For some people, why we get fat is an emotionally charged issue. If your opinion is fixed on this topic, then this post will likely agitate you. This blog isn’t part of any group or movement, so I take positions based on the evidence that I come across. I try to be as dispassionate as possible. If you disagree, I welcome and encourage you to post studies and perhaps present something that I haven’t seen. If you bring new information, I will surely modulate my position based on the new information.
I’d like to mention that data and studies don’t trump individual experiences. Studies only find effects that pertain to a majority/plurality of the population. If your experiences are different, then your biology might be different.
The Japanese Litmus Test
To understand why we get fat and debunk some myths related to why we get fat, it’s important to look at what the thinnest people in the developed world do: The Japanese. (a few countries in Africa with starvation problems are thinner) 
In 2010, the US had an obesity rate of 36%, with all of our diets and weight loss programs. On the other hand, the Japanese and Koreans had an obesity rate of about 3.5-4%, the lowest in the developed world [R, 2]. You can see the obesity explosion in the US and an insignificant uptick in Japan.
Since the Japanese are obviously doing something right, it’s probably a good idea to test our current theories of why we get fat against them.
So if someone comes along and says we get fat because of X, we should test this theory by comparing it to what the Japanese are doing. If they are also doing X, then it’s probably a good idea to scrap theory X, because the Japanese are the thinnest people in the developed world.
For fun, let’s say an arrogant, brash, zealous and overly-confident individual named “Gary” comes along and says he knows why we get fat. Gary is certain that we get fat solely because we’re eating carbs, not because we eat too much or because we’re exercising too little. If such a situation occurred, I’d tell Gary he has an interesting theory and that we should test this against the Japanese to see if it makes sense.
Other popular theories that we should test against the Japanese is the palatable food theory, the junk food theory, and the sugar theory, among others.
It’s important to recognize that the Japanese don’t have some distinct set of genes that protect them against obesity. When the Japanese come to America and become completely Americanized, they become just as fat as everyone else in their socioeconomic class, to my knowledge.
This litmus test isn’t a smoking gun, but it can help put some things into perspective. As general advice, it’s wise to be skeptical of individuals who come along and blame all of the society’s ills on one specific factor. The reality is always more complex.
Myth 1: Calories Don’t Matter For Weight Loss
According to studies, increased calorie intake is more than sufficient to explain the increases in obesity . If you see the graph below, our increased caloric intake matches up nicely with the obesity epidemic.
The Japanese eat 25% fewer calories than we do in the US [R, 4], which fits with the position that calories do matter.
I have a friend who meticulously ate less than 20g of carbs a day for three months, but ate as much as he wanted. At the end of it, he gained 25 pounds. So, yes, you can gain weight on a very low carb diet if you increase your caloric intake.
As you will see, I don’t think calories are the only thing that matters, but saying that they don’t matter at all is bizarre.
Myth 2: We Get Fat Because of Carbs
As you can see from the graphs, our carb intake has fluctuated up and down, while our fat consumption has gone up a bit. Some people say we became obese because of the low-fat recommendations, but people haven’t even largely been keeping to these guidelines, as the graph shows.
We’ve been eating the same amount of carbs at the beginning of the 1900s as we were in the early 2000s according to the graph, yet our obesity rate has sky-rocketed.
Looking at the Japanese, they eat a relatively high carb diet. As a percentage of calories, they eat 58% of their calories from carbs and we eat 49% . 58% is quite high. One study that I discuss below uses a high carb diet to mean 46% of calories are from carbs .
We actually decreased our carb intake by 4% from 1995-1997 to 2005-2007, yet our obesity rates still markedly increased over that period .
A recent study has found that a low-carb diet is more effective than a low-fat diet for weight loss , although both groups lost weight. What many don’t realize about the study is that both the “low-fat” and low-carb groups reduced their fat and calorie intake, so this refutes the popular claim that eating more fat leads to weight loss or that calories don’t matter .
In 2008, a high-quality study lasting 2 years was published by the New England Journal of Medicine that compared a low-carb diet, a low-fat diet, and a Mediterranean diet. The mean weight loss was 2.9 kg (6.4 pounds) for the low-fat group, 4.4 kg (9.7 pounds) for the Mediterranean-diet group, and 4.7 kg (10.3 pounds) for the low-carbohydrate group, demonstrating that the Mediterranean diet is equally as effective for weight reduction as a low carb diet.  The Mediterranean diet had a significant amount of carbs (45% vs 28%) , yet the weight loss was the same. This was the case even though the low-carb diet had 8% more protein (25% vs 33%), and protein is established to inhibit appetite. (I’ve created a modified Mediterranean diet template for people to build off of.) See the results below:
Looking at the totality of changes in health markers, I’d say the Mediterranean diet wins out, even though the low-carb dieters consumed fewer calories (Mediterranean diet had lower fasting glucose and insulin resistance) .
