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craving for sweets

Eating "junk food" may confuse the satiety control mechanisms in the brain and stimulate our appetite excessively / Paris Jaber

Obesity. Photo: shutterstock
Obesity. Photo: shutterstock

The article is published with the approval of Scientific American Israel and the Ort Israel network

Matthew Bryan has struggled with overeating for 20 years. When he was 24 years old, he was 1.77 meters tall and weighed 61 kilograms. Today, the weight of the licensed masseur reaches 104 kilograms and he finds it difficult to resist the temptation to drink carbonated drinks and eat bread, pasta, cookies and ice cream, especially when it is sold in compressed half-liter containers loaded with almonds and chocolate chips. He tried various weight loss programs that restricted food portions, but he was never able to stick with them for long. "It's almost ingrained in my subconscious," he says. "Have I finished lunch? Good, I need dessert. Someone else might settle for two scoops of ice cream, but I'm going to eliminate the whole container. I cannot restrain this need.”

Eating for pleasure and not just for survival is a known thing. But only in recent years have researchers fully understood how certain foods, especially fats and sugars, actually change the chemical mechanisms in the brain in a way that pushes certain people to overconsumption.

Scientists have given this craving phenomenon a new name: Hedonistic appetite, strong desire for food without any need; The longing we experience when our bellies are full but our minds are still hungry. A growing number of experts now claim that a hedonistic appetite is one of the main causes of the sharp increase in the rate of obesity in developed countries all over the world, especially in the USA, where Arabic desserts for the palate and junk food, gluttony in Hebrew, are cheap and plentiful.

"Moving the focus to [the study of] pleasure" is a new approach to understanding the feeling of hunger and weight gain, he says Michael Law, a clinical psychologist at Drexel University who coined the term hedonic appetite in 2007. "Most of the overeating, and perhaps everything that people eat above their energy needs, is based on the most Arab foods for the palate. And I think this approach is already having an effect in the treatment of obesity." According to Lau, the diagnosis that determines whether a person's obesity is mainly due to emotional cravings, and not from an innate defect in the body's ability to use energy, helps doctors choose the most appropriate medications and behavioral guidance for his treatment.

Anatomy of appetite

Traditionally, the reference of researchers to the regulation of hunger and weight has focused on what is called metabolic hunger or homeostatic. Such hunger arises from bodily needs and is usually identified as gurgling on an empty stomach. When we begin to utilize a portion of our energy reserves during the day or when we begin to drop below our typical body weight, a complex network of hormones and neural processes in the brain begins to operate that increases our feeling of hunger. When we overeat or gain extra pounds, the same hormonal system and brain pathways tend to suppress our appetite.

In the 80s, researchers understood how the central hormones and neural connections responsible for metabolic hunger work. They discovered that most of it is regulated by the hypothalamus, an area of ​​the brain that contains nerve cells that stimulate the production of various hormones and are also very sensitive to them.

As with many biological mechanisms, these chemical signals create an integrated system of checks and balances. When people consume more calories in their food than are needed for their immediate needs, some of the excess is stored in fat cells found throughout the body. When the fat cells begin to grow in size, they begin to rapidly produce large amounts of the so-called hormone Leptin. Leptin travels with the bloodstream to the brain and signals the hypothalamus to flush the body with appetite-reducing hormones that also increase the cellular activity that utilizes the unnecessary calories, thus restoring the balance to its original state.

Similarly, when cells in the stomach and intestine sense the presence of food, they secrete different hormones, such as Cholecystokinin וPeptide YY, which act to suppress hunger by migrating to the hypothalamus or by acting directly on the vagus nerve: a long and winding bundle of nerve cells that connect the brain, heart and intestines. In contrast, to the hormone Ghrelin, which is released from the stomach when it is empty and when the blood glucose level is low, has the opposite effect on the hypothalamus: to stimulate hunger.

In the 90s, following studies using brain imaging and experiments done on rodents, a second biological pathway, which underlies the process of eating for pleasure, began to be discovered. Many of the hormones participating in the metabolic hunger mechanism are also involved in this pathway, but the end result is the activation of a completely different area of ​​the brain: the reward center. This branched network of neural tracts has been studied primarily in the context of addictive drugs and, more recently, in the context of compulsive behavior, such as pathological gambling.

It turns out that very sweet or very fatty foods engage the reward center in the brain in exactly the same way that cocaine and gambling do. For most of our evolutionary past, such high-calorie foods were rare treats, providing much-needed nutrients, especially in hard times. So, when the opportunity arose to gorge on sweets or fats, it was a matter of survival. In today's society, when we have an abundance of cheap, high-calorie food, this instinct works against us. "For most of our history, humans had to forage for food in order not to die of starvation," says Lau, "but in the modern world many of us have to face a completely different challenge: to avoid overeating in order not to gain weight."

