Why You Should Never Eat Potato Chips With Diet Coke, According to Brain Science

The damage to the body inflicted by the Western diet, based heavily on processed food, extends to the mind, too, research at Yale and elsewhere shows

Credit: Aron Ehrlich and Ofir Hovav
Tzach Yoked
Tzach Yoked
Tzach Yoked
Tzach Yoked

Everyone is familiar with the experiment carried out near the turn of the 19th century by Ivan Pavlov, the Russian physiologist. It’s become synonymous with conditioning, or an involuntary response to a stimulus. Pavlov fed dogs and measured their salivation. He repeated this time after time, and observed that the sounds that preceded the food distribution began themselves to cause the dogs to salivate. That happened even when the bell he rang wasn’t followed by food.

Nearly a hundred years later, the Canadian scientist Harvey Weingarten conducted a similar experiment. He used rats instead of dogs, but he also sounded a tone while feeding them. The results were similar: the rats associated the sound of the bell with food. But then Weingarten expanded the experiment: He gave the rats a meal when they weren’t hungry. The rats saw the food but ignored it completely. Until he rang the bell, that is. Now, Weingarten discovered, the rats, which had only a short time earlier shown no interest in dining, ate up the food.

“The experiment demonstrates the extent to which cues in our environment influence what we eat and how much,” says Dana Small, a brain researcher and psychologist who heads the Modern Diet and Physiology Research lab at Yale University’s School of Medicine. According to Prof. Small, the results of the experiment attest to the unhealthy disparity that can exist between a sense of satiety on the one hand, and an uncontrollable instinct to eat even without being truly hungry.

Small has turned her career, she says, “into a quest to understand how the modern food environment causes obesity and affects eating habits” in the Western world. She constantly warns about the dangers that lurk in processed foods, and is determined to understand what it is about these foods that leads to obesity. The human brain, it turns out, can be tricked by aberrations in the form of dinosaur-shaped chicken nuggets, energy drinks containing artificial sweeteners, yogurts enhanced with weird flavors or breakfast cereals in shapes and colors that don’t exist in nature.

A Portland, Oregon, supermarket. Kicking the processed-food habit, says Avena, can be accompanied by symptoms similar to those familiar from breaking the nicotine habit.Credit: Lyza Danger Gardner / lyzadanger

You’re a psychologist by training – what led to your interest in processed food?

Small: “My mother suffered from epilepsy, and on many occasions lost consciousness. That was the starting point. Her illness brought about my interest in consciousness and how the brain creates conscious experiences. I decided to focus on the experience of pleasure – which led me to chocolate.”

I’m not sure I understand. How are consciousness and awareness connected to processed food?

“I wanted to know how the sensation of chocolate produces pleasure, and how that was coded in the brain. So I did a few studies on the subject of flavor and on the enjoyment of food, until, at a certain point I realized that the real driver of brain response came from signals from the stomach rather than sensory signals from the mouth. The metabolic system broadcasts the signals, and the brain encodes this metabolic information as reward separately from the conscious experience of pleasure. As a psychologist, I had no intention of dealing with the stomach or the digestive tract, but eventually I grasped that the answers to my questions lay precisely there. I wanted to know why our food environment triggers overeating, to understand what causes eating that leads to excessive weight.”

Brain researcher Dana Small. Credit: Frank Poole

What did you discover?

“That we humans evolved the sense of sweetness, and have a positive experience of sweet things because ‘sweet’ is accompanied by energy.

“From the evolutionary point of view, our brain learned to reward us for consuming sweet items – and not only us, it’s the same with other animals. The reason is that sweet foods are rich in calories, which are a condition of survival and are not so easily obtained in nature.”

Still, what was true across millennia has in the meantime lost its relevance. “Over the years, our food environment underwent various transformations,” Small notes. “The first revolution occurred with the discovery of fire and with the cooking of meat, the second stemmed from the agricultural revolution and the third happened with the invention of processed food. I find the dialogue we are having about obesity treatment misguided. Our response as a society to obesity is to remove [parts of] the stomach rather than change our food environment. We took food, broke it down, reconstructed it and added artificial sweeteners – and the result is a discrepancy between the sensation of flavor and the nutritional composition. What we don’t understand is that this mismatch exacts a steep price.”

Small gives another example. “In nature, either you have meat that contains fat and has no carbohydrates, or fruits and vegetables that contain carbohydrates but little fat. From the dawn of history, the brain learned how to accommodate sugar fat separately from each other, until processed food arrived.”

If carbohydrates and fat are so harmful in combination, why do they both exist in breast milk?

“It’s not harmful, it’s just really rewarding. Breast milk is apparently one of the only foods in nature that has substantial amounts of both fat and sugar. When you first learn how to eat, it’s logical that you’ll want to send the brain strong, rewarding signals that will encourage eating and facilitate breastfeeding. One of the reasons that processed foods are so popular is the high reward level to the brain. With their double signal, these foods exercise far more influence on the brain. The signals heighten our desire to consume processed food, even if we’re not hungry.”

