How recognizing sex differences is transforming biomedical research and creating better healthcare for everyone
Imagine this: a new medication shows remarkable promise in clinical trials, earning rapid approval and excitement from medical professionals. Yet within months, reports emerge of dangerous side effects occurring disproportionately in women—some life-threatening. This isn't hypothetical; it's exactly what happened with the sleeping pill zolpidem in 2013, when the FDA discovered women metabolized the drug so differently that they needed a halved dosage recommendation to stay safe 2 .
For decades, the male body was considered the default in biomedical research, from animal studies to clinical trials. The assumption was that beyond reproductive systems, male and female biology differed insignificantly. This scientific blind spot has affected everything from drug development to medical device design, with very real consequences for women's health.
The recognition of "sex as a biological variable" (SABV) represents a revolutionary correction to this historical oversight. This article explores how this paradigm shift is transforming biomedical research, the exciting discoveries already emerging, and the challenges that remain in ensuring healthcare truly serves everyone.
Sex as a biological variable refers to the integration of sex into research designs, analyses, and reporting across biological disciplines. It acknowledges that being male or female impacts health processes and outcomes from the cellular level to whole organisms 1 .
This approach begins with developing research questions and continues through study design, data collection, analysis, and reporting of findings. Proper consideration includes adequate representation of both sexes in experiments and disaggregating data by sex to allow for meaningful comparisons 1 .
While often used interchangeably in everyday language, science clearly distinguishes:
This distinction matters profoundly in medicine. While this article focuses on biological sex, researchers increasingly recognize that both sex and gender influence health outcomes.
A significant policy shift began when the National Institutes of Health (NIH) introduced its SABV mandate in 2016, requiring researchers to factor sex into vertebrate animal and human studies 1 . This policy emerged from the recognition that while women accounted for roughly half of clinical trial participants, basic and preclinical research still overwhelmingly focused on male animals and cells 1 .
Required inclusion of women and minorities in clinical research 2
Championed individualized healthcare approaches 6
Mandated consideration of sex in preclinical research designs 1
Required sex-specific data integration across medical device lifecycle
One of the most compelling demonstrations of sex differences comes from nanomedicine—the use of microscopic particles for drug delivery. Researchers discovered that supposedly "neutral" nanoparticles behave differently in male versus female biological environments.
In one pivotal study, scientists found that equal doses of nanoparticles don't undergo equal uptake and intracellular trafficking in males and females 2 . This discovery has profound implications for cancer treatments and other therapies using nanoparticle delivery systems, suggesting that sex-specific dosing might be necessary for optimal efficacy and safety.
Sex differences in physiology create distinct biological environments that affect how nanomedicines function. Variations in hormones, metabolic enzymes, immune function, and organ size all contribute to these differences 2 . For instance, the activity of cytochrome P450 enzymes, crucial for drug metabolism, differs between males and females, leading to variations in how quickly medications are processed and cleared from the body 2 .
The brain demonstrates some of the most fascinating sex differences. Contrary to popular belief, few brain regions are truly "sexually dimorphic" (having distinct forms), but sex differences in brain organization contribute to variations in behavior, cognitive strengths, and vulnerability to mental health disorders 8 .
After puberty, females are diagnosed with depression at nearly twice the rate of males, while males are more likely to develop Parkinson's disease and addiction 8 . These disparities aren't merely social—they reflect complex interactions between sex chromosomes, gonadal hormones, brain circuitry, and environmental factors.
Alzheimer's disease presents another striking example: more than twice as many females receive Alzheimer's diagnoses, and females show more rapid decline after menopause, suggesting protective effects of estrogen 8 . Females also show stronger associations between the APOE-ε4 genetic risk factor and cognitive decline 8 .
