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Diet willpower waning? It could be biological.

Findings from a study in the February Journal of Personality and Social Psychology (Vol. 9, No. 2) suggest that relatively small acts of self-control—passing up a cookie, stifling prejudice, controlling attention, even helping people—significantly reduce the body’s blood glucose level, leading to poor self-control on subsequent tasks.

Lead author Matthew Gailliot, a fourth-year social psychology graduate student at Florida State University, explains that glucose acts as brain fuel that supports complex psychological processes—in this case, the ability to override one’s urges or emotions. He suggests that a person’s capacity to mobilize glucose may help determine their success in pursuing their goals, adhering to their morals and sticking to a diet.

Researchers conducted a total of nine experiments with 329 undergraduates enrolled in an introductory psychology class to demonstrate the effects of glucose on a broad range of self-control behaviors. In one experiment, the researchers directed participants not to cringe while watching a two-minute video clip of animals in a slaughterhouse and not to laugh during a two-minute clip from the comedic “The Tonight Show with Jay Leno.” Participants then attempted to complete an unsolvable figure-tracing task.

The researchers found that—after controlling for glucose levels at the experiments’ start—participants with lower glucose levels after the first self-control task gave up much sooner on the figure-tracing task than those whose levels were higher. The researchers also found that administering sugary drinks to participants after their first self-control task counteracted the effects of the initial glucose depletion, allowing for better self-control performance on subsequent tasks.

For dieters, eating several candy bars to boost one’s self-control might sound too good to be true, and it is. A better solution, says Gailliot, lies in noshing on proteins or complex carbohydrates throughout the day to keep glucose levels stable, thus fueling willpower.

“The study adds yet more evidence to the importance of eating healthily and exercising,” Gailliot says.

—A. Cynkar