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THE HUMAN COMPUTER

The human brain is a three-pound pink gelatinous mass of tissue. It bears little resemblance to the hard metal and plastic components inside your computer. Yet they both perform the same basic function: information processing. Although they differ in many details of operation, brains and computers both collect information inputs, process them, and use them to generate useful outputs. I think this is a good starting point for understanding how the brain works.

The inputs to the brain come from two places: inside and outside the body. From outside, the brain receives information from our external sense organs, such as sight, hearing, smell, taste, and touch. From inside, it receives information from many different sensors that convey information about the position of our limbs, how our head is accelerating and rotating, our core temperature, the quantity and quality of our gut contents, bladder and rectum fullness, our blood concentration of ions and glucose, the amount of fat we carry, digestive distress, infections, tissue damage, and countless other variables.

As in a computer, the brain processes this information and uses it to generate useful outputs. Once again, these outputs affect two things: what happens inside the body and what happens outside the body. We call the first one physiology and the second one behavior. So your brain collects information from inside and outside your body and uses it to appropriately regulate what’s happening inside and around you, in order to benefit you. For example, if you see a basketball rapidly approaching your head, you’ll duck out of the way if you can. In that case, your brain collected information from your visual system and used it to generate movements that got you out of the way of the ball.

Here’s a less intuitive example that relates more closely to our subject of interest. When you lose weight, your brain detects falling levels of leptin and increases your motivation to eat. In that case, your brain is using an internal input to regulate a behavioral output, and it happens largely outside of your conscious awareness. Here’s the point I’m driving toward: The outputs of your brain, including your appetite and eating behaviors, are determined by the input cues it receives. Some of these cues are processed by conscious circuits, and we’re very much aware of how they affect us. Many others are processed by nonconscious circuits, which influence our physiology and behavior in ways that we have little awareness of, and little direct control over. Yet they can nevertheless have a substantial impact on our lives.

I’ve argued that these nonconscious circuits explain why we overeat, despite our best intentions. As Daniel Kahneman relates in his book Thinking, Fast and Slow, the brain’s thought processes can be roughly divided into two systems. System 1 is fast, effortless, intuitive, and nonconscious, while system 2 is slow, effortful, rational, and conscious. System 2 understands the long-term consequences of our choices and represents our rational intentions: it wants you to eat the right amount of nutritious food, get plenty of exercise and sleep, and stay lean, fit, and healthy to a ripe old age.

Unfortunately, system 1 isn’t so rational. It has its own agendas, crafted by millions of years of natural selection. It guides our behavior and physiology using a collection of hardwired heuristics that helped us survive and thrive in the world of our distant ancestors. It didn’t evolve to guide us through a world of credit cards, pornography, and addictive drugs. And it didn’t evolve to handle easy access to calorie-dense, highly palatable food. It often disagrees with system 2, and it can be extremely persuasive. System 1 is why we overeat, despite the fact that we know it makes us fat and undermines our health and well-being.

Although Kahneman doesn’t discuss in his book the brain structures that underlie systems 1 and 2, we know that system 1 represents more than one circuit. In fact, it’s a hodgepodge of many circuits that perform a variety of processing tasks. In this book, I’ve taken you on a tour of some of these processes that drive us to overeat in the modern world, despite our best intentions.

First, we encountered the reward system, centered in the basal ganglia, which teaches us how to get food properties—such as fat, sugar, starch, and salt—that the brain instinctively views as valuable. The reward system collects food-related cues from our external sense organs and our digestive tract, and guides us toward valuable foods by helping us learn and motivating our behavior. What we experience is that we crave and enjoy certain foods more than others, and we develop deeply ingrained eating habits around the foods we enjoy. Unfortunately, this system evolved in an ancient world where calories were hard to come by, and obtaining them required a lot of work, and therefore motivation. In the modern affluent world, where calorie-dense, highly rewarding foods are ubiquitous, our hardwired motivation to eat remains strong, and it drives us to overconsume.

