Junk food is tricking your brain: The depressing science behind your binging

Research reveals fats and sweets alter the brain's satiety-control mechanism, sending our appetites into hyperdrive...

FERRIS JABR, SCIENTIFIC AMERICAN

(Credit: AP)

This article was originally published by Scientific American.

Matthew Brien has struggled with overeating for the past 20 years. At age 24, he stood at 5′10′′ and weighed a trim 135 pounds. Today the licensed massage therapist tips the scales at 230 pounds and finds it particularly difficult to resist bread, pasta, soda, cookies and ice cream—especially those dense pints stuffed with almonds and chocolate chunks. He has tried various weight-loss programs that limit food portions, but he can never keep it up for long. “It’s almost subconscious,” he says. “Dinner is done? Okay, I am going to have dessert. Maybe someone else can have just two scoops of ice cream, but I am going to have the whole damn [container]. I can’t shut those feelings down.”

Eating for the sake of pleasure, rather than survival, is nothing new. But only in the past several years have researchers come to understand deeply how certain foods—particularly fats and sweets—actually change brain chemistry in a way that drives some people to overconsume.

Scientists have a relatively new name for such cravings: hedonic hunger, a powerful desire for food in the absence of any need for it; the yearning we experience when our stomach is full but our brain is still ravenous. And a growing number of experts now argue that hedonic hunger is one of the primary contributors to surging obesity rates in developed countries worldwide, particularly in the U.S., where scrumptious desserts and mouthwatering junk foods are cheap and plentiful.

“Shifting the focus to pleasure” is a new approach to understanding hunger and weight gain, says Michael Lowe, a clinical psychologist at Drexel University who coined the term “hedonic hunger” in 2007. “A lot of overeating, maybe all of the eating people do beyond their energy needs, is based on consuming some of our most palatable foods. And I think this approach has already had an influence on obesity treatment.” Determining whether an individual’s obesity arises primarily from emotional cravings as opposed to an innate flaw in the body’s ability to burn up calories, Lowe says, helps doctors choose the most appropriate medications and behavioral interventions for treatment.

Anatomy of appetite
Traditionally researchers concerned with hunger and weight regulation have focused on so-called metabolic or homeostatic hunger, which is driven by physiological necessity and is most commonly identified with the rumblings of an empty stomach. When we start dipping into our stores of energy in the course of 24 hours or when we drop below our typical body weight, a complex network of hormones and neural pathways in the brain ramps up our feelings of hunger. When we eat our fill or put on excess pounds, the same hormonal system and brain circuits tend to stifle our appetite.

By the 1980s scientists had worked out the major hormones and neural connections responsible for metabolic hunger. They discovered that it is largely regulated by the hypothalamus, a region of the brain that contains nerve cells that both trigger the production of and are exquisitely sensitive to a suite of disparate hormones.

As with so many biological mechanisms, these chemical signals form an interlocking web of checks and balances. Whenever people eat more calories than they immediately need, some of the excess is stored in fat cells found throughout the body. Once these cells begin to grow in size, they start churning out higher levels of a hormone called leptin, which travels through the blood to the brain, telling the hypothalamus to send out yet another flurry of hormones that reduce appetite and increase cellular activity to burn off the extra calories—bringing everything back into balance.

Similarly, whenever cells in the stomach and intestine detect the presence of food, they secrete various hormones, such as cholecystokinin and peptide YY, which work to suppress hunger either by journeying to the hypothalamus or by acting directly on the vagus nerve, a long, meandering bundle of nerve cells that link the brain, heart and gut. In contrast, ghrelin, a hormone released from the stomach when it is empty and blood glucose (sugar) levels are low, has the opposite effect on the hypothalamus, stimulating hunger.

By the late 1990s, however, brain-imaging studies and experiments with rodents began to reveal a second biological pathway—one that underlies the process of eating for pleasure. Many of the same hormones that operate in metabolic hunger appear to be involved in this second pathway, but the end result is activation of a completely different brain region, known as the reward circuit. This intricate web of neural ribbons has mostly been studied in the context of addictive drugs and, more recently, compulsive behaviors such as pathological gambling.

It turns out that extremely sweet or fatty foods captivate the brain’s reward circuit in much the same way that cocaine and gambling do. For much of our evolutionary past, such calorie-dense foods were rare treats that would have provided much needed sustenance, especially in dire times. Back then, gorging on sweets and fats whenever they were available was a matter of survival. In contemporary society—replete with inexpensive, high-calorie grub—this instinct works against us. “For most of our history the challenge for human beings was getting enough to eat to avoid starvation,” Lowe says, “but for many of us the modern world has replaced that with a very different challenge: avoiding eating more than we need so we don’t gain weight.”

Research has shown that the brain begins responding to fatty and sugary foods even before they enter our mouth. Merely seeing a desirable item excites the reward circuit. As soon as such a dish touches the tongue, taste buds send signals to various regions of the brain, which in turn responds by spewing the neurochemical dopamine. The result is an intense feeling of pleasure. Frequently overeating highly palatable foods saturates the brain with so much dopamine that it eventually adapts by desensitizing itself, reducing the number of cellular receptors that recognize and respond to the neurochemical. Consequently, the brains of overeaters demand a lot more sugar and fat to reach the same threshold of pleasure as they once experienced with smaller amounts of the foods. These people may, in fact, continue to overeat as a way of recapturing or even maintaining a sense of well-being.