Compared to glucose, fructose doesn't do as well at telling our brains we're full.
PROBLEM: The question of to what degree fructose is responsible for rising rates of obesity is still up for (hotly contested) debate. While the monosaccharide appears naturally in fruit, it's also used as an added sweetener -- most contentiously in the form of high fructose corn syrup (HFCS) -- in processed foods. And in its ubiquity, it's previously been associated with an increased risk of heart disease and diabetes.
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METHODOLOGY: Lab animals fed fructose, past experiments have demonstrated, actively seek out and eat more food. Researchers at Yale recruited 20 average-sized adults to see if they'd react similarly. The participants were given drinks containing high concentrations of either given glucose or fructose, and sat for MRI sessions before and after consuming them.
RESULTS: The participants who drank the glucose showed reduced blood flow to areas of the brain associated with appetite, motivation, and reward processing -- the parts that signal hunger. They also tested for higher levels of the hormones that contribute to satiety.
In other words, the brain appeared to know when to shut down cravings for glucose once the body's requirement for the energy source had been met. Fructose, however, did not have this effect.
CONCLUSION: Fructose appears to affect the brain differently than does glucose, failing to signal fullness and thus potentially contributing to overeating.
IMPLICATIONS: It's not only because of HFCS that fructose is so high in our diets -- regular table sugar only contains slightly less fructose than HFCS does, in an equal ratio with glucose. To further complicate things, agave nectar, which is popular in health food circles, has one of the highest levels of fructose of any sweetener around. Because it's less sickly sweet, glucose on its own is rarely used as a sweetener. So anything with added sugar is going to contain fructose and will, according to these preliminary findings, cause us to eat more.
The full study, "Effects of Fructose vs Glucose on Regional Cerebral Blood Flow in Brain Regions Involved With Appetite and Reward Pathways," is published in the Journal of the American Medical Association.