Next, we explored the economic choice system, centered in the orbitofrontal cortex and the ventromedial prefrontal cortex, which integrates the costs and benefits of possible actions and selects the one that’s the best “deal.” This system contains both conscious and nonconscious elements, because it integrates costs and benefits from all parts of the brain that relate to the decision at hand. Some of the costs and benefits it considers relate to system 2 processes, such as predicting the future impact on your waistline of eating a pastry, or calculating whether you can afford to buy it. Yet much of what it considers is nonconscious, and in many animals, including humans, it appears to be wired to value calories above all other food properties. And the easier the calories are to get, the more of them we eat. Another way of saying this is that when it comes to food, its primary cues are calories and convenience. This becomes a liability in a world where calorie-dense foods are more convenient than ever to purchase, prepare, and consume.

The lipostat is a third system, located primarily in the hypothalamus, which nonconsciously regulates adiposity by influencing appetite, our responsiveness to seductive food cues, and our metabolic rate. It takes its cues primarily from the hormone leptin, which is produced by fat tissue, although it also responds to food reward, protein intake, physical activity, stress, and possibly sleep (as well as other factors that are beyond the scope of this book). The lipostat has one job: to prevent your adiposity from decreasing. And it’s very good at what it does, because in the world of our ancestors, losing weight meant having fewer offspring. This is the system that makes weight loss difficult and often temporary, and it may also be part of the reason why our weight tends to creep up over the years. It partially explains why people with obesity tend to eat more than lean people, making obesity a self-sustaining state. When the lipostat squares off with our best intentions to eat the right amount and stay slim, it usually wins in the end.

Working in parallel with the lipostat, the satiety system regulates food intake on a meal-to-meal basis by making us feel full and reducing our drive to continue eating after we’ve had enough. Located primarily in the brain stem, the satiety system takes its cues from the digestive tract, which relays how much volume we’ve eaten, and the protein and fiber content of our food. The satiety system also receives cues from the reward system, which tends to shut down the feeling of satiety when we eat highly rewarding foods, such as pizza, french fries, and ice cream. And finally, it takes important cues from the lipostat, which increases or decreases satiety to help maintain the stability of body fat stores. One of the reasons why modern food tends to be so exceptionally fattening is that it doesn’t provide the cues the satiety system needs to appropriately regulate calorie intake. We tend to use the sensation of fullness as an intuitive signal that we’ve eaten enough, so when we eat calorie-dense, low-fiber, low-protein, highly palatable foods that provide little satiety per calorie, we overeat substantially without even realizing it.

The genetic details of how each person’s lipostat and satiety system are constructed go a long way toward explaining why some of us develop obesity in the modern world, while others remain lean. Each person’s brain has a unique genetic blueprint, and this nudges us to interact differently with food and defend different levels of adiposity. Some people are naturally resistant to overeating even in a very fattening food environment, while most of us are susceptible. Certain lucky folks don’t gain weight even if they do overeat, because their lipostat just burns off the excess calories. This makes it very difficult to judge people for their weight, since we’re all born with different predispositions. At the same time, for most of us, our genetic makeup is just a predisposition—not an inevitable fate. Our ancestors four generations ago carried the same genes we do, yet they rarely developed obesity because they lived in a different context that provided profoundly different cues to the brain and body.

The sleep and circadian rhythm systems, located in the hypothalamus, the brain stem, and other brain regions, nonconsciously influence eating behavior and adiposity in large part by interacting with the reward system, the lipostat, and the economic choice system. The amount of restorative sleep we get, the timing of our sleep, the blue-wavelength light we see, and the timing of our meals are key cues that regulate these systems. When we don’t sleep enough, or don’t sleep well enough, it increases the reward system’s responsiveness to food cues, and we often end up eating more calories. It affects our economic choice system, promoting an “optimism bias” that makes us think more about the current benefits of eating a pastry than the future costs. And it nudges us to become less faithful to our rational, constructive daily goals, such as eating healthy food. When our sleeping and eating behaviors are misaligned with the day-night cycle of the sun, or with our own internal clocks, it can also cause us to gain weight and undermine our metabolic health. In the modern affluent world, with its time-consuming responsibilities, high rates of sleep disorders, abundant artificial light at night, stimulating media, rotating shift work, and travel between time zones, the sleep and circadian rhythm systems receive cues that often send our eating behavior and weight in the wrong direction.

