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Every year, about 1.3 million women in the United States reach menopause, most around 51 years of age. More than 80% experience symptoms such as hot flashes, trouble sleeping, mood changes, or shifts in memory and focus (Carlson & Vadakekut, 2026). For many women, hot flashes alone can last for years (Avis et al., 2015).
October 18 is recognized as World Menopause Day. It was established in 2009 by the International Menopause Society (IMS) to raise awareness about menopause and the support people can find to improve their health and wellbeing during it (International Menopause Society, 2026). There is still a lot we don’t know about menopause and perimenopause, partly because women were left out of much of medical research for many years (NIH Office of Research on Women’s Health, 2024). Efforts are underway to close that gap, and animal research can help answer some of the open questions. Rhesus macaques are especially useful here because they have monthly menstrual cycles and go through menopause much like women do (Downs & Urbanski, 2006).
Here in Oregon, collaborating divisions at the Oregon National Primate Research Center have spent decades studying menopause from a number of angles, including hormones, brain health, mood, and immune aging.
Hormone Therapy May Hold Promise
Memory
One of the most popular discussions around supportive treatments for perimenopausal and postmenopausal individuals is hormone therapy. There have been a few studies at ONPRC examining such treatments, a major one being from 2016 by Dr. Steven Kohama, in which 28 aged female rhesus monkeys (18 to 25 years old) were trained on cognitive tests. Twenty-two of them then experienced surgical menopause via ovariectomy (removal of the ovaries) and were given either a placebo, estradiol, or estradiol and progesterone over the course of 12 months, while the other six kept their ovaries for comparison. The results indicated possible cognitive benefits when treated with estradiol, with the strongest effects seen in animals given estradiol alone. Adding progesterone appeared to weaken those benefits (Kohama et al., 2016).
Stress
Research by Dr. Cynthia Bethea found that ovariectomized female macaques experienced a boost in serotonin function and protection for serotonin neurons when supplemented with estrogen and progesterone. The researchers suggest this could translate to women, with hormone therapy aiding in mood and stress resilience (Bethea et al., 2002; Bethea et al., 2009).
Immune Aging
A 2011 collaborative study involving Drs. Ilhem Messaoudi and Steven Kohama examined the effect of surgical menopause on immune function and vaccine response in female rhesus monkeys. Ovariectomized females showed signs of accelerated immune aging and a weaker vaccine response, but in older animals, hormone therapy partly restored their T-cell response to vaccines.
The researchers think losing ovarian hormones may play a part in weaker immune responses after menopause, and that hormone therapy could help offset those effects (Engelmann et al., 2011).
Diet
In 2020, a collaborative study between researchers at Oregon Health & Science University and the ONPRC looked into whether the timing of hormone therapy, particularly estradiol, was important to its effectiveness in aged female rhesus monkeys who had undergone surgical menopause (removal of the ovaries and uterus) while on a Western-style (high fat, high sugar) diet. They found that animals who started estradiol right after surgery did better on a spatial maze than placebo animals (Zimmerman et al., 2020).
An earlier 2019 study of the same group of animals looked into possible metabolic benefits of estradiol. Starting estradiol right away slowed the rise in insulin resistance and helped maintain bone density, while starting it two years later did not significantly change body composition or blood sugar measures (Purnell et al., 2019).
Looking Ahead
Alzheimer’s Disease
Almost two-thirds of Americans living with Alzheimer’s disease are women (Alzheimer’s Association, 2026), and researchers are working to understand whether the loss of estrogen at menopause plays a role. In a 2024 study at the ONPRC, aged ovariectomized macaques that received estradiol had significantly fewer amyloid plaques, a hallmark of Alzheimer’s disease, in the amygdala, a part of the brain involved in emotion and memory. This was true whether the animals ate a standard diet or a Western-style diet (Appleman et al., 2024). It’s still unclear whether hormone therapy could help prevent Alzheimer’s in women, but these findings point to an important area for future study.
The Menopause Transition
Another open area is the menopause transition itself. Most of the studies mentioned above used surgical menopause, where the ovaries are removed and hormone levels drop suddenly. Natural menopause is different, since hormone levels shift gradually over several years during perimenopause, so results from surgical models may not fully reflect what most women experience. Like women, female rhesus macaques go through a natural perimenopause, with hormone changes that begin before their cycles stop entirely around 25 years of age (Downs & Urbanski, 2006), which gives researchers a way to study this gradual change more closely. ONPRC researchers have already begun this work, finding that in older perimenopausal macaques, estrogen levels in the hippocampus, a memory center of the brain, were similar to those of younger animals, even though levels in the blood were lower (Urbanski et al., 2017).
