The No. 1 Underlying Cause of the Obesity Epidemic


Childhood obesity is a growing problem, largely because children don’t make a conscious decision about what to eat, but let innate preferences for calorie-dense foods rich in fat, sugar, and salt guide their choices.

The obesity epidemic has quickly become one of the greatest health challenges humans have ever faced. Worldwide obesity rates have steadily climbed over the last several decades, and in many westernized nations, more people are now classified as being either overweight (BMI 25-30) or obese (BMI >30) than normal weight. What is going on? Why do we keep putting on the pounds?

As you’re undoubtedly aware if you’ve been following this blog, obesity is nonexistent among hunter-gatherers and traditional people minimally affected by modern lifestyle habits (1, 2, 3). Even when they have access to an abundance of food, non-westernized people eating ancestral diets (e.g., the Kitavans) stay fairly lean (1, 4).

Studies consistently show that if these types of non-industrialized populations adopt a more westernized lifestyle, obesity rates climb and diseases of civilization emerge (1, 2, 3). This clearly suggests that it’s something about our modern environment that’s fueling the obesity epidemic. But what?

Some blame the obesity problem on the high intake of starch, fructose, and other carbohydrates in the modern world, saying that it’s all about insulin and blood glucose levels and the impact food has on our hormonal system. Others point out the widespread consumption of “fast food” in both developing and developed nations and may say that it doesn’t matter what we eat as long as we stick to minimally processed, whole foods. Others again focus on non-diet related factors and suggest that insufficient sleep, chronic stress, and antibiotic-induced damage to the gut microbiome are largely to blame.

While all of these things are clearly important and can account for a large portion of the problem with our modern diet and lifestyle, they don’t explain why we can’t seem to stay in control of our food choices and caloric intake. After all, we all have the option of avoiding donuts, crackers, candy, and pizza. So, why do so many people have trouble sticking to a healthy diet and maintaining a lean frame?

Is it all about willpower?

When I’ve discussed the obesity problem with people who are lean and either eat a healthy diet or don’t seem to gain any weight despite their poor diet, they often say that weight control is all about willpower; basically implying that people who are overweight are lazy and lack self-control. Is that it? Should we just talk about sloth, gluttony, and laziness when we try to figure out why we seem to get fatter and fatter every year? No, that’s clearly a massive oversimplification of the problem.

Nobody wants to be overweight. If you ask someone who’s carrying a solid extra couple of pounds whether he is perfectly happy with his situation, he’ll most likely tell you no, and if you keep digging, he’ll probably tell you that he has tried sticking to numerous diets and exercise regimes in an attempt to lose weight, but without success.

We humans have a tendency to think of ourselves as in complete control of our actions. After all, we do have the option of making conscious choices regarding what we eat, when we go to sleep, and how much we exercise. This clearly implies that the “willpower theory” of obesity does hold some truth. If we all just made a conscious decision to eat a Paleo-style diet, exercise for 1 hour every day, and dial in our sleep – and stuck with it no matter what, the obesity problem would largely be eliminated.

However, as anyone who’s taken the time to do a little more research on the matter than merely watching a couple of episodes of the Biggest Loser will tell you: Telling people to exercise more and eat less rarely works – and simply blaming the obesity problem on a lack of individual self-control doesn’t get us very far.

The evolutionary explanation for the obesity epidemic

To really get an understanding of why things are like they are, we have to take into account the fact that the human body was sculpted for a very different environment than the one we currently live in. Through millions of years, natural selection acted upon the human genome to adapt our primal ancestors to various and diverse natural, ancestral environments. Needless to say, in these types of environments, fast food joints, soda machines, and grocery stores were nowhere to be found.

Our preagricultural ancestors had to work for their food, often spending hours each day walking and digging in an attempt to get a hold of meat and tubers, which were among the most calorie-dense foods available. This is in stark contrast to how things are like today, in the sense that we can now buy foods rich in sugar, salt, and fat at every street corner.

