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.