Many doctors recommend the consumption of fat and carbohydrates together, because it helps slow down the body’s absorption of sugar.

“The problem is that the moment we consume fat and carbohydrates together, the stomach sends a double signal to the brain – of carbohydrates and of fat – and the brain releases more dopamine, which ‘stamps’ the reward value of the food into our brain circuits. Experiments we did in our lab at Yale, and in collaboration with colleagues at the Max Planck Institute for Metabolism Research, showed that people will be ready to pay more for foods with fat and carbohydrates compared to foods with the same level of sweetness and the same amount of calories but containing only carbohydrates or only fat.”

Not every brain responds to stimuli the same way; cultural experience also enters into the equation. Whereas children in the West may show stimulation in a brain scan after being shown a chocolate bar, similar tests done with children in Indonesia have shown high levels of stimulation when they are shown a photo of insects deep-fried in oil. The brain of a child in the West who has never eaten fried insects won’t show a similar response. According to Small, the intensity of the response stems from eating habits, genes, accompanying emotional experiences and from the usual combinations of ingredients in processed food.

Small refutes the idea that it is pleasure that drives overeating. It’s not the flavor that stimulates the brain, she maintains, but the digestive process itself, which breaks down glucose and converts it into energy. “As soon as we consume sugar, the cells start to break it down and send positive signals from the liver region. Those signals are received in the brain, which in turn secretes dopamine.”

Small describes a study conducted in 2008 by Ivan de Araujo, her colleague at Yale Medical School. “One group was comprised of regular mice, but the other included mice that had been genetically altered so that they could not taste sweet. Then he placed two sippers in front of the mice, one containing sweet water, the other regular water. Even though the [second group of] mice couldn’t sense the difference in taste, after time, they began to drink only the sweet water. Their preference for sugar was just as strong as the animals that could taste, and they released similar amounts of dopamine. The conclusion is that it is not sweet taste that drives behavior and dopamine release.”

We took food, broke it down, reconstructed it and added artificial sweeteners – and the result is a discrepancy between the sensation of flavor and the nutritional composition. This mismatch exacts a steep price.

Dana Small

De Araujo also conducted the reverse experiment, in order to ground the hypothesis. This time he injected the mice with a chemical substance that neutralized the cells’ ability to break down sugar into energy, while allowing the taste buds to function normally. Even though the mice could clearly distinguish the taste of the sweet water, their brains remained indifferent. “It’s amazing,” Small says. “It teaches us that the essence of sugar reward is energy and not sweetness.”

Small decided to continue investigating in this direction. She undertook to compare the cerebral response to two drinks bearing an identical level of sweetness but different energy levels. She sweetened two drinks with an artificial sweetener, and added calories to one of them. The results confirmed the hypothesis, says Small: “The brain responds more to the drink with the energy.”

Seemingly, the more energy that is produced, the more satisfied the brain will be. But it’s not so simple, as Small learned subsequently. “Our working assumption was that the more calories the subject consumes, the more intense the reward system in the brain will be, and the more dopamine will be be secreted.” Putting this hypothesis to the test, Small engineered five different drinks that all tasted similar, but had different energy levels. As in the previous experiment, she made use of maltodextrin to add calories. The participants were asked to consume five flavored drinks over a period of a couple of weeks, each with a different caloric quantity – 0, 37.5, 75, 112.5 and 150. Later their brains were scanned while tasting each of the drinks. The results were surprising.

“In light of our previous experiments, I was certain that we would see the greatest response to the taste of the drink that had the most calories, but I was wrong. The strongest responses were observed when the subjects tasted the drinks that had 75 calories.”

It took Small and her team no less than two years to solve the mystery. Finally, she explains, they concluded that, “when subjects drank beverages where sweetness and calories did not match, the energy was not being metabolized and the brain did not learn to respond.”

These findings by Small and her colleagues led to a conclusion that made waves in the United States: Consuming dietetic drinks together with carbohydrates might be hazardous to one’s health. “When the sensory experience doesn’t match the energy, metabolism of that energy is altered,” says Small.

If you consume a sugared drink, the body knows it needs to break down the glucose and convert it into energy. If you have a drink with no sugar but with artificial sweeteners, the body understands that there is no energy it needs to break down. But the moment you mix the two – have a dietetic drink and at the same time consume carbohydrates – you are sending signals that the brain doesn’t know how to deal with. In the end, the confusion harms and disrupts the entire reward system in the brain. If you feel like having a diet cola from time to time, that’s fine; but do yourself a favor: wait an hour before eating potato chips.

My son is capable of finishing a large bag of chips or an entire chocolate bar by himself. I’m not. Is that because our cerebral response is different, or do I just have more self-control?

“Self-control is important. When you are trying to maintain a healthy way of life, your brain interprets the signals that arrive from food differently. On the other hand, the obesity epidemic doesn’t stem from the fact that we lack willpower and self-control; it comes from the profound changes in our menu and from the amounts of processed food around us.”

