Currently, roughly two out of three U.S. adults are overweight and one out of three is obese. Changes in lifestyle and dietary intake during the past quarter century are thought to be the main culprits, with the increase in fructose consumption of particular concern.

Fructose is the simple sugar found in fruit, but it is added to many foods as a "refined sugar" in the form of high-fructose corn syrup. By comparison, glucose, the primary energy source for the body, is usually produced through the breakdown of complex carbohydrates.

Fructose ingestion produces smaller increases in circulating satiety hormones than glucose ingestion. Further, administration of fructose directly into the brain provokes feeding in rodents, whereas glucose administered this way promotes satiety, or the feeling of being full. Preliminary studies in people have also shown that glucose reduces activity in the hypothalamus, an event that is associated with metabolic satiety, whereas fructose does not.

Using functional magnetic resonance imaging (fMRI), Kathleen Page at the Keck School of Medicine of the University of Southern California (USC) and her colleagues in the Department of Psychology at the University of Southern California extended this work. They examined brain responses and motivation to eat when research volunteers viewed images of food (like chocolate cake) after they drank a beverage containing either glucose or fructose.

The food cues produced activation in the nucleus accumbens, a part of the brain's "reward circuit," and increased the desire for food. Activation in the nucleus accumbens was greater after consuming the fructose drink compared to the glucose drink. The fructose drink also resulted in greater ratings of hunger and motivation to eat compared to the glucose drink.

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