"We've all been told at one point in our lives that we should avoid late-night snacks as they will lead to weight gain. In reality, however, this belief is not based in fact. We conducted a review of previous data on the topic and found no real evidence that this was true. In addition, our research in rhesus monkeys, which are considered an excellent model for studying primate (man and monkey) obesity issues, showed that eating at night is no more likely to promote weight gain than eating during the day," said Judy Cameron, Ph.D., a senior scientist in the Divisions of Reproductive Sciences and Neuroscience at the OHSU Oregon National Primate Research Center. Cameron also is a professor of psychiatry at the University of Pittsburgh.

To conduct this research, scientists studied 16 female rhesus monkeys that were placed on a high-fat diet similar in composition to the diet normally consumed by humans in the United States and other Western countries. During the study, all of the monkeys had their ovaries removed -- this simulates a menopause-like state in female monkeys similar to human female menopause. In lower animals both high fat diet and decreased ovarian function lead to weight gain.

The researchers then observed the monkeys for one year. In addition to studying their weight gain, researchers noted how much and when the animals ate, which varied dramatically among the animals observed. Specifically, the researchers found that the monkeys ate between 6 percent and 64 percent of their total calories at night. This is comparable to reports in humans who take in approximately 24 percent to 65 percent of total calories at night.

"We were not surprised to find that as a group, the monkeys in this study gained weight when they were placed on this very palatable diet," said Elinor Sullivan, an OHSU graduate student conducting research along with Cameron at the Oregon National Primate Research Center. "However, what did surprise us initially was the fact that there was no clear correlation between caloric intake and weight gain. In other words, the monkeys that ate more didn't necessarily gain more weight. In addition, while some monkeys preferred to eat during the day and others ate most of their calories at night, neither of these groups gained more weight than the other."

Initially, the research project was conducted to gather data about the role of menopause in weight gain. These results were presented at the 2003 Society for Neuroscience Meeting. The data in regard to weight gain and night eating were secondary, unexpected discoveries that occurred in the midst of gathering this information.

"Overall, this research shows that caloric intake does not specifically correlate to weight gain," added Cameron. "We're now trying to determine what the cause of weight gain is. Some of our preliminary data presented at the 2005 Society for Neuroscience Meeting shows that a person's activity level is a better predictor of weight gain and loss. In other words, for those wishing to lose weight, simply changing your diet may not be enough."

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