FOR IMMEDIATE RELEASE
Contact: Donna Krupa
Phone: 703.527.7357
Cell: 703.967.2751
djkrupa1@aol.com
Gender Issues Related to
Spaceflight: A NASA Perspective
November Edition of the
Journal of Applied Physiology Offers Review
November 25, 2001 -- Bethesda, Md.— Twenty-two percent of the
active astronaut corps are women (35 of 158). The average female astronaut
is 42 years old (vs. 43 years for men) and weighs 60.7 kg (vs. 81.2 kg for
men). Insufficient data exist in most of the discipline areas at the present
time to draw valid conclusions about gender-specific differences in
astronauts or to determine their impact on the health of male and female
astronauts.
The inability to draw statistical comparisons based on gender differences
is linked to the small number of female astronauts, resulting in an
insufficient study sample. In addition, individual differences in
physiological responses within genders are usually as large as, or larger
than, differences between genders, so individual characteristics usually
outweigh gender differences per se.
Despite these obstacles, a minireview in the November edition of the
Journal of Applied Physiology, a publication of the American
Physiological Society (APS), offers a summary of gender-specific
physiological changes and health issues in astronauts for future
consideration and in light of the expectation that the number of female
astronauts will grow. The summary is derived from a special task-force
report developed by Deborah L. Harm, Janice V. Meck, Michael R. Powell,
Lakshmi Putcha, Clarence P. Sams, Suzanne M. Schneider, Linda C.
Shackelford, and Scott M. Smith, of the Human Adaptation and Countermeasures
Office, NASA Johnson Space Center, Houston, Tx.; Richard T. Jennings,
Department of Obstetrics and Gynecology, University of Texas Medical Branch,
Galveston, Tx., and Peggy A. Whitson of the Astronaut Office, NASA Johnson
Space Center, Houston, Tx.
The review is part of the APS’s fall focus on gender differences in
physiology. It provides an overview of known and potential gender
differences in physiological responses to spaceflight and covers
cardiovascular and exercise physiology, barophysiology and decompression
sickness, renal stone risk, immunology, neurovestibular and sensorimotor
function, nutrition, pharmacotherapeutics, and reproduction.
Highlights of the article appear below.
PHARMACOTHERAPEUTICS
Gender-related differences are known to exist with respect to clinical
efficacy and adverse effects of drug treatment. There is a general consensus
among clinical pharmacologists that pharmacokinetics and pharmacodynamics in
women are different from those in men. Gender differences in
gastrointestinal physiology and hepatic metabolism may contribute to
differences in drug dynamics. In addition, hormonal changes during the
menstrual cycle, renal blood flow, and body composition also play roles in
gender-specific drug disposition.
Gastrointestinal Physiology and Hepatic Metabolism
Women may have adverse effects on their drug absorption, biovariability,
metabolism, and elimination due to gender differences in gastrointestinal
physiology and hepatic metabolizing enzyme systems. They also have altered
bile composition, slower intestinal transit time, and higher gastric pH than
men. During the first few days of spaceflight, there is a high incidence of
space motion sickness (SMS) accompanied by decreased gastrointestinal
motility. The combination of initial slower intestinal transit time and
decreased gastrointestinal motility associated with SMS may adversely affect
absorption and bioavailability of orally ingested medications more in female
than in male astronauts. Clinically significant gender differences have been
reported for drug elimination processes; these were predominantly linked to
the gender-specific expression of metabolic enzyme systems. Differences
between men and women in hepatic phase I and phase II metabolism have an
important influence on drug metabolism. Men have higher levels of certain
metabolizing enzyme isoforms, whereas women have higher activity levels of
different isoforms. These differences in drug metabolism may partially
account for the higher incidence of adverse reactions to drugs in women than
in men.
Hormonal Changes
Menstrual cycle hormonal changes can also influence drug absorption,
distribution, metabolism, and elimination, and oral contraceptive use can
interfere with the metabolism of many drugs. Due to the absolute preclusion
of pregnancy in space, many female astronauts choose oral contraceptives
during the training period, and most continue to use them while on orbit.
