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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) (R)

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, R2).  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 protects 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, an increased calorie intake is more than sufficient to explain the increases in obesity (R).  If you see the graph below, our increased caloric intake matches up nicely with the obesity epidemic.

Obesity and Calorie Consumption

The Japanese eat 25% fewer calories than we do in the US (R, R2), 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 1900’s as we were in the early 2000’s according to the graph, yet our obesity rate has sky-rocketed.

Looking to 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% (R).  58% is quite high.  One study that I discuss below uses a high carb diet to mean 46% of calories are from carbs (R).

We actually decreased our carb intake by 4%  from 1995-1997 to 2005-2007, yet our obesity rates still markedly increased over that period (R).

The Japanese eat 29% of their calories as fat, while we eat 38% (R).  We eat roughly the same percentage of protein (R).

A recent study has found that a low-carb diet is more effective than a low-fat diet for weight loss (R), 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 (R).

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. (R) The Mediterranean diet had a significant amount of carbs (45% vs 28%) (R), 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:

Shai, et al. 2008.

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) (R).

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.  (R, R2)

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.  (R)

A variety of health markers were similar for both diets, but they tended to be a 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

Food Spending, Smaller

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 a fast food restaurant  (R).

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% (R). Other relatively thin countries are in the top 10 as well. (R)

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 (R, R2).

There is also a plethora of observational studies showing that the people who eat the most sugar are at a much greater risk of getting type 2 diabetes, heart disease and even cancer (R, R2, R3).

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.

Many studies show that excess fructose has adverse effects on hormones related to obesity (R, R2, R3, R4) and we know glucose can spike insulin.

Studies show that people who eat the most sugar are at a high risk of future weight gain and obesity (R).

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 1950’s, 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

Food Price Trends as Percentage of Disposable Income

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

Historical Sleep Trends in Hours Per Night

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.

Poor sleep has negative effects on various hormones that are related to weight gain, and can contribute to increased hunger and cravings (R, R2, R3).

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 (R).

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 (R).

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:

  1. 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.
  2. 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

  1. When you increase caloric consumption, your body is supposed to increase caloric expenditure through generating heat by 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.
  2. 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.

Health Tools I Wish I Had When I Was Sick

At SelfHacked, it’s our goal to offer our readers all the tools possible to get optimally healthy. When I was struggling with chronic health issues I felt stuck because I didn’t have any tools to help me get better. I had to spend literally thousands of hours trying to read through studies on pubmed to figure out how the body worked and how to fix it.

That’s why I decided to create tools that will help others cut down the guesswork:

  • Lab Test Analyzer – a software tool that will analyze your labs and tell you what the optimal values are for each marker — as well as provide you with actionable tips and personalized health and lifestyle recommendations to help you get there.
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  • SelfHacked Secrets – an ebook where we examine and explain the biggest overlooked environmental factors that cause disease. This ebook is a great place to start your journey if you want to learn the essential steps to optimizing your health.
  • SelfHacked Elimination Diet course – a video course that will help you figure out which diet works best for you
  • Selfhacked Inflammation course – a video course on inflammation and how to bring it down
  • Biohacking insomnia – an ebook on how to get great sleep
  • Lectin Avoidance Cookbook – an e-cookbook for people with food sensitivities
  • BrainGauge – a device that detects subtle brain changes and allows you to test what’s working for you
  • SelfHacked VIP – an area where you can ask me (Joe) questions about health topics

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  • Vladimir Krasnobaev

    Author, your logic make me cry.
    Your conslusion: We Get Fat Because of Carbs – Myth

    So now look at those 2 graphs and NOTICE that obesity from 1980 y. are correlated at the same proportion to increased carb intake. not FAT. in this time range 1980 – 2000 , fat consumption goes down, and only carbs UP, so YES CARBS MAKE YOU FAT.

  • Terrance

    ” 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 ”

    Both of those studies should be removed as neither are actually low in fat. 30% of kcals is not low fat at all ( which is the lowest in fat either of them went ). When compairing the effects of macros on health parameters and lowering carbohydrate to 4% intake but not willing to lower fat equally as low is moronic and not science.

  • Ryan

    I think we can add Leptin to the list of myths above.

    One of the things we know about obese individuals is that muscle mass beneath all of the fat is significantly compromised. High amounts of stress and cortisol is known to be catabolic to muscle and anabolic to fat. High cortisol also destabilizes blood sugar, raising insulin, eventually leading to insulin resistance.

    When people lose all of this weight, the reason they need fewer calories to maintain the same weight as a forever thin adult is because of REDUCED MUSCLE MASS.

    Muscle mass raises our basal metabolic rate burning more calories.

    A 190 lb male with a lean body mass of 173lb will be stable at 2200 k/cals than a 190 lb man with a lean body mass of 163 lbs at 2200 k/cals.

    The weight reduced person is handicapped not because of leptin, but because obesity has eaten away at their muscle and thus there basal metabolic rate will be permanently lowered until lean body mass is increased by replenishing muscle on the body…

  • Solomon

    Further evidence. Leptin, insulin dysfunction. Obesity is medical disease.

    1. Joseph M. Cohen


    2. Michael Attanasio

      Obesity is a “medical disease” caused by lifestyle decisions.