There were 2 studies done looking at weight loss differences between a high carb diet and a low carb diet when people were instructed to eat the same amount of calories (an isocaloric diet). There were no statistically significant differences between the diets. [6, 12]
I should mention, though, that in both studies the low-carb group lost more weight than the low-fat group, even though the results weren’t statistically significant. The low carbs lost 4 pounds more in the 6 months study and ~7 pounds more in the 12-month study. However, people on the low fat/high carb diets still lost 22.3 pounds and 25.3 pounds in the 6 and 12-month study, respectively. 
A variety of health markers were similar for both diets, but they tended to be a bit better for the high carb diet, though they weren’t statistically significant. The high carb diets had better LDL numbers and the low carb diets had better HDL numbers.
The nonstatistical difference in weight between the diets in the referenced studies can be more than explained by the fact that this wasn’t a controlled environment and these people prepared their own food.
What these studies tell me is that you can lose weight with any diet, even when you relatively up your carb intake, as the low-fat diets do. Also, we should ditch low-fat diets for weight loss.
Myth 3: We Get Fat Because of Fast Food/Junk Food/Palatable Food
In the graph above, you see how the population changed its eating habits in the past 120-130 years.
At the turn of the 20th century, people were eating mostly simple, home-cooked meals. Around 2009, about half of what people ate was fast food or other foods away from home.
It may seem then like fast food is a key driver of our obesity epidemic based on this graph. However, let’s compare this to the Japanese.
The Japanese have the second most Mcdonalds in the world (the US is number 1). They are number 6 when it comes to people per fast food restaurant .
Japan, South Korea, and France are 3 of the top 6 countries that consume the most fast food, according to one source. Japan and South Korea are the thinnest countries in the developed world and France is one of the thinnest. France has the lowest obesity rate in Europe at~12% . Other relatively thin countries are in the top 10 as well. 
It’s also interesting that many of the countries that consume the most fast food are also the healthiest (R).
The Japanese eat plenty of junk food as well. The bulk of their calories come from refined rice, noodles, and beer, according to observers. They’re not big into brown rice or whole grains, though they eat veggies (including fermented veggies). They also eat food that tastes very good, from what I hear. This information comes from people who I know that spent time there.
It’s possible that these factors (junk food, fast food) play some role in making us fat, but it seems like these theories can’t explain in a significant way why we get fat. It surely isn’t a sufficient cause of obesity.
Myth 4: We Get Fat Because of Sugar
The sugar theory states we get fat because of added sugar. According to the theory, added sugar spikes insulin and causes weight gain.
Numerous studies show that eating excess amounts of added sugar can have harmful effects on metabolism, leading to insulin resistance, belly fat gain, high triglycerides and small, dense LDL cholesterol [16, 17].
Excess sugar is harmful to our health, but the question is if excess sugar causes more weight gain than an excess of some other food with the same caloric value and if it does if the difference is significant.
Studies show that people who eat the most sugar are at a high risk of future weight gain and obesity .
Given these studies, we might jump to the conclusion that sugar is a significant source of our weight troubles. However, few of us eat the amount of fructose in these studies and if we do it’s because we’re eating too much in general.
When we eat within our caloric needs, these problems don’t occur. If we ate less, we’d naturally reduce our level of fructose because we’d preferentially crave protein. I don’t know about you, but eating sugar or sweet foods all day isn’t appealing. I like foods that satisfy other gustatory desires like chicken, beef, etc…
Also, it’s no surprise that people who eat the most sugar are the most obese because they’re also eating the most calories.
If we see the picture below we will see that France’s sugar content isn’t much different than America’s, yet they are pretty thin there (1/3 of the obesity rate compared to us). And while Japan eats 40% less sugar, they also eat 25% fewer calories. Australia consumes more sugar and has a lower obesity rate than the USA.
The fact is that the Japanese are still consuming more sugar than many of us would think of as healthy. In addition, if we were to map out the history of their sugar consumption from the 1950s, it almost certainly would show a marked increase, without an accompanying rise in obesity.
I can only conclude that sugar isn’t a very significant factor in our obesity epidemic (although it’s probably a minor factor).
Myth 5: We Get Fat Because Food is Cheap
One factor that some have argued has contributed to the obesity epidemic is that food has become cheaper relative to our income.
From the graph above, you see that food prices have dropped from 25% of disposable income to about 10% of disposable income in the past 80 years.
However, Japan, South Korea and many of the thinnest nations are very rich and their income and wealth are more spread out than it is in the USA, which should result in a higher obesity rate. Also, the poorest people with the least disposable income in the US are usually the most obese.