Studies have shown that the brain begins to react to sweet and fatty foods even before they enter the mouth. The sight of a desirable object is enough to stimulate the reward center. As soon as such food touches the tongue, the taste buds send signals to different areas of the brain, which in response secrete the neurotransmitter dopamine. The result is a feeling of intense pleasure. Often, overeating an evening meal saturates the brain with such a large amount of dopamine that it eventually adapts to it by lowering the level of sensitivity, that is, by reducing the number of cellular receptors that recognize and respond to the neurotransmitter. The brains of people who binge eat therefore require much larger amounts of sugar and fat to reach the same pleasure threshold that they previously reached with smaller amounts of food. These people, in fact, continue to overeat as a way to regain or maintain their sense of well-being.

New evidence shows that some of the hunger hormones that normally act on the hypothalamus also affect the reward center. In a series of studies conducted in 2007-2011, researchers at Gothenburg University in Sweden demonstrated that the release of ghrelin (the hunger hormone) in the stomach directly increases the release of dopamine in the reward center of the brain. The researchers also found that drugs that prevent the binding of ghrelin to nerve cells reduce overeating in very obese people.

Under normal conditions, leptin and insulin (which become common in the body with the consumption of excess calories) suppress the release of dopamine and reduce the feeling of pleasure as the meal continues. But new research in rodents shows that the brain stops responding to these hormones as the fat tissue in the body increases. And so, when you continue to eat, the brain is flooded with dopamine even as the pleasure threshold continues to rise.

restraining the passions

A certain surgery that overweight people go through to lose weight highlights the importance of the hormone ghrelin in weight control and provides some biological insights into the question of why many of us eat much more than our physical needs. Bariatric surgery, also known as gastric bypass surgery, is the last resort for people suffering from extreme obesity. The surgery dramatically reduces the size of the stomach, either by removing tissue or by tightly tightening the stomach with a sleeve that prevents it from being able to contain more than small portions.

Within a month after such surgery, patients are usually less hungry in general and are no longer drawn to foods high in sugar and fat. This is apparently due to the change in the amount of hormones that their shrunken stomach can produce. Recent studies in brain scanning reveal that this reduced desire reflects changes in the neural pathways: after the surgery, there was a marked decrease in the response of the reward center in the brain to images and names of tempting foods, such as chocolate cake, and the sensitivity to smaller amounts of dopamine is renewed.

"The idea is that by changing the anatomy of the stomach and intestines, the levels of hormones that these organs secrete and that eventually reach the brain are also changed," she says Kimberly Steele, a surgeon at the Johns Hopkins University School of Medicine. Some studies have reported that after surgery the levels of ghrelin, the hunger-stimulating hormone, decreased, and the levels of peptide YY, the hunger suppressant, increased. And as recent experiments have shown, these hormones do not only work in the hypothalamus but also in the reward center. "In the long term, we will probably be able to imitate the effect of gastric bypass surgery using drugs," he says Bernd Schultz From the eSwiss Center for Medicine and Surgery in St. Gallen, Switzerland. "This is a huge dream."

Meanwhile, some clinics are using new discoveries about hedonic appetite to help people like Brian. Yi-Ho Yu, one of Brian's doctors at Greenwich Hospital in Connecticut, suggests at least two distinct forms of obesity, which sometimes overlap: metabolic and hedonic. Since Hugh believes Brian is mainly struggling with hedonistic obesity, he recently decided to treat him with the drug Victoza, known to reduce pleasure-driven eating. On the other hand, drugs that usually target the hypothalamus will work better if the main problem is a defect in the body's ability to maintain a constant weight.

On the other hand, Lau from Drexel University focuses on a new approach to behavioral change. "The traditional idea is that we can teach overweight people to improve their self-control," says Lau. "The new idea is that the foods themselves are the essence of the problem." In some people, appetizing foods trigger such a strong response in the reward center of their brain, changing their biology so dramatically, that they rarely, if ever, have enough willpower to resist eating them when they are within reach. Instead, says Lau, "the dining environment must be reengineered." And in practical terms, first of all, do not bring any fatty or very sweet food into the home and avoid as much as possible places where such foods are offered.

Elizabeth O'Donnell, a 53-year-old store owner from Wallingford, Pennsylvania, applies these lessons. She learned to change the food environment at home and on the street, after participating in one of Lau's weight loss studies. She says she is helpless in the face of sweets and pastries, so she has pledged to keep them out of her house and to avoid restaurants that offer all-you-can-eat desserts, which in the past lured her into consuming "an excess of 3,000 to 4,000 calories." On her last visit to Disney World, for example, she avoided the buffet restaurants common in the park in favor of small food stands where she settled for a salad. This is exactly the simple change that can make a big difference in the fight to maintain a healthy weight.


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