For our ancient forebears, the problem wasn’t to limit their caloric intake to avoid fat gain, but rather to maximize the amount of calories they got from the amount of work they had to do. An attraction towards calorie-dense foods and an ability to store fat for scarcer times provided a survival advantage, and hence, it doesn’t come as surprise that we have evolved a tendency to seek out palatable and safe foods that are rich in calories.

Also, since expending more calories than necessary to acquire food, build shelter, etc. had negative survival value in an ancestral environment, it’s no surprise that we’re wired to take it easy when possible and that so many modern people find it hard to get off the couch to exercise.

Up until very recently, having these evolved instincts was an advantage, but in the modern obesogenic environment, where calorie-dense food is everywhere, these ancient adaptations that helped our ancestors seek out rich sources of calories and store as much fat as possible are working against us. They are mismatched to modern environments. No wonder so many people have trouble sticking to a diet.

Combine this with the fact that we’ve engineered highly-processed foods with an unnatural nutrient composition and we can start to understand why obesity rates have skyrocketed these last couple of decades. Donuts, cupcakes, and other similar hyper-palatable foods contain a potent combination of fat, starch, sugar, and salt that our ancient ancestors never encountered. These foods affect our body and brain in ways that natural selection never prepared us for and are highly addictive.

So, when we look at the things from an evolutionary perspective we can quickly grasp why we have such a massive obesity problem today. An obesity epidemic is just what you’d expect to get when you put humans into the evolutionarily abnormal environments we’ve created for ourselves. When you boil it down, we’re really just doing what our genes are “telling us” to do.

Clearly, we can’t just go and eat crap and gain weight and blame everything on the innate tendencies that were passed down to us from our primal ancestors, as we do have the possibility of making conscious choices. However, it could be argued that the next time you didn’t manage to resist the urge to eat a box of crackers or a jar of ice cream, it wasn’t “your” fault.

Okay, I think I’ve managed to convey the main message of this article, so to avoid making this into a very long post I won’t go deeper into this whole thing today. If this is a topic that interests people I might do another post in the future where I talk about possible solutions to all of this, and what we as individuals can do to overcome cravings for unhealthy food, avoid overeating, and stick to a healthy diet.

Let’s finish with a quote from a paper titled “How is Darwinian medicine useful?” that really hits the message home:

… traditional clinical medicine looks at the problem of obesity in terms of individual differences that explain why one person becomes obese and another does not. These factors may be due to genes, early environment, or current lifestyle. Now that one half of Americans are overweight, however, it is time to answer the evolutionary question: why are our bodies designed so that most of us eat too much and exercise too little?

An initial answer is simple. In the environment in which we evolved, natural selection shaped appetite regulation mechanisms to ensure that we survived periods of famine. In those ancient times, eating required walking for hours each day to get food, a caloric cost that made it impossible for most people to accumulate much surplus as fat. Exposure to intermittent periods of food shortage sets off a system that prepares for a coming famine by increasing appetite and basal weight above the starting point. Dieting activates the same system, so weight can rebound to above what it was when the diet began. When young people try to lose weight by using willpower to drastically limit their food intake, their regulation mechanisms react with a response that is adaptive: they often gorge themselves. These episodes of uncontrolled eating can make the dieter even more fearful of becoming obese, so still further efforts of will arouse the mechanism more strongly, setting in cycle the positive feedback spiral we see in anorexia and bulimia.

As for our food preferences, one would think we would be designed to eat what is good for us. The system would work fine if we lived on the African savanna. In the natural environment, fat, salt, and sugar are in such short supply that when they are encountered, the useful response is to consume them. Fat provides twice as many calories per gram as carbohydrates. Sugar is often associated with ripe fruits, and seeking it out was usually beneficial. Now that we can choose our foods, we prefer what was in short supply on the African savanna.

