What Sugar Does to the Brain
Sugar has been considered a dietary enemy for a long time—and with good reason. Excessive consumption of sugar has been linked to a variety of adverse health outcomes, including weight gain, obesity, heart disease, type 2 diabetes, and even cognitive decline.
Understanding how sugar interacts with our brains and whether it serves as a friend or foe can help us manage our intake, sidestep the serious health consequences of eating too much of it, and win back our self-control. Here’s what the latest science says about sugar’s effect on the brain.
Ingesting some sugar is vital to our survival, because it’s the brain’s main source of fuel, note the authors of a review published in Frontiers in Bioscience. Sugars in the form of glucose are transported via astrocytes and epithelial cells of the blood-brain barrier, and the glucose is then absorbed by our brains’ neurons.
But sugar also triggers the mesocorticolimbic system, known as the “reward pathway,” in the brain. When we consume sugar, the prefrontal cortex, amygdala, ventral-tegmental area, and nucleus accumbens are stimulated, releasing dopamine. After sugar triggers this system, we’re left craving more, because these parts of the brain are responsible for behavior reinforcement, pleasure-seeking, and addiction, according to a study published in Neuroscience & Biobehavioral Reviews in 2019.
From an evolutionary standpoint, this system had its advantages, because it encouraged the consumption of foods that would allow us to store fat—a useful behavior during periods when food sources were scarce. For many modern humans, however, there seems to be no shortage of foods high in sugar and fat.
So what happens when we eat those sweet foods? The sugar acts on the brain’s reward pathway in ways similar to drugs like cocaine and heroin—when overstimulated, the pathway becomes primed to require that stimulus, noted the study authors.
According to another study using animal models, published in Scientific Reports in 2019, increased consumption of energy-dense foods prompts increased cravings for those foods, because of this reward pathway. Researchers examined the effects of intermittent sucrose access on a group of anesthetized mini-pigs, by monitoring their μ-opioid and dopamine D2/3 receptors with PET scan imaging over 12 days.
Findings indicated that, following that period of sucrose access, the voxel-wise binding potentials of both receptors had declined significantly in the striatum, nucleus accumbens, thalamus, amygdala, cingulate cortex, and prefrontal cortex, consistent with down-regulation of receptor densities. What does that mean? Researchers concluded that in their models, sucrose affected reward mechanisms in a manner similar to that of abusable drugs like opioids.
“Sucrose consumption is associated with obesity, and sucrose is increasingly considered an addictive substance,” the authors wrote.
New research is shedding light on how this process works. For example, a 2020 study published in Nature, found that in addition to triggering specialized taste buds, sugar molecules switch on an additional neurological pathway, which begins in the intestines—known as the gut-brain axis.
Investigators monitored the brain activity of rodents, which were given sugar, artificial sweeteners, or water. They found that the caudal nucleus of the solitary tract in the brain stem of the rats responded only to the sugar. This response begins in the lining of the intestine, where signals are then sent to the brain via the vagus nerve, which provides a direct communication line between the intestines and the brain.
“This gut-to-brain circuit favors one form of sugar: glucose and similar molecules,” the study authors wrote. “It ignores artificial sweeteners—perhaps explaining why these additives can’t seem to fully replicate sugar’s appeal. It also overlooks some other types of sugar, most notably fructose, which is found in fruit.”
They continued, “Uncovering this circuit helps explain how sugar directly impacts our brain to drive consumption,” he says. “It also exposes new potential targets and opportunities for strategies to help curtail our insatiable appetite for sugar.”
The harms of excessive sugar consumption
The researchers of the 2019 Scientific Reports study made the case for the link between excess sugar consumption and the obesity epidemic. They argued that the addictive nature of sugar can result in calorie overconsumption and, ultimately, obesity.
“As a hallmark of the metabolic syndrome, obesity is associated with type 2 diabetes, cardiovascular disease, respiratory problems, and risk of depression and possibly dementia,” the authors noted.