A crucial 2021 experiment examined whether sex affects nanomedicine efficacy and toxicity 2 . The research team designed a comprehensive approach:
Researchers created therapeutic nanoparticles loaded with active compounds
Established cell lines from both male and female donors
Administered identical nanoparticle doses to both male and female animal models
Used fluorescent tags and mass spectrometry to track nanoparticle behavior
The findings revealed significant sex-based variations that could dramatically impact treatment efficacy:
| Parameter | Findings in Males | Findings in Females | Clinical Implications |
|---|---|---|---|
| Cellular Uptake | Moderate uptake in target cells | Enhanced uptake in same cells | Females may require lower doses for same effect |
| Liver Clearance | Faster clearance | Slower clearance | Dosing frequency may need sex-specific adjustment |
| Tissue Distribution | Preference for muscle tissue | Preference for fatty tissue | Drug delivery efficiency varies by target tissue |
| Protein Corona | Different protein adsorption patterns | Distinct corona composition | Nanoparticles effectively become different entities |
Perhaps most strikingly, researchers observed that the "protein corona"—the layer of biomolecules that forms on nanoparticles in biological fluids—differed significantly between sexes, effectively creating sex-specific nanoparticles from identical starting materials 2 .
| Therapeutic Area | Key Sex Difference | Potential Impact |
|---|---|---|
| Cardiovascular Disease | Higher prevalence in men pre-menopause; higher in women post-menopause 6 | Sex-specific prevention strategies needed |
| Autoimmune Conditions | 70-90% of lupus and rheumatoid arthritis cases occur in females 6 | Female-focused research urgently required |
| Drug Metabolism | At least 40% of drugs show sex differences in pharmacokinetics 2 | Sex-specific dosing for many medications |
| Adverse Drug Reactions | Females experience nearly twice the rate of adverse drug reactions 6 | Improved safety through sex-aware prescribing |
These findings fundamentally challenge the "one-size-fits-all" approach to nanomedicine and suggest that sex-specific formulations might be necessary to optimize treatments for everyone.
Incorporating SABV into research requires specific tools and approaches. Here are key elements in the modern researcher's toolkit:
| Tool/Reagent | Function | Sex-Aware Application |
|---|---|---|
| Sex-Specified Cell Lines | Cells with documented chromosomal sex | Avoids false conclusions from using cells of only one sex 6 |
| Hormone Assay Kits | Measure estradiol, testosterone, progesterone levels | Accounts for hormonal influences on experimental outcomes 9 |
| Estrous Cycle Tracking | Determines stage of female reproductive cycle | Controls for female physiological variability 9 |
| Sex-Chromosome Specific Antibodies | Identifies proteins from X or Y chromosomes | Prevents false reports of Y chromosome proteins in female tissues 7 |
| Animal Models of Both Sexes | Includes male and female subjects in experiments | Enables detection of sex differences in basic research 1 |
Despite progress, significant challenges remain. Many researchers still don't consistently include both sexes in experimental designs. A survey of biomedical engineering research found that only about 4% of studies reported cell sex, and none included it as a variable of interest 6 .
This translational gap has real consequences. As UVA Health researcher Bradley Gelfand notes, "We know so much about the consequences of being male or female for many disease conditions. What we don't yet have a complete handle on is why sex plays such a crucial role in human health" 7 .
Current research tools often can't adequately distinguish sex-specific effects. Protein databases sometimes wrongly report the presence of Y chromosome proteins in tissues that don't have Y chromosomes, leading to flawed conclusions 7 .
Statistical approaches also need refinement. Many studies still don't design experiments with sufficient power to detect sex differences, or they fail to report sex-specific analyses even when both sexes are included in research 9 .
Perhaps the most persistent barrier is cultural. The perception that female animals introduce more variability due to estrous cycles continues to influence research design, despite evidence showing that data variability is generally comparable between males and females 9 .
As one meta-analysis demonstrated, male hormones also fluctuate significantly—testosterone levels can be five times higher in dominant versus subordinate male mice—yet this variability is rarely considered 9 .
The recognition of sex as a biological variable represents more than a policy requirement—it's a fundamental transformation in how we conduct science and practice medicine. From revealing why women experience adverse drug reactions nearly twice as often as men to explaining sex disparities in diseases from Alzheimer's to autoimmune conditions, this approach is filling critical gaps in our understanding of human health 6 .
The future of medicine is personalized and precise, and biological sex represents just one dimension of human diversity that must be considered. As researchers continue to embrace SABV, we move closer to healthcare that truly serves everyone, regardless of sex.
As biologist Lixing Sun reminds us, "Biological sex and gender are far more complex than we know. And many of our current understandings are wrong or will be proven wrong in the future. We should look to science to learn more" 5 .