09/06/2013 01:57 pm ET Updated Nov 06, 2013

The Impact of Sex in Space

It is part of human nature to explore. Just as this is woven into our DNA, so are the changes that our bodies experience while in space. It is critical to understand how the body adapts and how to minimize the risks such as radiation and microgravity which astronauts will be exposed to during long duration missions such as to Mars.

Men and women adapt to the challenging environment of space differently Many of these changes are similar to what happens to people as they age on Earth. From every system in our bodies including the cardiovascular or heart, the musculoskeletal or muscles and bones, the neurovestibular or the brain and nerves, the genitourinary or reproductive system and urinary tract, the immunological or the cells that fight infection and even the behavioral health or mental health system, there are differences.

The differences between men and women are defined as sex or gender differences. When we use the word "sex" we are considering the biological aspects of being male or female at the most basic chromosomal level. When we use the "gender", it's directed at the psychosocial level whether someone thinks they are male or female. It is exciting to study these differences in space because the findings present themselves rather quickly and are quite dramatic. By understanding these differences, we may even improve our health and aging processes on Earth.

Consider musculoskeletal adaptation in space. Both men and women lose muscle and bone. Generally, people lose more than 1% of bone each month in space compared to 1% per year on Earth. Now consider that women start off with less bone mass than men, these losses in space may have more significant impact on women when they return to the gravity environment of Earth and are at risk of fracturing their bones. Women on Earth are 4 times more likely compared to men to have osteoporosis or thin bones and that is even before they go into space. However, when a man fractures a bone, he is more likely to die in that year compared to a woman. Both sexes lose muscle mass in space too, but the loss of muscle strength may be greater for men compared to women.

Women respond to cardiovascular stress with increased heart rates, whereas men respond with increases in vascular or blood vessel resistance. Upon return to Earth, more women compared to men have orthostatic hypotension or a drop in blood pressure that may cause them to faint. This may be occurring because women have more estrogen then men. Estrogen, a sex hormone, causes blood vessels to expand or dilate thus blood pressure drops. In space, astronauts may experience Visual Impairment Intracranial Pressure Syndrome (VIIP) in which there is higher pressure in the brain resulting in problems with eyesight. Interestingly, only male astronauts are significantly impaired. The etiology has yet to be determined. It will be critical to understand why it happens and how to prevent or treat it. Perhaps, the role of sex hormones may also be important in this syndrome.

On Earth, women are 80% more likely to have autoimmune diseases compared to men. In space, there is bone marrow suppression in which blood cells that fight infections and even cancer cells are reduced which puts people at risk for developing infectious diseases and cancer. We don't know what the impact of sex will be on long term space travel and whether women may be more protected because their immune system mount robust responses from the start.

Women are at higher risk for motion sickness and imbalance problems on Earth compared to men. The nervous systems of both sexes adapts to the space environment and actually rewire. In the first few days of space flight, astronauts may have space sickness and when they return to Earth, they have problems with walking and balance. More studies need to be done to see if women are at higher risk for these problems. The brains of men and women are different in which women tend to use both sides and men may predominately use just one side. On Earth, women recover from strokes and brain injuries faster compared to men. What the impact of space does on the brain needs to be further examined.

Both sexes are at risk for radiation exposure including an impact on their sperm and ova or eggs which may result in chromosomal defects that can be passed down to their offspring. Women may be more susceptible to radiation exposure overall but their thyroid glands, lungs, stomachs and ovaries are especially sensitive. It may also be harder for women to void in space because of the challenges of using bathroom facilities, thus women may have more urinary tract infections.

Everyone experiences stress when they go into space. On Earth, women are twice as likely to have depression compared to men. Whether this translates clinically in space needs to be examined. The gender composition of the crew will be important as well as assessing how crew members live and work together. Crew selection is very important to the success of any mission especially a long duration mission to Mars.

There are interesting differences between men and women on Earth and in space. Understanding these differences will help promote better health and well being for both sexes on this planet and on Mars. Sex and gender-based medicine can serve as a foundation for personalized or 'precision' medicine which will help define the most appropriate countermeasures to keep people healthy on a trip to Mars and their successful return to Earth. As humans, both men and women, explore the cosmos, the defining element will not be the differences between the sexes, but the connection that unites all of us to the stars since we are all made from exquisite stardust from the beginning of time.