      Leptin resistance occurs due to excess adipose tissue. Excess adipose tissue is created by eating too much and eating poorly (and a lack of exercise). Insulin resistance is caused by excess carbohydrate consumption (particularly simple carbohydrates). Obesity is as much a medical disease as laziness in most cases (not all).

  • Solomon

    Here are some reasons, perhaps the most important reasons, as posited by some Nobel-winning obesity researchers (discovery of leptin), as to why we gain weight and more importantly why it’s so hellishly hard for people who’ve successfully lost weight to maintain their losses. This is perhaps the most authoritative thinking on the subject, to date.

    1. Joe


  • MachineGhost

    I think meal size, meal duration, meal timing and iodine intake/hypothyroid rates need to be looked at in the three countries and added as myths, if appropriate. And don’t forget the viral hypothesis or flora imbalance. Obesity has been cured by one bloke simply by fixing patient’s gut bugs.

  • bosiljosar

    Joe, thank you for all the great blog posts! I check for updates, almost every day.

    Regarding this blog post.
    How about; Sugar (fructose) makes us fat by causing insulin resistance > which causes excess insulin in the blood > which blocks leptin in the brain > which makes us not feel full after a meal and messes up our energy balance > which makes us fat?

    1. Joe

      Thanks. I obliquely mention this in the post, but I also say that if we restrict calories – fat AND carbs- insulin resistance isn’t an issue. I’ve done a lot of research on insulin resistance and excess fat or excess carbs can cause insulin resistance. Restrict calories and this isn’t an issue. See causes of insulin resistance.

      1. bosiljosar

        Thank you for your quick reply!
        I will look into your blog post on insulin resistance.

        Regarding your thoughts on sugar, you ask the question if excess sugar causes more weight gain than excess of some other food with the same caloric value, I was wondering if you have found studies specifically on that topic? I did a quick google search and I found one that compares HFCS to sucrose and chow consumption in rats. The findings indicate that the group that had access to HFCS 24/7 gained the most weight, even though they were eating as many calories as the other groups.
        I am not arguing that these findings are conclusive but high fructose corn syrup consumption might play a role in we are getting fat, at least according to this study:
        “Rats with 12-h access to HFCS gained significantly more body weight than animals given equal access to 10% sucrose, even though they consumed the same number of total calories, but fewer calories from HFCS than sucrose. In Experiment 2, the long-term effects of HFCS on body weight and obesogenic parameters, as well as gender differences, were explored. Over the course of 6 or 7 months, both male and female rats with access to HFCS gained significantly more body weight than control groups.”

        Another thing I have been thinking about recently is that disruption of the metabolic cycle (or not fasting) might play a big role in weight gain (along with the excessive calorie intake). People are disrupting their metabolic cycle / not fasting / eating too frequently today, and that might contribute to our fat accumulation. A recent study supports this hypothesis.

        Mice who ate frequently throughout the day gained weight compared to the control group.
        They developed high blood glucose and cholesterol, motor control diminished and they even had some liver damage..
        The time restricted eating control group ate just as many calories, (they followed the same diet and ate the same food) but they still weighed 28 percent less and showed no adverse health effects.

        I just wanted to drop this in here as well; high carb diet might contribute to a greater weight gain compared to high protein according to this study:
        “Postprandial thermogenesis is increased 100% on a high-protein, low-fat diet versus a high-carbohydrate, low-fat diet in healthy, young women.” “These data indicate an added energy-cost associated with high-protein, low-fat diets and may help explain the efficacy of such diets for weight loss.”

        1. Joe

          Thanks, your points aren’t counter to what I say.

          The studies use a fructose level that barely anyone consumes and if you reduce calories your fructose load is lower. I don’t rule out fructose playing a some role, but it’s overplayed.

          More posts in the future will clarify my positions in more detail.

          1. Michael Attanasio

            Do you have any studies suggesting fat contributes to insulin resistance? I guess fat in could exacerbate the effects of excess carbohydrates. But fat, on its own or with a reasonable amount of carbohydrates, does not appear to cause insulin resistance. In fact, ketogenic diets reduce fasting glucose and increase insulin sensitivity.

            reply icon
          2. Joseph M. Cohen

            Excess fat causes insulin resistance. See my post and also this.

            Realize that genetics plays a role in what will be considered excess. Many other factors at play as well.

            reply icon
  • Jean

    Well written. This sums it all up really well. The only thing that is important for me in order to get an aesthetic body is to: Eat high protein (really dampens food cravings) and get sufficient amounts of fats. The rest of the calories really doesn’t matter. I’ve experimented with this for years and micro managing is just a waste of time. From personal experience, I’ve noticed that I get a plateau if I’m stressed/have elevated cortisol. I just won’t lose weight. This is the time for refeeding, allowing oneself to unwind and relax. By simply eating a bit more and relaxing can give a boost when one is stuck doing caloric restriction. I’m quite sure that caloric restriction will give one hormonal imbalances in the long run, similar to bulimics and anorectics. It is important to keep in mind that a caloric deficit is also a recovery deficit. This is why I advocate caloric restriction and intermittent fasting in cycles.

    Also, early morning fasted walks is one of the healthiest and best things to do if one wants to lose weight. There is no point in doing any kind of fasting if one is sedentary and doesn’t take advantage from it. Being sedentary while fasting won’t transform your body.

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