SBN Public Positions

Inclusion of Both Sexes in Biomedical Research

NIH leadership stated an intention to develop and implement policies requiring applicants to consider sex as a biological variable in the design and analysis of NIH-funded research involving animals and cells.   On behalf of SBN, Rae Silver and the SBN Public Education Committee submitted this response to the NIH Request for Information on this topic

Comment 1: Whether consideration of sex as a biological variable is an issue affecting the reproducibility, rigor, and/or generalizability of research findings.
There is no question: sex is a biological variable that can affect reproducibility, rigor, and/or generalizability of research findings because indeed, males and females are different. 

Differences between males and females may be expressed in different ways that will contribute to this variability; these include:
  • Qualitative Differences (e.g., during a sexual encounter females exhibit lordosis while males exhibit mounting).
  • Quantitative Differences in the magnitude of the response (e.g., female rats show more activity in response to cocaine than do male rats).
  • Distribution of incidence of a behavior (e.g., more female than males rats prefer cocaine to tasty pellets).
  • Different Mechanisms mediating the same outcome (e.g., different signaling pathways mediating response to CRF-R1).
  • Combinations of the above mechanisms are also possible

It is important that investigators not only include both males and females, but also that they be aware of the various ways in which variability can be introduced into a data set.

Comment 2 Areas of science (e.g., cancer, neuroscience) or phases of research (e.g., basic, translational) conducted with animals that have the greatest opportunity or need for considering sex.

It can not be predicted whether there are any areas of science in which sex can not be excluded as a variable that can influence results.  In neuroscience/neuroendocrinology it has become increasingly evident in the last decade that sex can influence the most basic mechanisms in the brain, and that experiential factors such as stress can interact with sex.  If we had to prioritize where the greatest opportunity and need for considering sex, it would be in respect to neuroscience, development, and translational/clinical research.  We recognize that this is a point on which the stakeholders in the scientific community are unlikely to completely agree.

Heart disease is the number one killer of women.  One significant concern is that a heart attack presents so differently in females and often goes unrecognized because the picture we all carry around in our heads is the one that is characteristic for males.  Not only do we need to understand why heart attacks present so differently in males and females, and whether the same treatments strategies that work in one sex work in the other, but considerable work has to be done to educate the public and the medical professionals that care for us.  It is not only a mistake but a tragedy that research on female health issues has for the most part been limited to female-specific body parts.  When we do study both sexes, findings more often than not indicate that the processes underlying disease are different and that different treatment strategies are called for when treating men and women.

In some instances it is important to understand the sources of variability: For example, in females stage of the cycle may be an important parameter. In other work, reproductive experience may be an important parameter (for example in parental behavior). In males, testosterone levels change as a function of time of day, and recent physical activity. The International Olympic Committee has proposed to use testosterone levels to determine who can compete as a man vs. woman. Testosterone levels fluctuate as a function of time of day and previous exercise exertion. The FDA has recommended a different dose of sleep medication be given to men and women. The research is being used. It needs to be based on evidence, and on knowledge of mechanisms!

Epigenetics studies suggest that sex-relevant experiences (for instance, differential parental care, etc.) can affect methylation patterns and hence expression of genes differently in males and females, even for genes where there are no sex differences in variants distributed in the population. For instance, both male and female rodents have identical glucocorticoid receptor (GR) genes but their expression in the hippocampus is affected by early parental stimulation; and males receive more of this stimulation than do females. The implication is that there is no difference in genetics, but there is a difference in epigenetic effects.  

Comment 3. Areas of science or phases of research conducted with cells and/or tissues that have the greatest opportunity or need for considering sex as a biological variable.

Medical breakthroughs will be based on understanding how one sex is protected and the other susceptible. We have a chance of such a breakthrough in diseases such as MS where epidemiological evidence points to substantial sex differences in incidence. How pain affects men and women and the biological mechanisms that can be harnessed to control pain are likely different for the two sexes.

Stem cells also are sexually dimorphic, both in growth and in efficacy of treatment when transplanted. The future use of stem cells as therapeutics treatments depends on a thorough understanding of how biological sex functionally affects stem cells.

Comment 4 Main impediments (e.g. scientific, technical, and other) to considering sex as a biological variable in research. 

The impediments are many. They include items listed on BOX 6-1 “Barriers to Sex-Based Research in:  Diseases of the Nervous System” in the report by the Institute of Medicine (2010) “Sex Differences and Implications for Translational Neuroscience Research” provides a thoughtful and fairly comprehensive summary of impediments. Needless to say, if the experiments are to be properly powered to examine both sexes, it is most likely that the time and the associated costs are one of the most significant impediments. These barriers (time and money) apply to every selection process, and are not a reasonable argument for ignoring sex differences.

Comment 5 Ways in which NIH can facilitate the consideration of sex as a biological variable in NIH-supported research. 

Education on why sex differences are important in all realms of basic, translational, and clinical research would be a good start.  This is something Society for Behavioral Neuroendocrinology and Organization for the Study of Sex Differences could assist with. 

NIH could mandate that investigators use males/females/both in their grants/projects. The NIH could expand current funding for studies specifically looking at sex differences. 

The NIH could require that investigators include in their published work results for both males and females, analyzed for the presence or absence of sex differences.  Study sections would use data produced by sex as evidence of compliance.

Comment 6 Any additional comments you would like to offer to NIH about the development of policies for considering sex as a biological variable in research involving animals, tissues, or cells.

It is unlikely that there will be consensus in the scientific research community on one way to prioritize which areas of research should focus on male/female differences. The decision that is likely to get support is a mixed strategy where some areas of research are targeted by NIH for analysis of both sexes and some are result of investigator initiated hypotheses.