Throughout much of medical history, the stereotypical “average patient” has looked a lot like the average male. Medical research, from clinical trials for new treatments to studies on disease mechanisms and diagnostics, has traditionally focused on studying male subjects as the default human model. This male-centric approach reflected societal gender norms and power dynamics and stood largely unchallenged as part of scientific tradition. Without studying female subjects, however, a critical gap emerged in our understanding of sex differences—how biological factors such as hormones, genetics, and physiology can influence disease progression and treatment outcomes. While it’s simpler to just assume that male and female bodies work in the same way, reality is more complex.
This long-standing inattention to sex differences in health research led to them being often overlooked or dismissed as inconsequential. A striking example is the study of estrogen, the primary female sex hormone produced by the ovaries. While estrogen defines reproductive characteristics in women, it is also critical to functions throughout the body, including cardiovascular health. Yet, for decades, estrogen’s broader impact on women’s health was underexplored. “The first major study on the effects of estrogen on coronary heart disease, conducted in the 1970s, included 8,341 men and not one woman,” said Carolyn Mazure, the Norma Weinberg Spungen and Joan Lebson Bildner Professor in Women’s Health Research at Yale School of Medicine.
Other sex hormones, as well as genetic differences in the X and Y sex chromosomes, also influence individual biology. The historical underrepresentation or exclusion of women in research has made it challenging to fully understand how these biological differences influence disease mechanisms and treatment responses. Biological sex, while not a comprehensive explanation on its own, provides an important framework for understanding divergences in health outcomes between the sexes.
Early Efforts to Change
In the early 1990s, the U.S. Government Accountability Office (GAO) published a report highlighting the stark underrepresentation of women subjects in clinical trials for recently approved drugs approved in the country. Of the fifty-three drugs reviewed, more than half had female participation rates that were at least ten percentage points lower than what would be representative based on the proportion of women typically affected by the condition being studied. Notably, in trials for cardiovascular drugs, only one in thirteen newly approved drugs included a representative number of female participants.
Such biases had clear consequences. In the 1990s, women with hypertension (high blood pressure) were often prescribed diuretics, which increase urine production to eliminate excess salt and fluid, whereas men with hypertension tended to be prescribed medications that relax blood vessels and reduce heart strain, such as angiotensin-converting enzyme (ACE) inhibitors and calcium channel blockers. At the time, this clinical practice was at odds with the research; according to clinical guidelines, sex did not affect hypertension treatment responsiveness. It seemed that clinicians had observed important sex differences in medication responses that had not yet been recognized by researchers, indicating a lag in the research. Almost three decades later, we now understand that these drugs cause distinct adverse effects depending on sex—only after women were included in hypertension research.
Policy Shifts: Integrating Sex
Throughout the 1980s, the need for inclusivity in medical research was increasingly recognized by advocacy groups, researchers, and policymakers. In 1993, this attention culminated in the American Congress passing federal legislation mandating that trials funded by the National Institutes of Health (NIH) include women and minorities in their cohorts. The change acknowledged that diverse representation would not only enhance the accuracy of treatments but also respond to the unique health needs of historically marginalized groups.
In 1998, Mazure founded Women’s Health Research at Yale (WHRY) to advance interdisciplinary research on the health of women and the interplay between sex and health. “Our goal is [to] integrate those findings into professional practice and clinical education and shape public policy to advance health,” Mazure said.
One of the first WHRY-funded projects investigated the link between BRCA1/2 gene mutations and increased risk breast cancer recurrence. The project was led by Bruce Haffty, who was a professor of therapeutic radiology at Yale School of Medicine and is now Chair of Radiation Oncology at Rutgers Cancer Institute of New Jersey. The study challenged the prevailing standard of care for breast cancer, which involved tumor removal and breast conservation. This procedure aimed to remove cancerous tissue while preserving as much of the breast as possible, following a generalized cancer treatment model. However, Haffty’s research showed that women with BRCA1/2 gene mutations faced a higher risk of cancer recurrence in the remaining tissue. “This helped women in the decision-making process regarding breast conservation if they were carriers of the [mutated] BRCA1/2 gene. The funding allowed us to test a larger cohort of women as the testing was quite expensive at the time,” Haffty said.