Menopause is a part of life for millions of people, yet for a long time it received far less attention in research than it deserved. This World Menopause Day, we hope you’ll take some time to learn more about it, whether that means looking through the International Menopause Society’s resources or simply starting a conversation with the women in your life. The research happening here in Oregon is one part of a larger effort to make sure people going through menopause have better answers and more options for care.
Learn More
Want to learn more about menopause and the research behind it? Visit the International Menopause Society’s World Menopause Day page for resources. Each year they choose a theme, and for this year they’ve chosen: chronic pain in midlife.
The Alzheimer’s Association’s Facts and Figures report covers how Alzheimer’s affects women. You can also find the studies reviewed here in the References section below, most of which are free to read online.
If you found this post helpful, share it with a friend or loved one to help spread awareness this World Menopause Day.
References
- Alzheimer’s Association. (2026). 2026 Alzheimer’s disease facts and figures. https://www.alz.org/alzheimers-dementia/facts-figures
- Appleman, M. L., Thomas, J. L., Weiss, A. R., Nilaver, B. I., Cervera-Juanes, R., Kohama, S. G., & Urbanski, H. F. (2024). Effect of hormone replacement therapy on amyloid beta (Aβ) plaque density in the rhesus macaque amygdala. Frontiers in Aging Neuroscience, 15, 1326747. https://doi.org/10.3389/fnagi.2023.1326747
- Avis, N. E., Crawford, S. L., Greendale, G., Bromberger, J. T., Everson-Rose, S. A., Gold, E. B., Hess, R., Joffe, H., Kravitz, H. M., Tepper, P. G., Thurston, R. C., & Study of Women’s Health Across the Nation. (2015). Duration of menopausal vasomotor symptoms over the menopause transition. JAMA Internal Medicine, 175(4), 531–539. https://doi.org/10.1001/jamainternmed.2014.8063
- Bethea, C. L., Lu, N. Z., Gundlah, C., & Streicher, J. M. (2002). Diverse actions of ovarian steroids in the serotonin neural system. Frontiers in Neuroendocrinology, 23(1), 41–100. https://doi.org/10.1006/frne.2001.0225
- Bethea, C. L., Reddy, A. P., Tokuyama, Y., Henderson, J. A., & Lima, F. B. (2009). Protective actions of ovarian hormones in the serotonin system of macaques. Frontiers in Neuroendocrinology, 30(2), 212–238. https://doi.org/10.1016/j.yfrne.2009.04.003
- Carlson, K., & Vadakekut, E. S. (2026). Menopause. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK507826/
- Downs, J. L., & Urbanski, H. F. (2006). Neuroendocrine changes in the aging reproductive axis of female rhesus macaques (Macaca mulatta). Biology of Reproduction, 75(4), 539–546. https://doi.org/10.1095/biolreprod.106.051839
- Engelmann, F., Barron, A., Urbanski, H., Neuringer, M., Kohama, S. G., Park, B., & Messaoudi, I. (2011). Accelerated immune senescence and reduced response to vaccination in ovariectomized female rhesus macaques. Age, 33(3), 275–289. https://doi.org/10.1007/s11357-010-9178-0
- International Menopause Society. (2026). World Menopause Day 2026. https://www.imsociety.org/education/world-menopause-day-2026/
- Kohama, S. G., Renner, L., Landauer, N., Weiss, A. R., Urbanski, H. F., Park, B., Voytko, M. L., & Neuringer, M. (2016). Effect of ovarian hormone therapy on cognition in the aged female rhesus macaque. The Journal of Neuroscience, 36(40), 10416–10424. https://doi.org/10.1523/JNEUROSCI.0909-16.2016
- NIH Office of Research on Women’s Health. (2024). History of women’s participation in clinical research. https://orwh.od.nih.gov/toolkit/recruitment/history
- Purnell, J. Q., Urbanski, H. F., Kievit, P., Roberts, C. T., & Bethea, C. L. (2019). Estradiol replacement timing and obesogenic diet effects on body composition and metabolism in postmenopausal macaques. Endocrinology, 160(4), 899–914. https://doi.org/10.1210/en.2018-00884
- Urbanski, H. F., Sorwell, K. G., Prokai, L., & Kohama, S. G. (2017). Effect of short-term DHEA supplementation on serum and hippocampal estrogen concentrations in perimenopausal female rhesus macaques. Neurobiology of Aging, 55, 172–174. https://doi.org/10.1016/j.neurobiolaging.2017.03.026
- Zimmerman, B., Kundu, P., Liu, Z., Urbanski, H. F., Kroenke, C. D., Kohama, S. G., Bethea, C. L., & Raber, J. (2020). Longitudinal effects of immediate and delayed estradiol on cognitive performance in a spatial maze and hippocampal volume in menopausal macaques under an obesogenic diet. Frontiers in Neurology, 11, 539. https://doi.org/10.3389/fneur.2020.00539