Let’s Not Sugar Coat It: Evidence Is Emerging that Fructose and Glucose Elicit Different Brain Responses
Researchers have found that in humans, ingestion of glucose, but not fructose, activates brain pathways and stimulates hormones that promote feelings of satiety, fullness, and reward. Consumption of the simple sugar fructose in the United States has risen significantly over the past few decades, concomitant with an increase in the prevalence of obesity. Several studies have provided evidence suggesting that fructose consumption does not have the same effects as glucose on circulating hormones that modulate hunger and satiety. While research using animal models has demonstrated that fructose affects regions of the brain (e.g., the hypothalamus) that help regulate feeding behavior, corresponding analyses in humans have been technologically challenging to perform. Scientists have now used a new method of determining the effects of fructose or glucose on specific regions of the human brain by using functional magnetic resonance imaging (fMRI) to quantify cerebral blood low—a proxy measure for brain activity. Twenty normal‑weight participants, after fasting, drank beverages containing either glucose or fructose; the resulting effects on the brain were analyzed, along with circulating hormones (from blood samples), and other changes. Glucose consumption led to reduced blood low in the hypothalamus and other regions of the brain associated with hunger, changes which were not seen with fructose consumption. Glucose ingestion led to a signfiicantly greater induction of circulating insulin and GLP‑1, two hormones that promote satiety, than did fructose consumption. Although both sugars induced functional connectivity—a statistical measurement of the communication between parts of the brain—between the hypothalamus and various other brain regions, only glucose consumption induced functional connectivity to the striatum, a reward center of the brain. Participants who drank the glucose beverage reported feeling full and satiated, but those who drank the fructose beverage did not.Together, these data support the idea that in people, glucose, but not fructose, promotes satiety, in part because of its effects on the activity of specfiic regions of the brain, which differ from those of fructose. This study was limited by several potential issues, including technical constraints, the small number of participants, and the complicating fact that the study design does not reflect real-world conditions (e.g., people typically consume sugars and other nutrients in combination, not individually). Although the role of fructose’s effects on the brain in the development of obesity was not directly explored, these results suggest that previous findings in animal models are relevant to humans, with respect to differences between fructose and glucose. Future research could shed light on how these effects may relate to food consumption and health.