Sex Difference Research at Yale
In addition to initiating investigations on women’s health, WHRY has also funded many projects that examine sex differences between female and male biology that can inform clinical care. The center invites Yale faculty to submit research proposals on a yearly basis. Their current portfolio encompasses a wide range of research topics, reflecting their commitment to addressing gaps in women’s health across disciplines.
One example of a key project responding to an urgent health need comes from the lab of Akiko Iwasaki, Sterling Professor of Immunobiology at the Yale School of Medicine, which studies sex differences in immune response to pathogens like COVID-19. Based on emerging data that men with COVID-19 experienced more severe symptoms and higher mortality than women, Iwasaki’s team set out to determine if there was a sex difference in the immune response to SARS-CoV-2. “Distinct immune responses correlate with the distinct symptom profiles that females and males experience,” said Julio Silva, an MD/PhD student at the Yale School of Medicine. These findings provided clinical information that could be used at the start of the pandemic. The Iwasaki Lab is also investigating whether similar sex-specific immune signatures appear in other post-acute infectious syndromes, such as long COVID-19, Chronic Fatigue Syndrome, and long Lyme disease.
A better understanding of sex differences in immune response enables the development of more targeted treatments. “This means that we should tailor our diagnostic approaches and treatment strategies to account for the patient’s biological sex,” Iwasaki said.
Caroline Johnson, an associate professor of epidemiology at Yale School of Public Health, studies sex differences in colorectal cancer (CRC) metabolism. Her lab uses untargeted metabolomics, a large-scale analysis of the entire biochemical profile of tissue samples, including metabolites involved in various metabolic processes. They found that tumors from female patients with CRC have an enhanced asparagine synthesis pathway. Asparagine is one of the twenty standard amino acids universal to all biological organisms, and previous studies have shown that when asparagine is overproduced by tumors, it can be a sign that the local environment is under stress, supporting the tumor’s ability to survive and spread. “We have found that high levels of asparagine are linked to poorer prognosis for female patients only, and the regulators of asparagine production are sensitive to local stress and help tumor growth in the absence of other molecules,” Johnson said.
The asparagine synthesis pathway is already the target of some cancer treatments. Asparaginase, an enzyme that breaks down asparagine, is an approved first-line therapy for acute lymphoblastic leukemia. The drug is undergoing clinical trials for other cancers, including CRC, with the hypothesis that asparaginase can alleviate cellular stress by digesting asparagine, promoting conditions closer to normal tissue homeostasis. If proven effective, Johnson’s research could guide precision medicine approaches for CRC, using sex or asparagine production profiles as indicators to inform treatment decisions. Clearly, investing research effort and funding into studying sex differences can yield tangible benefits that have the potential to improve patient outcomes and advance medical practice.
Preclinical Research: Does Cell Sex Matter?
In 2016, the NIH published its policy Sex as a Biological Variable. The policy acknowledged that women now represented about half of all participants in NIH-supported clinical research. However, the NIH raised another issue: basic and preclinical biomedical research often relied on male animals and cells. They suggested that sex as a biological variable should be factored into research in vertebrate animal and eukaryotic studies.
It is well-understood that sex hormones can influence animal models in preclinical research in ways similar to humans. Less intuitively, researchers have also suggested that the sex of eukaryotic cells may interfere with preclinical trial results independently of hormonal exposure. For example, the Dolan Lab at the University of Chicago characterized 233 human lymphoblastoid cell lines—a model system for human genetic studies—and identified ten genes not located on the sex chromosomes that are expressed differently in male and female cells. These genes guide diverse cellular processes, including cell adhesion, programmed cell death, transcription regulation, and cell structure. These findings underscore that even at the most basic biological level, sex differences can influence cellular processes, which in turn affect the overall physiology of an organism.
Future Directions in Sex Difference Research
Over the past three decades, progress in sex-based research has advanced health outcomes not only for women but for all patients. “It is now evident that prior historical efforts using male health as a normative model is not an effective approach to the whole population,” Mazure said.
As researchers like Mazure—who is currently on leave from Yale to serve as the Chair of the White House Initiative on Women’s Health Research—continue to drive this field forward, we move closer to a future where healthcare truly serves everyone. Mazure emphasized that advancing research in women’s health strengthens healthcare systems and leads to better, more inclusive care for all populations. “When women’s health improves, the nation thrives,” Mazure said.