Can a brain scan reveal who has a greater weakness for processed food?

“Yes. For example, we know that the smell of hot bread in the oven will generate a cerebral response in a hungry person, but not in a person who’s full. When we examined that in the lab, we discovered that in people with excess weight, there is a far smaller difference in the cerebral response to food when they are hungry versus when they are full. We also found that the strength of response in reward regions of the brain to food cues in the sated state predicted weight gain one year later.”

Radical change

Dana Small, 49, grew up on Vancouver Island, in British Columbia. She was a vegetarian most of her life, but when she moved to the United States for her post-doctoral studies, her weight rose by 15 kilos (33 pounds), and she developed early signs of diabetes. To address this sharp switch, she changed her eating habits radically. She also began running 100 kilometers (62 miles) a week. She even ran the New York Marathon once. Ultimately she shed 30 kilos, and has maintained her weight for more than a decade.

Her partner is Pieter van Dokkum, a professor of astronomy and physics at Yale, who studies galaxies and builds telescopes. Her first husband, David Sasso, is the son of Dennis and Sandy Sasso, who made history by becoming the first rabbinical couple ever. (Both are Reconstructionist rabbis.) The fact that her son is their grandson remains a source of pride to her.

The first thing Small noticed when she moved to the United States was the great difference in the food, she recalls. “Ever since I can remember, I loved cucumbers. I was absolutely astounded to discover how tasteless the cucumbers in the United States are. I didn’t understand how the flavor of a vegetable could differ so much from one country to another.”

She wasn’t alone: Her son also found it hard to believe. “When he was 7, I asked him one time what his favorite food was, and he replied ‘Montreal.’ I didn’t know what he meant, so I asked him what exactly in Montreal, and he said ‘everything, all the grocery stores and restaurants.’ A year later we visited Montreal and I remember making him a simple supper, and he said, ‘You know, mom, if we lived in Montreal, I might even eat the broccoli.’”

After all, broccoli and cucumbers are, in the end, no more than supporting players on the American culinary stage. And apropos American eating habits, one of the most sensitive issues for Small is the local breakfast. From pancakes with chocolate chips and maple syrup, to muffins and Belgian waffles. In every other country those items are considered desserts that one might pamper oneself with once every few weeks, but in the United States, you’ll find them on the standard breakfast menu of every hotel, restaurant or café.

“Quite a few studies have been conducted on the connection between breakfast and the behavior of young children. Soyoung Park, from the German Institute of Nutrition, found that the first meal of the day affects how altruistic or aggressive the children were later in the day,” Small notes. “Children who ate a breakfast rich in proteins were less aggressive than children who ate sugary cereals. That’s especially problematic, because kids eat those cereals first thing in the morning, when the stomach is empty, so they consume large amounts that generate a stronger stimulus in the brain – and an increased secretion of dopamine. That makes it more likely that they will want to eat those cereals again.”

Aggressiveness is one result, but poor nutrition can also cause depression. In a 2015 article referred to by Small, Harvard scientists compared traditional eating habits identified with Japanese and Middle Eastern cuisine, with Western eating habits, which are characterized by a large quantity of processed food. The article, which examined an array of studies, found that a Western diet increases the likelihood of depression by about 30 percent.

In the past two years, a series of additional studies have established a connection between depression and a high consumption of processed food. French scientists tracked the data of 27,000 people across five years, and found that an addition of 10 percent in the daily consumption of ultra-processed food (such as pizza, cheeseburgers, chicken nuggets) increases the likelihood of depression by around 20 percent. Extensive studies carried out last year in the United States and Spain also presented unequivocal findings.

Small and her colleagues frequently quote a 2019 study by Dr. Kevin Hall, a senior investigator at the National Institute of Diabetes in the United States. Hall divided 20 people into two groups. One group ate a menu based on 80 percent processed food, while the second was put on a diet completely free of processed foods. Two weeks later, those who ate processed food had gained on nearly a kilogram average (2.2 pounds), whereas those in the second group had lost almost a kilo.

“Earlier studies that drew a link between processed food and various diseases were unable to neutralize completely variables such as socioeconomic status or residential environment,” Hall told Haaretz. “Our study is the first to be based on random controls.”

Despite the large difference between the two menus, Hall emphasizes that he made sure to maintain an almost identical nutritional composition. “People tend to say that processed food contains larger quantities of sugar and salt, so we created menus for the two groups with exactly the same quantity of those components.”

Then what actually caused the differences?

“My theory is that we’re talking about two main factors. First, ultra-processed food is far more concentrated in terms of the proportion of calories per gram. Second, ultra-processed food is almost always far softer and easy to digest. It’s much easier to stuff your mouth with chips than to chew a vegetable salad. From this point of view, when you eat fast you reduce the amount of time the stomach has to signal the brain that you have consumed enough calories and that you’re full. By the time that signal is relayed to the brain, it’s frequently already too late.”