"Perhaps most importantly, this research pertains to the country's worsening obesity epidemic. Currently about 30 percent of Americans are considered obese. In addition, significant weight gain has been tied to various serious health conditions, including diabetes, stroke, heart disease, joint problems and various forms of cancer," explained Cameron, a scientist in the divisions of Reproductive Sciences and Neuroscience at the OHSU Oregon National Primate Research Center. "In women, it has been demonstrated that major weight increases often occur during menopause, the time in a woman's life in which cyclic ovarian function ends and the ovarian hormones estrogen and progesterone decline. The goal of this research was to determine whether, and to what extent, the decline in these hormones have an effect on body weight in an effort to better understand and proactively treat obesity."
To conduct this research, Cameron along with graduate student Elinor Sullivan and other colleagues studied 47 adult female monkeys. Nineteen of the monkeys had their ovaries surgically removed, a common procedure that results in a drop of estrogen and progesterone levels, much like menopause. While monkeys, like humans, go through menopause naturally during their lifetimes, surgical removal of the ovaries during an animal's healthy adult life allows scientists to study hormone effects in a controlled manner and in the absence of other environmental factors related to aging. The other 28 monkeys in the study were observed as a control group.
"What we witnessed was that the absence of these hormones resulted in a 67 percent jump in food intake and a 5 percent jump in weight in a matter of weeks," said Cameron. "We would expect weight gain to continue over time. Additionally, we noted an increase of the hormone leptin, which is produced by fat cells and has been shown to play a role in food intake."
A related finding by the researchers argues against the commonly held belief that food intake at night results in weight gain.
"Time and time again we've been told that eating late at night should be avoided because it will cause weight gain. However, there isn't a lot of research to back up this commonly held belief, which may in fact be somewhat of an urban myth," explained Cameron. "In conducting this study, we noted the times that animals ate. Some of the monkeys ate most of their food during the evening and nighttime hours. However, weight gain and the time of day that the animals were feeding had no correlation whatsoever."
Another issue highlighted by the study involved changes in metabolism. The scientists noted a relationship between the loss of ovaries and changes in metabolism, which may also be related to menopause. Cameron and her colleagues plan to continue research in this area to obtain a better understanding of metabolism changes throughout life related to menopause and other factors.
This research was funded through GlaxoSmithKline, Inc. and a cooperative center grant provided by the National Institute of Child Health and Human Development (NICHD).
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 (AAALAC).