LETTERS TO THE EDITOR
Aviation, Space, and Environmental Medicine: Vol. 70, Page 939 September 1999

Gender Differences in Astronaut Selection

Dear Sir:
In mankind's long march through time, females have had, until recently, the disadvantage of being the "weaker sex" in various hostile environments. Now with the space age, the tables are turned. Women have unquestionable physical advantages over men as astronauts, particularly for very prolonged space missions (2). This is not an altruistic plea for equal rights for women, but a brief analysis, as I see it, of the facts.

Strength and endurance are no longer criteria for superiority, as in the distant past. The male's protective advantage for survival on Earth, provided by greater bursts of adrenaline for "fight-flight" is no longer a necessity; in space indeed it is a decided disadvantage. The delicate lining of the blood vessels is particularly vulnerable to injury in the young male. How else can one explain the six fold increased incidence of coronary artery disease of males compared to females in the third and fourth decade, or the rarity of cardiovascular death of a premenopausal woman (1)? With weightlessness (microgravity) and limited physical activity on a space mission, there is an invariable loss of vital magnesium because its storage sites in the bone and skeletal muscle are invariably depleted (2,3) no matter how much an astronaut exercises. This loss of magnesium triggers the release of adrenaline which in turn causes a further reduction in the availability of magnesium, resulting in a persistent vicious cycle. Too much or too little exercise will only aggravate this vicious cycle and it can begin with only a 12-day mission, as for example on Apollo 15 (3), or persist for an anticipated duration of two to three years round trip to Mars (2).

This potential vicious cycle can cause spasm of blood vessels, clots, serious heart rhythm disturbances, and because of impairment in the repair process, permanent heart muscle injury (2,3).

Young female astronauts would have a decided advantage over their male counterparts, since young females retain magnesium better than young males (4,5) and, as a result, there is less risk of instigating a space-related vicious cycle. Furthermore, because young females have potentially higher magnesium, not only would their repair mechanism be superior to males, but they would have greater antioxidant protection (2).

Of course estrogens would play a vital role as well, in enhancing these female attributes, through numerous vascular mechanisms, but oral contraceptives must be withheld since their excessive estrogen content might incite clotting (5). We can't have it both ways; males are trapped with inferior genes for space travel, whereas their superior genes for survival long ago served to protect the family units. Our genes have probably changed very little in the past 50,000 years. We could alter them by genetic engineering, decades if not centuries from now. But for the present, we must be willing to adapt, if we are to succeed in the topsy-turvy environment of space.

On long space missions, since there is invariably a potential for a significant magnesium deficiency (2)ówith, in turn, the potential for multiple adverse cardiovascular consequences (6)óshouldn't we profit by the gender difference in magnesium retention, in the selection of astronauts?

William J. Rowe, M.D
Former Clinical Assistant Professor of Medicine
Medical College of Ohio at Toledo

REFERENCES
1.- Kauser K, Rubany GM. Vasculoprotection by estrogen contributes to gender differences in cardiovascular diseases; potential mechanism and role of endothelium. In: Rubany GM, Dzau VJ, eds. The endothelium in clinical practice. New York: Marcel Dekker 1997; 439-67.
2. Rowe WJ. Interplanetary travel and permanent injury to normal heart. ACTA Astronautica 1997; 40:719-22.
3. Rowe WJ. The Apollo 15 space syndrome. Circulation 1998; 97: 119-20.
4. Seelig MS. Increased need for magnesium with the use of combined oestrogen and calcium for osteoporosis treatment. Magnes Res 1990; 3:197-215.
5. Seelig MS. Interrelationship of magnesium and estrogen in cardiovascular and bone disorders, eclampsia, migraine and premenstrual syndrome. J Am Coll Nutr 1993; 12:442-58.
6. Seelig MS. Cardiovascular consequences of magnesium deficiency and loss: pathogenesis, prevalence and manifestationsóMagnesium and chloride loss in refractory potassium repletion. Am J Cardiol 1989; 63:4G-21G.