Scientists at the Oregon National Primate Research Center at Oregon Health & Science University believe they have helped dispel the myth that late-night eating causes weight gain. The research is published in the current edition of the journal Obesity Research.

"We've all been told at one point in our lives that we should avoid eating meals late at night as it will lead to weight gain. However, 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. Of course this research does not suggest that snacking at night after eating your normal daily ration of calories is a good idea, " 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 contribution of menopause to weight gain in middle-aged women has not been well established, perhaps because many other life style changes generally occur during this period of life, such as changes in eating habits and exercise habits as children grow up and leave home. In this study monkeys did gain about 5% more weight after their ovaries were removed providing clear evidence that ovarian hormones contribute to weight balance in primates, as well as in lower animals. These results were presented at the 2003 Society for Neuroscience Meeting.

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.

"It was really interesting to see that the monkeys who ate most of their food at night were no more likely to gain weight than monkeys who rarely ate at night," said Elinor Sullivan, an OHSU graduate student conducting research along with Cameron at the Oregon National Primate Research Center. "This suggests that calories cause weight gain no matter when you eat them."

The ONPRC is a registered research institution, inspected regularly by the United States Department of Agriculture. It operates in compliance with the Animal Welfare Act and has an assurance of regulatory compliance on file with the National Institutes of Health. The ONPRC also participates in the voluntary accreditation program overseen by the Association for Assessment and Accreditation of Laboratory Animal Care International.