guides to the books mentioned in this article:

• Ultra-Processed People
• In Defense of Food
• How Not to Diet
• Never Binge Again
• The Hacker’s Diet