A study presented at the 2011 conference of the Society for the Study of Ingestive Behavior sheds new light on the age-old advice: "Don't go shopping when you're hungry."

Researchers injected participants with either ghrelin, a hormone that increases hunger, or a saline solution. Then they created their own little eBay in the laboratory, asking participants to bid on both edible treats and non-food items. While participants were contemplating their choices and calculating their bids, the researchers were observing their brain activity in an fMRI machine.

Grehlin had a specific effect: it increased what participants were willing to pay for food, but decreased what they were willing to pay for everything else. The brain's reward system was also more responsive to the food than other goodies. It was as if the hunger hormone biased the brain toward craving calories, and dampened the appeal of any other reward.

The neurobiological finding isn't too surprising; it makes perfect sense from an evolutionary point of view. If you're starving, you better not be distracted by the latest designer doodad on the way to the nearest food source. The only reward that matters now is one that will fuel your physical survival.

You may know this state well, if you've ever loaded up a shopping cart with high-price groceries under the influence of an underfed appetite. And if you've ever been on a diet, you know what it's like to be suddenly and surprisingly obsessed with food.

Previous research has shown that the level of ghrelin circulating in your body is influenced by factors other than when you last ate. For example, people who sleep less than six hours a night have chronically elevated ghrelin levels. A sleep-deprived brain is energy hungry, and the body will try to trick you into refueling with a surge of ghrelin. Psychological stress can also lead to a burst of ghrelin -- once again, nature's trick to get you to consume energy (to handle whatever triggered the stress).

So although this study directly manipulated levels of ghrelin in participants, it also tells us something about why being tired or stressed makes us especially susceptible to temptation. One more reason that self-care is as important as self-control when it comes to losing weight and creating health.

Kelly McGonigal, PhD, is a health psychologist at Stanford University and the author of The Willpower Instinct: How Self-Control Works, Why It Matters, and What You Can Do to Get More of It (Penguin, 2011).

Studies mentioned:

1. Press release from the Society for the Study of Ingestive Behavior (2011, July 15). Ghrelin increases willingness to pay for food. ScienceDaily. Retrieved July 17, 2011.
2. Taheri, S. et al. (2004). Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS Med, 1(3):e62.
3. Kristenssson, E. et al. (2006). Acute psychological stress raises plasma ghrelin in the rat. Regulatory Peptides, 134, 114-7.

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