Changes in the renal, cardiovascular, hematological, and immune systems
during menstruation are well known, and these physiological changes could
influence the pharmacokinetics and pharmacodynamics of drugs by altering
such variables as protein binding and volume of distribution of drugs, which
could significantly worsen disease severity.
REPRODUCTION
Normal Menstrual Function
Because space shuttle flights are considerably shorter than the average
menstrual cycle length, no on-orbit studies have been done to determine the
impact of microgravity on normal hypothalamic/pituitary/ovarian axis
function. The primary concern is that anovulation might occur, resulting in
continuous estrogen exposure, endometrial hyperplasia, and possibly
menorrhagia. Second, there is some concern that hypothalamic amenorrhea and
reduced estrogen levels could occur. The reason for concern is that the
exercise necessary for long-term cardiovascular and musculoskeletal fitness
may be strenuous enough to cause hypothalamic-induced hypogonadism with
reduced serum estrogen levels. The combined effect of hypoestrogenemia and
spaceflight-related calcium loss could lead to increased osteoporosis risk.
Menstrual Efflux and Retrograde Menstruation
Many women normally experience some retrograde intra-abdominal bleeding
during menses. Because of the effects of gravity, the blood products and
cellular debris usually stay confined to the pelvis. The development of
endometriosis is multifaceted, but exposure of the pelvic peritoneum to
menstrual blood products is thought to be the primary cause of its
development.
Endometriosis is also primarily a pelvic problem in part because gravity
keeps the menstrual products confined to the pelvis.
Although medical debriefing data from shuttle flights have not supported
concern that retrograde menstruation increases during spaceflight, the role
of gravity in menstruation should be investigated to determine whether
retrograde menstruation is increased and how peritoneal fluid is
distributed. In addition, radiation exposure at varying doses has been
associated with the development of endometriosis in certain primate species.
RADIATION, FERTILITY
Radiation concerns for women and men are generally similar except for
exposure of the gonads and breast and thyroid tissue. Although the radiation
exposure levels found in long-duration spaceflight may present an
infertility problem for men, this is not likely for the more
radiation-resistant ovary. However, the effect of space-based radiation on
the chromosomes of oocytes is of considerable importance to women who may
desire future pregnancies.
Compared with women, men are at considerably increased short-term risk
from damage to gametes. However, the effect of neutrons, high-energy
particles, and other radiation from space needs to be assessed for both men
and women on prolonged missions so that they can make informed decisions
regarding cryopreservation of gametes before flight.
The participation of women in spaceflight piques the interest of the
public in issues relating to pregnancy and fertility in space. However, the
radiation levels associated with spaceflight in low Earth orbit or deep
space preclude pregnancies at this time. The National Council on Radiation
Protection and Measurements guidelines limit radiation exposure to 500 mrem
for an entire pregnancy and to only 50 mrem per month. The International
Commission on Radiological Protection guidelines are more restrictive. On
the International Space Station, radiation exposure to a pregnancy may
approach 35,000 mrem or more. Radiation exposure on the International Space
Station varies with altitude, solar cycle, and location of the astronaut in
the space station. Thus, during a nine month pregnancy, exposure could range
from 10,500 to 36,000 mrem, depending on the altitude of the station and the
solar cycle.
CARDIOVASCULAR PHYSIOLOGY
Generally, women have lower blood pressure and peripheral vascular
resistance and higher heart rates than men. In addition, women respond to
cardiovascular stress with greater heart rate increases, whereas men respond
primarily with greater increases in vascular resistance. In one study
designed to examine postflight orthostatic intolerance, the presyncopal
astronauts (five women and three men) were found to have greater increases
in heart rate, greater decreases in blood pressure, and less of an increase
in peripheral resistance in response to the postflight stand test than their
nonpresyncopal counterparts (two women and 19 men). It was suggested that
indirect vasodilatory effects of estrogen in premenopausal women may
contribute to smaller vasoconstrictive responses in women compared with men
during orthostatic stress.