This theory doesn’t seem to have much merit. It’s possible, however, that eating more vegetables can help with weight loss and veggies are more expensive, but this is a different argument and I don’t think the price of veggies has increased relative to our income. Also, most of the poor can afford enough non-organic veggies.
Myth 6: We Get Fat Because We’re Getting Less Sleep
A popular theory for our obesity epidemic is that we’re getting less sleep. This is true and it sure isn’t healthy, but whether this is causing our obesity epidemic is another question.
In recent decades, average sleep duration has decreased by 1-2 hours per night. In the US, short sleep duration is one of the strongest individual risk factors for obesity. It is linked to an 89% increased risk in children, and a 55% increased risk in adults .
Sounds convincing, right? However, the Japanese sleep the least in the world at 6 hours and 22 min, compared to 6 hours and 31min in the US .
We can only conclude from this that if sleep plays a role, it’s not a significant one and can be compensated by other factors. I do think that this may play a small role, though.
There Are No “Good Calories”
To lose weight, we should try to eat less of everything: sugar, carbs, fat, protein, fructose – you name it. Sure, you should try to reduce added sugar more than other things, but don’t mistakenly believe that you can increase “good calories” without consequence.
If you eat too much, you will gain weight, regardless of the macronutrient profile of the food.
While I think that sugar is a bit worse than other calories if overdone, the question is how significant is this? I think the significance seems to be hyped up.
At the end of the day, though, people need to listen to their bodies and do what works for them. People should follow a diet that reduces calories and that they can stick to for the long haul.
The Calorie Debate
Increasing caloric intake does make it more likely that you will gain weight, but it’s not necessary that you will gain weight.
Here are two facts to ponder:
- Some people try to gain weight but are extremely thin. They have reduced hunger and increased energy expenditure, which they didn’t choose to have.
- Some people are obese no matter how little they eat or how much they exercise or how much energy they put toward losing weight.
For much of the US population, people become overweight because they don’t care too much. You know, that guy eating Mcdonald’s and junk food for 3 meals a day and downing cokes and beer while he watches TV.
But if you are reading this blog, it’s likely that you care and want to have a normal weight.
And so we have a lot of readers that are too thin or overweight, even though they’re trying to just be fit.
If you struggle with your weight – either you are trying to put on or lose weight, it indicates that you have a metabolic problem, which means that your metabolism isn’t working right (either too fast or too slow).
Facts About Calories
- When you increase caloric consumption, your body is supposed to increase caloric expenditure by generating heat from burning brown fat and increasing metabolism. Increased caloric intake is also supposed to increase your drive to move around and exercise to burn those calories. In addition, calories can go towards energy used to build muscle, for your immune system or for fat storage. For some people, instead of increasing muscle mass, they increase fat mass.
- Your body is supposed to have an appetite based on your level of fat mass and energy levels. If you have enough energy in the form of body fat, your appetite is supposed to be reduced.
Again, when you are trying to lose or gain weight and you are not capable of doing this or it is very difficult, it means you have a metabolic problem.
However, realize that metabolic problems result from living unhealthily for a while. And once you have a problem, it’s not so easy to reverse it. That’s the situation that a lot of the country find themselves in.
You might indeed be eating too much or too little, but this is beyond “your” control and is controlled by your physiology, which controls your appetite.
People are differentially motivated to lose weight as well. If you are in Thailand or Japan people will shame you for gaining weight, which increases motivation.
However, I realized that people who are highly motivated and really want to look good, sometimes expend an inordinate amount of time and physical and psychological energy focusing on losing weight. This indicates a metabolic problem. You shouldn’t have to eat 1500-1800 calories a day to lose weight.
Your weight should be relatively stable by not thinking about it. That’s when you know you’re healthy.
Calories Matter, But Your Physiology Matters More
If you check out some of the linked posts below, you will see that there are so many ways that appetite and calorie expenditure are controlled by physiology. The interactions are often complex.
In each of the 132 different biological mechanisms uncovered, animals or humans would change their weight by having just one variable in their physiology altered.
Do Carbs Make You Fat?
Now there are people who say that you can only gain weight if you eat carbs, but this is wrong if you look at the big picture.
People can gain weight even if they don’t eat carbs, even though many people do lose weight from a low carb diet.
Insulin is one of the pillars of weight gain (one of the big 4 hormones), but there are 122 other mechanisms involved in weight control, many of them that operate independently of insulin.
Some of them are influenced by insulin, so insulin is a very important factor, but it’s far from the only one.
Also, some people don’t realize that in people insulin causes satiety. The problem is when you are insulin resistant, it takes more insulin to have the same satiating effect. Insulin resistance is one of the metabolic problems that can disrupt normal weight physiology.
Did I Miss Something?
If you have a study you’d like to share, please do. I am always open to changing my opinion based on new information.