Finally, the threat response system, rooted in many parts of the brain but coordinated in large part by the amygdala, is a largely nonconscious collection of processes that help us manage challenging situations by altering our behavior and physiology. This system takes its cues from a variety of sensory inputs that convey information about potential threats, such as vision and hearing, but also from abstract concepts, such as the possibility of being laid off. In the modern world, most of the threats it deals with are psychological, but in many ways it still reacts as if we’re fighting off wild beasts. It’s not clear that we have more to be stressed about today than our ancestors did, yet it’s possible that we cope with our stress less effectively than we did in times past. In certain people, psychological stress sharply increases cortisol levels, and this may reduce the sensitivity of the lipostat to leptin, in turn increasing food intake and the accumulation of body fat. This is particularly true when we feel like we have little control over a stressful situation. Stress also shifts our eating preferences toward comfort food—which is usually calorie dense and highly palatable—because it helps dampen the activity of the stress response system and makes us feel better.

The brain is a complex organ, and it contains many circuits that influence eating behavior besides those I described in this book—and I’m certain that more remain to be discovered. Yet those we’ve explored in these chapters play key roles in determining our calorie intake and adiposity. These circuits respond to the cues they receive, and the modern affluent food environment provides a perfect storm of cues that promote overeating. I believe these circuits caused Yutala, the fattest man on the island of Kitava, to develop a body shape unfamiliar to his traditionally living community, after adopting a modern lifestyle in the small Papua New Guinea city of Alotau. I also believe these are the circuits that pushed our own culture from leanness to obesity, as our way of life departed radically from that of our ancestors.

TAKING THE NEXT STEPS

Where will the research go from here? As I hope I’ve conveyed, neuroscience is a vibrant field that is rapidly expanding our understanding of the human brain. Yet as the most complex object in the known universe, the brain has many secrets left to divulge. Within the sphere of eating behavior and obesity research, many scientists are continuing to explore the nonconscious circuits that drive us to overeat and gain fat. Much of this work revolves around identifying the input cues these circuits respond to, understanding the molecular details of the circuits themselves, and determining how each of these can be manipulated to prevent and reverse obesity.

One excellent example is the scientific quest to understand why certain types of weight-loss surgery are so effective at treating obesity. While many people with obesity struggle to lose fat using diet and lifestyle approaches, usually without much success, those who undergo specific surgical procedures, such as Roux-en-Y gastric bypass or sleeve gastrectomy, experience substantial and durable fat loss with much less effort.

Initially, surgeons assumed the obvious: These procedures work because they reduce stomach volume and digestive efficiency, so the recipient simply can’t fit much food into his stomach and also ends up flushing calories down the toilet instead of absorbing them. But when researchers took a closer look, the story turned out to be much more interesting. People who undergo these procedures still have the digestive capacity to eat enough calories to sustain obesity, and they absorb calories nearly as well as they did before surgery. Yet they simply lose their desire to eat rich foods. While before surgery, they may have craved large portions of hamburgers, fries, and soda, after surgery they gravitate toward smaller portions of lighter fare, such as vegetables and fruit. And as they lose weight, they never show signs of the starvation response that usually accompanies substantial weight loss, implying that weight-loss surgery turns down the set point of the lipostat. Just in case you think the change in food preferences might be a conscious choice on their part, let me add a critical detail: Researchers see similar effects on appetite and food preferences in obese rats and mice who have undergone the same procedures.

Clearly, something about weight loss surgery alters how the brain nonconsciously regulates food intake and adiposity. And these effects result from changes in the jumble of connections and chemicals that the brain uses to do its job. So which changes, exactly, are responsible for the remarkable efficacy of certain weight-loss surgeries? No one knows for certain yet, but researchers like Randy Seeley of the University of Michigan and Hans-Rudi Berthoud of the Pennington Biomedical Research Center are closing in on the answer. And once we have that answer, it may empower us to manipulate the same circuits using tools that are less risky and less permanent than surgery, such as drugs, or, ideally, diet and lifestyle. We have much to learn about the brain, and within that unknown realm lies powerful tools for preventing and treating obesity.

At the same time, we already have quite a bit of information about how the brain works and which cues influence the nonconscious processes that drive overeating and fat gain. In the next chapter, I’ll take what we’ve covered so far and see what practical value we can derive from it. The objective is to give the nonconscious brain the right cues, thereby aligning it with our conscious goals of leanness and health.