Fructose, a sweetener found in many packaged desserts, has been found to cause obesity.
Yale University researchers found that fructose triggers regions of the brain that increase appetite, leading to overeating in some.
The study used brain imaging to measure activity after participants ate both glucose and fructose - common sweeteners, said Bloomberg.
Each sweetener had diverse effects on the brain.
Revealingly, only glucose had the ability to tell the brain it was no longer hungry, while fructose did not have that effect.
The brain scans showed that people who ate glucose had increased levels of hormones that play a role in feeling full, reported the Associated Press.
Glucose is brain food, with the body requiring it to feed cells.
Those who consumed fructose did not see much change in their hormone levels.
Fructose, sweeter than glucose, is often manufactured from sugar canes, beets and corn.
It has become ubiquitous in the modern Western diet, particularly through high-fructose corn syrup.
"This increased intake of added sugar containing fructose over the past several decades has coincided with the rise in obesity in the population, and there is strong evidence from animal studies that this increased intake of fructose is playing a role in this phenomenon," said Jonathan Purnell, of the Oregon Health and Science University in Portland, who co-authored an editorial about the study, said HealthDay.
Yet, fructose is also found in fruits, and honey but in much lower doses.
It was the first study to compare the brain's response to both glucose and fructose.
The findings were published in the Journal of the American Medical Association.