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We blame our food cravings on hunger, but stress may be the real culprit.
Another study has shown that low blood sugar makes you more susceptible to temptation. The idea itself isn't new. People who have low blood sugar are more likely to take risks, spend impulsively, cheat on a test, crave a cigarette, and even flirt with someone they have no business flirting with.
There have been several scientific theories about why. One common explanation: the brain needs high levels of energy (i.e., glucose) to exert self-control. When blood sugar runs low, the brain can't (or won't) spend the energy to override impulses.
Another theory has it roots in evolutionary psychology: when your blood sugar drops, your brain shifts into survival mode. It instinctively pushes you to consume more energy (increasing food cravings). And just in case that low blood sugar reflects a scarce food supply, it will push you to take more risks in general. After all, in time of a food crisis, you may need to fight for your next meal, or try a strange berry you've never tasted.
But the new study, conducted by neuroscientists at Yale University and the University of Southern California, provides another important clue. It's not just an energy problem, or a survival instinct to fill up on more food. The researchers manipulated participants blood sugar levels intravenously, then measured what was going on in people's brains and bodies as they tempted them with high calorie foods. They found that the brain's reward system, which produces cravings, was more responsive to the food when blood sugar was dropping. The prefrontal cortex-which is needed to override those cravings-was underactivated.
The surprise effect was what seemed to be driving the effect. When blood sugar dropped, participants' stress hormones increased. And it was the stress hormones that had biggest direct effect on the brain--not the observed changes in insulin, grehlin, or other hormones associated with hunger.
This is consistent with another body of research showing that stress is the biggest saboteur of self-control. I've written about this idea before, including studies showing that stress hormones play a major role is alcohol addiction and dieting failures.
Many of the students in my "Science of Willpower" course love the low-blood-sugar studies because it gives them an excuse to snack all day long, or indulge in a high-calorie "willpower boost." But this study is a good reminder that you don't just need to feed your brain. When it comes to willpower, we most of all need to find healthy ways of managing stress.
Study Reference:
Page KA et al. Circulating glucose levels modulate neural control of desire for high-calorie foods in humans. J Clin Invest. 2011;121(10):4161-4169. doi:10.1172/JCI57873.
Kelly McGonigal is a psychologist at Stanford University. Her latest book is The Willpower Instinct: How Self-Control Works, Why It Matters, and What You Can Do to Get More of It.