We also choose our levels of exercise to minimize caloric expenditure—a wise strategy in the Paleolithic era when wasting calories could bring death. This tendency to be sedentary, in combination with our preferences for large amounts of high-calorie, high-fat food, has resulted in an epidemic of atherosclerotic disease. Natural selection will eventually fix such design problems, but it will take hundreds or thousands of generations to do so.

Picture: Creative Commons picture by Joe 13. Some rights reserved.


  1. tedhutchinson says:

    While I appreciate the example paper below is a rat study, might it not also be the case that the tendency to be sedentary and the compulsion to consume more than is good for us, may be a result of the nature (including the fructose/glucose ratio) of the processed foods that are so readily and cheaply available particularly for those with limited incomes?
    Fructose:Glucose Ratios—A Study of Sugar Self-Administration and Associated Neural and Physiological Responses in the Rat

    • Thanks for sharing the study, Ted!

      Absolutely. In my three part series on sugar cravings I discussed some of the problems with processed, hyper-palatable foods, including why they are so addictive.

      To make it perfectly clear that I’m not suggesting that this is the only underlying cause/explanation for the obesity epidemic I’ve modified the title a tiny bit, so it now has the title I was first thinking about giving this post.

  2. I always look forward to your posts. Please keep up the great work

  3. Jennifer says:

    This makes total sense. One thing I wish is for our friends and family to dig deep and find answers to their problems (heart disease, Type 2 diabetes, obesity, high blood pressure, ulcers and digestive disorders). Clearly the examples we set and the gentle guidance we give them is not enough; maybe they just can’t accept that the food industry has changed and is harmful to them. Maybe some of them are not “sick and tired of being sick and tired” or they just don’t believe that the food they eat is the problem… Or perhaps a combination of all of the above, or simply just apathetic to their situation. Hopefully with your information becoming more apparent one day they will truly understand.

    • Agreed, Jennifer!

      I suggest you throw a lacto-fermentation party to get your friends more interested in real food – and to give them a boost of probiotics. Haha 🙂

    • Jennifer, I agree. I also dislike the current trend of putting “spin” on the issue in order to place the blame on some entity that’s beyond human control. Rationalizing doesn’t solve anything. Moreover, it’s causing obesity to become the new “normal.” I’ve heard/read dozens of reasons why a person is fat, and some of them are pretty far-fetched. But I’ve never heard anyone come right out and admit they are fat because they eat too much of the wrong kinds of food, which is almost always the case. This, of course, involves loss of face and constitutes a personal failure that most people don’t like to admit to, not even to themselves. At the bottom of it, maintaining a healthy weight really is about desire and willpower.

  4. Jennifer says:

    LOL! Great idea, Eirik 🙂

  5. BeesMakeHoney says:

    N=1 Warning.
    I could write a screed in response to this – but a few points instead.
    1. Culturally, eating and food have become obsessions from a social standpoint. Overindulgence is the norm rather than the exception. Appetite and hunger are no longer indicators of the need to eat. Unless a person concerned with their weight/health reigns this in – change will not happen.
    2. Despite all the debates about this Calories in = Calories out for weight loss.
    3. The focus on weight and not health causes more harm than good. It might reduce the incidence of overweight/obese individuals, but it does nothing to address health issues.
    Quality of life should be the focus, and that can be had at both ends of the spectrum, as well as all points in between.

  6. This makes total sense. One thing I wish is for our friends and family to dig deep and find answers to their problems (heart disease, Type 2 diabetes, obesity, high blood pressure, ulcers and digestive disorders). Clearly the examples we set and the gentle guidance we give them is not enough; maybe they just can’t accept that the food industry has changed and is harmful to them. Maybe some of them are not “sick and tired of being sick and tired” or they just don’t believe that the food they eat is the problem… Or perhaps a combination of all of the above, or simply just apathetic to their situation. Hopefully with your information becoming more apparent one day they will truly understand.

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