MONDAY, June 28, 2004 (HealthDayNews) -- In thin people, levels of the hormone that makes you hungry -- ghrelin -- vary wildly throughout the day and peak at night. But overweight people maintain more constant levels of the hormone throughout the day and don't experience a nighttime spike.
That's one of the findings of a study appearing in the June 28-July 2 issue of the Proceedings of the National Academy of Sciences. The researchers also found differences in the levels of two other hormones that may play a role in obesity -- leptin and adiponectin -- when they compared lean and heavy people.
"Ghrelin is secreted by the stomach to tell the brain you have to eat," explained one of the study's authors, Dr. Julio Licinio, a professor of psychiatry and biobehavioral sciences and medicine/endocrinology at the David Geffen School of Medicine at the University of California, Los Angeles. "In lean men, the peak of secretion is at night when sleeping. In obese subjects, the peak wasn't there at all."
Obesity is a growing concern in America. According to the U.S. Centers for Disease Control and Prevention, 30 percent of adults are obese, which puts them at a higher risk of developing serious health problems such as high blood pressure, heart disease, stroke, diabetes, and even some types of cancer.
"The thing that's important to know from this study is that while obesity has been posed as a weakness, it appears there's going to be some reason why it is so hard not to eat and why [obese] people are always hungry -- a chemical reason," said Dr. George Fielding, a surgeon at the New York University program for surgical weight loss.
For this study, Licinio and his colleagues recruited five lean men and five obese men. Lean was defined as a body mass index of 20 to 25, and obese was a body mass index greater than 30. All of the men were Mexican-American.
The men gave blood samples every seven minutes for a 24-hour period. During that time, all of the men were given three full meals and an evening snack.
The researchers were looking to see what variations were present in three hormones believed to play a role in obesity -- ghrelin, leptin, and adiponectin.
Levels of all three of these hormones were different in the lean and obese study participants. Ghrelin levels were fairly constant throughout the day for the obese volunteers, while the lean volunteers saw a dramatic jump at night.
Licinio said it wasn't clear how this could affect obesity, but explained that it could be that obese people are hungry all day long because of the constant levels of ghrelin. Lean people, on the other hand, are at their hungriest in the middle of the night while they're sleeping and can't do anything about their hunger.
The researchers also found that levels of leptin were significantly higher in the obese volunteers, and spiked at night, while these levels remained steady for the lean volunteers.
Adiponectin levels were higher in lean people, and though this level varied somewhat throughout the day for both the lean and obese participants, there were no dramatic spikes in this level as there were for ghrelin and leptin.
Licinio said the researchers also learned that leptin is secreted in an orderly, organized fashion, while adiponectin levels appear to change randomly.
He noted that the researchers weren't clear if the differences in these hormone levels were a result of obesity or the cause of obesity, and said much more work needs to be done to clearly understand the effects of these hormones.
"This is an exciting avenue to pursue," said Licinio. "Here, we see a qualitative difference, and I think this potentially gives us an idea that changing someone's hormone profile could impact weight loss or the ability to keep weight off."
Added Fielding: "This study is fascinating and shows that obesity is a genuine physiological brain-mediated thing. It's good to show evidence that some of the causes of obesity are intrinsic in people."
If you're overweight or obese, visit the American Obesity Association and take this quiz to see how your weight may be affecting your health.