Kathleen Page and colleagues at the University of Southern California, studied the effects of glucose and fructose on the hypothalamus—the appetite control center, which responds to hormones such as Leptin that tell the brain you are full (metaphorically think of leptin as the brake pedal). When study participants consumed a drink containing only glucose, blood flow and activity in the hypothalamus decreased and they reported feeling full. When the same participants were fed a fructose drink, the hypothalamus remained active and they did not report feeling full. The brain still thought the body was hungry.
(Gameau, Damon. The Sugar Book. P. 121. NY:Flatiron Books, 2015)