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.
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. [8 Reasons Our Waistlines Are Expanding]
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. [7 Foods Your Heart Will Hate]
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.
Christopher Wanjek is the author of the books "Bad Medicine" and "Food At Work." His column, Bad Medicine, appears regularly on LiveScience.