Ventricular Dysrhythmias
New data suggest that cardiac dysrhythmias may be of greater concern
during long-duration than short-duration spaceflight. In the general
population, men in this age group have a greater risk of ventricular
dysrhythmias than women. It would, therefore, be expected that in the
astronaut population this would hold true as well.
EXERCISE PHYSIOLOGY
At the present time, 22 percent of the active astronaut corps are women
(35 of 158). The average female astronaut is 42 years old (43 years for men)
and weighs 60.7 kg (81.2 kg for men). In general, the average woman is 10 cm
shorter and 13 kg lighter and has 11 percent more body fat, 8 percent less
muscle mass, 10-14 percent less hemoglobin mass, and a lower level of
aerobic fitness than her male counterpart. These gender differences can be
expected to influence exercise capacity and thus the ability to perform
specific tasks during spaceflight.
Aerobic Fitness
The average aerobic fitness, expressed as the maximal oxygen uptake of
adult women is 2.0 l/min, compared with 3.5 l/min for men. When adjusted for
differences in body weight, the average maximal uptake for women is 40 vs.
50 ml/ kg for men. (These differences can be reduced still further.) Thus,
for any task requiring a given absolute oxygen uptake, the average woman is
working at a higher percentage of her exercise capacity than the average
man.
This would result in a higher heart rate, higher body temperature,
greater stress, and a quicker onset of fatigue during the exercise. These
more severe exercise responses may result in a greater number of injuries
and less tolerance for a stressful environment. For example, in a study of
124 men and 186 women during basic combat training, the women had a 51
percent injury rate compared with 27 percent for the men.
The average woman is less active and less fit than the average man.
Therefore, when exercise data are normalized for fitness, the gender
differences often are greatly reduced. If allowed to work at a similar
percentage of their maximal exercise capacity, men and women would have
similar cardiovascular and thermoregulatory responses. However, men tend to
be faster than women during aerobic events due to their greater muscle
strength and the mechanical advantage of their longer arms and legs. Women,
on the other hand, tend to have a greater endurance capacity due to a
greater reliance on fat metabolism during exercise; thus a glycogen-sparing
effect might delay fatigue during long-duration events.
At this time, there is not sufficient data to compare the degree of
aerobic deconditioning after spaceflight between men and women. However, in
response to bed rest, the relative changes in aerobic capacity are similar
between men and women despite the marked differences in absolute values.
Strength
There are obvious strength differences between the average man and the
average woman. Body strength of the adult woman is about two-thirds that of
the adult man. Upper body strength of the woman is 50 percent that of the
man, whereas lower-body strength is 70 percent. Few data exist concerning
strength changes in women after spaceflight or bed rest. Regional decreases
in muscle volume were similar in two men and two women after eight days of
spaceflight.
Some reports, however, indicate that women have a greater percentage of
slow-twitch muscle fibers than men. Slow-twitch muscle fibers appear to be
more affected by spaceflight than the fast-twitch fibers. If this is the
case, women may be more susceptible to changes in muscle mass and endurance.
This hypothesis has yet to be tested. In response to strength training,
women and men have a similar proportional increase in lean body mass and
strength, yet the total muscle girth of the women is less. These consistent
differences in muscle mass of men and women are believed to be attributable
to the anabolic effect of the 20- to 30-fold greater concentration of
testosterone in men.
-end-
The American Physiological Society (APS) was founded in 1887 to foster
basic and applied science, much of it relating to human health. The
Bethesda, MD-based Society has more than 10,000 members and publishes 3,800
articles in its 14 peer-reviewed journals every year.
***
Editor’s Note: For the full
text of the research cited above, or to set up an interview with a member of
the research team, please contact Donna Krupa at 703.527.7357 (direct dial),
703.967.2751 (cell) or djkrupa1@aol.com.
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