What makes a milkshake so irresistible? Is it the sweet flavor that our taste buds are after? Or the smooth and creamy texture? Or perhaps it is the copious blend of fat and sugar?
An intriguing new study suggests that what really draws people to such treats, and prompts them to eat much more than perhaps they know they should, is not the fat that they contain, but primarily the sugar.
The new research tracked brain activity in more than 100 high school students as they drank chocolate-flavored milkshakes that were identical in calories but either high in sugar and low in fat, or vice versa. While both kinds of shakes lit up pleasure centers in the brain, those that were high in sugar did so far more effectively, firing up a food-reward network that plays a role in compulsive eating.
To their surprise, the researchers found that sugar was so powerful a stimulus that it overshadowed fat, even when the two were combined in large amounts. High sugar shakes that were low in fat ramped up the reward circuitry just as strongly as the more decadent shakes that paired sugar and fat in large quantities, suggesting that fat was a runner-up to sugar, said Eric Stice, the lead author of the study, which was funded by the National Institutes of Health and published in The American Journal of Clinical Nutrition.
“We do a lot of work on the prevention of obesity, and what is really clear not only from this study but from the broader literature over all is that the more sugar you eat, the more you want to consume it,” said Dr. Stice, a senior research scientist at the Oregon Research Institute. “As far as the ability to engage brain reward regions and drive compulsive intake, sugar seems to be doing a much better job than fat.”
The new findings add to a growing number of brain studies that are providing a more complex understanding of what drives people to overeat in the first place.
Heavily processed foods loaded with fat and sugar activate and potentially alter the same reward regions in the brain that are hijacked by alcohol and drugs of abuse. Though the extent to which these foods can provoke addictive behavior remains controversial, the results may help explain why millions of people who diet and struggle to lose weight ultimately fail.
“The obesity epidemic and the problems with overeating don’t have too much to do with people overeating fruits and healthy foods. They have a lot to do with people overeating excess sugars and fats,” said Nicole Avena, a faculty member at the New York Obesity Research Center at Columbia University, who was not involved in the new study.
Dr. Avena said that people “can have all the willpower in the world. But if the brain reward system is being activated in a way that causes them to have a battle against their willpower, then it can be very difficult for them to control their intake.”
Public health officials often tell Americans to avoid fattening sodas, snacks and fast foods, but the relative roles of fat and sugar in influencing the brain and potentially behavior have remained uncertain.
Dr. Stice and his colleagues sought to learn more by recruiting 106 healthy teenagers — 47 male and 59 female — and asking them to sip different milkshakes as they lay in functional magnetic resonance imaging machines.
The milkshakes were all made with chocolate syrup and an ice cream base. But the fat content was manipulated in different conditions by using either half and half or 2 percent milk, and the sweetness was manipulated by varying the simple syrup content.
Low fat, low sugar milkshakes activated regions of the brain associated with taste and sensation, but they had no impact on reward regions.
Relatively high fat, low sugar milkshakes, however – with 9 grams of fat and 7 grams of sugar per 100 milliliters (less than half a cup) – did engage part of the reward circuitry. And a high sugar shake that had triple that amount of sugar but only a quarter of the fat had an even greater impact, lighting up such brain structures as the putamen, insula and rolandic operculum.
These brain regions, called the food-reward system, control our desire for food: the more active they are, the more we want to eat. The researchers found that increasing the fat content of a high sugar shake did not activate the reward region any further.
Dr. Stice said he was surprised because he expected fat would be a stronger stimulus than sugar. But he also noted that the human brain is hardwired to prefer sweet flavors.
“When we’re children, we prefer high sugar foods right away, but not high fat,” he said. “We develop preferences for fat, but we’re basically born with a preference for sugar.”
Dr. Stice said the bulk of brain research on fat and sugar was pointing to the idea that addressing societal problems with overeating should start with sugar.
“If you look at our American diet, most people are consuming considerably more sugar than fat,” he said. “We’ve really ramped up the sugar in our diets, but we’ve backed off on fat.”
In her own research, Dr. Avena has found that sugar and fat influence brain chemistry and behavior when consumed in large quantities. Animals given small amounts of each do not show many changes. But when they are given unlimited access to either, those that gorge on sugar in particular show changes in their opiate receptors — which help regulate pain, reward and euphoria — and when the sugar is suddenly taken away, they show signs of withdrawal.
Another researcher who has studied the impact of fat and sugar on the brain in people, Dr. David Ludwig, said it was not so much the “immediate hedonic response” to junk food that drives overconsumption, but its impact on the brain and the body over several hours.
Dr. Ludwig and his colleagues showed in brain imaging studies on adults that high sugar milkshakes stimulate reward regions but also cause spikes and drops in blood sugar levels. Four hours later, they showed, these blood sugar drops stimulate changes in brain activity that induce cravings — and those cravings are typically for foods that can quickly restore blood sugar levels, like desserts, starches and sweets.
“These foods themselves are not inherently addictive through their taste per se,” said Dr. Ludwig, the director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. “It’s the effects that they have on our metabolism. If our blood sugar is stable, we can walk by a bakery and be much less tempted than if our blood sugar is crashing. Your brain knows these foods are going to rescue low blood sugar. But then this sets up the next cycle.”