There is no single thing that causes it... various factors contribute, both internal (our biology) and external (our environment).
Not only that, but the combination of contributing factors may vary greatly between individuals. What causes obesity in one person may have no effect on another.
Modern obesity research is increasingly pointing to the brain playing a dominant role in the way our energy (fat) stores are regulated.
In the video above, Dr. Stephan Guyenet, an obesity researcher and one of my all-time favorite bloggers, explains how the brain is supposed to regulate energy balance... and why it's currently NOT working.
He makes a compelling case for the brain, specifically the "reward" centers and an area called the hypothalamus, being among the key players in obesity.
If you're interested in nutrition, health and the causes of obesity, then I highly recommend that you watch this video.
This is a critical piece of the puzzle to understand why some foods, but not others, make people fat.
The numbers are staggering... from 1960 to 2009, obesity increased from 13 to 34% and extreme obesity (BMI > 40) increased from 1 to 9%.
Although obesity had been creeping up slowly throughout the 20th century, it started to skyrocket around the year 1980.
This has been paralleled by a massive increase in type 2 diabetes, which often travels with obesity.
There have been many different theories about what it is that caused the obesity epidemic, but one inescapable fact is that calorie intake has gone up drastically at the same time (1).On the graph, the blue line shows the increase in obesity, while the green line shows the increase in calorie intake.
The increase amounts to about 363 calories per day. Sources vary on the exact figures, but pretty much everyone agrees that there has been a major increase.
Bottom Line: Obesity has increased drastically in the past few decades, correlating almost perfectly with an increased calorie intake in the population.
Of course, we're not just eating more food... we are eating more processed, commercially prepared food.
A century ago, we were eating mostly simple, home-cooked meals. Today, a large percentage of people's food intake is coming from fast foods: Keep in mind that this graph underestimates the true effect, because a lot of what people are eating at home these days is processed food.
The truth is, everywhere Western processed foods go, obesity soon follows.
Bottom Line: The increased calorie consumption in the past few decades is explained by an increase in processed, packaged foods and sugar-sweetened beverages.
The human brain is the most complex object in the universe, gram for gram.
The biggest part of it is the outermost layer, called the cerebral cortex.
This is where most of our "advanced" functions take place... such as logic, creativity, language, mathematics, etc.
However, the logical part of our brain isn't always in control.
There are other brain areas that take care of physiological functions like breathing, heart rate, body temperature and others.
These physiological functions are controlled subconsciously... we don't have to think about them because everything is happening on autopilot.
It turns out that our body weight is also controlled in large part by the brain (3). This involves a brain area called the hypothalamus, which regulates hormones and all sorts of internal functions.
It also involves a system called the reward system, which is activated when we do things that give us pleasure, such as laugh or eat.
Unfortunately, the brain cortex (logic and reason) doesn't really have full control over food intake, which is strongly affected by other more "primitive" areas of the brain.
The brain cortex can try... but there are other parts of the brain constantly trying to exert their influence, putting pressure on us to take actions that we may have previously decided aren't in our best interest.
As it turns out, junk foods directly affect some of these brain centers that control and regulate appetite, hunger and body fatness.
When the brains of people who have a tendency to gain weight are affected in this way, a strong physiological drive to eat more (and burn less) is created.
The strength of cognitive restraint ("willpower") pales in comparison.
This is one of the key reasons why junk foods, but not "real" foods, drive obesity... and perhaps other diseases as well.
Bottom Line: In people who have a tendency to gain weight, eating junk foods can lead to a strong physiological drive within the brain to eat more and get fat.
The main area in the brain that regulates energy balance is called the hypothalamus.
It senses various signals, including hormones, then either makes us feel satiated or hungry.
The brain regulates food intake both on a short-term (meal to meal) basis, as well as on a long-term basis (4).
The main hormone involved in long-term energy balance is called leptin, which is produced by the body's fat cells (5).
The bigger the fat cells, the more leptin they produce... and this functions as a signal to the brain that we have plenty of energy stored and that we don't need to eat.
When we lose weight, our fat cells get smaller and start making less leptin. This is interpreted by the brain as starvation, so we become more hungry and start burning fewer calories.
Conversely, if we gain excess fat then our fat cells secrete more leptin, which makes our brains realize that we don't need to eat, so we feel full and burn calories at a normal rate.This is how the brain "defends" its fat mass and is supposed to help us avoid starvation or becoming fat, both of which would have negative effects on our ability to survive in nature.
The problem is... this very clever system that has been designed throughout evolution to regulate our energy balance is broken, because the brain isn't sensing the leptin signal. This is called leptin resistance and is believed to be among the root causes of obesity.
When the brain doesn't "see" the leptin coming from the fat cells, it doesn't see that the fat cells are full of fat. In other words, the brain thinks we're starving, even though we have plenty of fat stored.
But if leptin resistance is among the key drivers of obesity, then what drives leptin resistance? According to Dr. Guyenet, inflammation in the hypothalamus may be what is causing the brain to become leptin resistant.
His lab has done studies in rats, where they compared mice fed a standard diet (rat chow) with mice fed a fattening diet, for 7 days. The results were staggering: The bright green and red colors are stains that emphasize inflammatory cells in the brain. As you can see, their number, size and activity increases drastically from eating a fattening diet for only 7 days.
But this goes beyond just rodents... they have also shown that there is probably also low-level inflammation occurring in the brains of obese humans (6).
They have also done studies showing that rodents that lack an inflammatory response don't become leptin resistant, giving further support to this hypothesis.
Interestingly, this does NOT happen in mice fed a normal diet... and when mice are switched from a fattening diet to a healthy diet, these changes can reverse completely.
This indicates that this process is reversible in humans as well, if they manage to stick to a healthy, real food based diet.
But that brings us to another problem... actually managing to avoid or eat less of the harmful foods in the modern environment, foods that happen to be downright addictive.
Bottom Line: Consumption of highly processed junk foods can cause inflammation in the hypothalamus of the brain, causing leptin resistance. This makes the brain think that the body is starving.
Another problem with junk foods, is that they are highly rewarding. They give us pleasure.What we perceive as pleasure is actually a flood of dopamine in the reward system of the brain.
This functions as a signal to the brain that this behavior is good, and the brain is hardwired to seek out behaviors that stimulate dopamine release.
Although this system worked well in the natural food environment, modern junk foods are so-called "superstimuli." They cause an intense dopamine release, similar to the way abusive drugs like cocaine work.
For people who are susceptible, this can lead to downright addiction, where people completely lose control over their consumption.
Put simply, the hyper rewarding effects of junk foods "hijack" the pathways in the brain that were designed to help us seek out behaviors that give us pleasure and are good for our survival.
But even people who aren't "addicted" to junk foods, many do experience various addiction-like symptoms such as cravings and obsessive thoughts about food, which drives increased energy intake and fat gain (12).
The food companies are well aware of all this... and use all sorts of tricks to make their foods as "rewarding" as possible.
I highly recommend that you read it.
Although the causes of obesity are complex and diverse, the evidence is clearly pointing to problems in the brain being among the dominant contributors.
The best thing you can do for yourself is to eat a real food based diet and live a healthy lifestyle, one that doesn't promote low-grade inflammation.
Bad food makes you fat and sick... but whole, single ingredient foods are the key to good health. Period.