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Why Do Some Brains Cave To Cravings

Accepting food cravings and keeping them in check is an important component of weight management. For normal-weight people, an empty tummy triggers the brain to tell the body to get some food. When the tummy gets filled, the tummy gets happy; and that's the end of that for about five hours or more. Some obese people, however, find themselves eating again only an hour or so after a meal. Now scientists think they know why.

Supplemental results from the Comprehensive Assessment of the Long-term Effects of Restricting Intake of Energy (CALERIE) trial provide new insights into food cravings, specific types of foods craved, and their role in weight control.

"Cravings are really normal; almost everyone has them," says corresponding author Susan Roberts, PhD, director of the USDA HNRCA's Energy Metabolism Laboratory. At the start of the study, 91 percent of the participants reported having food cravings, which are defined as an intense desire to eat a specific food. "Most people feel guilty about having food cravings," says Roberts, "but the results of this study indicate that they are so normal that nobody needs to feel they are unusual in this respect."

In addition, the results indicate that cravings don't go away during dieting. "In fact, 94 percent of the study participants reported cravings after six months of dieting. However,"Roberts says, "participants who lost a greater percentage of body weight gave in to their cravings less frequently. Allowing yourself to have the foods you crave, but doing so less frequently may be one of the most important keys to successful weight control," she adds.

Brain-imaging MRI scans of healthy subjects, some of whom were obese, reveal that when levels of glucose, or blood sugar, drop, the brain region that regulates impulses can't control the desire for high-calorie sweets and snacks. This craving for high-calorie food is particularly acute among the obese.

The study, conducted by researchers at Yale University, helps explain why some obese individuals with daily, wildly fluctuating blood sugar levels have difficulty controlling their appetite for junk food and desserts.

Their study was published online Sept. 19 in The Journal of Clinical Investigation.

The rise and fall of cravings

Blood sugar levels fluctuate naturally during the day. Levels are lowest in the morning before breakfast, triggering hunger. Levels peak an hour or so after a meal and then soon return to a base level for several hours after that, as the stomach is satiated.

The Yale researchers carefully controlled blood sugar levels of 14 subjects intravenously and then monitored their reactions to images of food as the subjects underwent a MRI brain scan. For nonobese subjects, a state of low blood sugar, called hypoglycemia, triggered a craving for high-calorie foods such as ice cream. That craving went away as the blood sugar level rose to its normal baseline, called euglycemia.

The obese subjects, however, experienced far greater cravings than the nonobese did while hypoglycemic. Most surprising, the researchers said, that craving did not diminish as blood sugar levels rose to baseline euglycemia. The obese apparently have lost their glucose-linked restraining mechanism, the researchers said, leading them to crave snacks just a few hours after a meal even when blood sugar levels are normal.

Making matters worse for the obese, the researchers said, is the inescapable vision of high-calorie foods, from advertisements on television and billboards to the proliferation of fast-food outlets themselves. This makes overeating nearly inevitable, the researchers added.

Junk food begets junk food

The study results imply that one strategy for weight management for the obese would be to eat smaller, more frequent meals. This might help keep blood sugar levels within a narrower range and reduce cravings. In general, foods such as high-fiber vegetables and whole grains are digested more slowly than low-fiber, processed foods and only gently raise blood sugar levels.

Conversely, junk food — which is usually sugar-laden or heavily processed and low in fiber — causes blood sugar levels to spike and then drop dramatically. This, in turn, increases the craving for even more junk food.


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