Sir, Robert Fitts et al have just published a space-breaking paper which showed that after six months on the International Space Station astronauts develop profound weakness of the legs with a 40 percent loss of skeletal muscles. The studies were obtained from the leg muscles with evaluations of the structure and function of slow and fast fibers of nine subjects. The paper stresses the point that the deterioration of the skeletal muscles in these astronauts is comparable to an accelerated aging process of the muscles — equivalent to that of an 80-year-old. 
This should not be surprising since there is a loss of skeletal muscles, shown in experimental animals and humans, within just a couple of weeks in microgravity. In this environment the circulation is significantly impaired, with evidence in experimental animals of injuries to the lining of the blood vessels (the endothelium). In humans the endothelium weighs about 2 kg and would cover an area equivalent to about seven tennis courts. It is vulnerable to injuries because of reduced magnesium stored in skeletal muscles, soft tissues and bone with about 60 percent stored in the latter. With invariable elevations of adrenaline in space, and in turn magnesium ion reductions, there are persistent vicious cycles which can ultimately damage the endothelium. [2-4]
What can be done to prevent this accelerated aging process? I have stressed the fact that the endothelium does not heal adequately after age 30 because by this time, the endothelial cells which replace those that are defective are inadequate; ideally astronauts should return from a Mars mission, for example, before this age. 
Regular exercise will reduce the potential for inadequacy of aging endothelial cells. [5,6] This is reason enough for all of us to exercise regularly the rest of our lives!
Fitts et al  have suggested a change in the current spaceflight exercise programmes which might involve increasing the frequency, duration and intensity of exercise.
However, both too much  and too little  exercise can damage even a perfectly normal heart. Also if one exercises above 85 percent of one’s maximum heart rate —determined by one’s age — the risk of sudden death increases as a result of a chaotic heart rhythm disturbance (ventricular fibrillation). 
Studies will be required to determine whether exercise in space will not only reduce the potential for loss of skeletal muscles and bones but also to determine whether this exercise will reduce the potential for age-related deterioration of the endothelial cells.
The bottom line is that with microgravity, perfusion of the skeletal muscles must be watched just as carefully as perfusion of the internal organs. After all our legs are not only necessary for locomotion but serve as important reservoirs for magnesium as well [3,7]
Since exercise for 15 minutes out of every waking hour would be intolerable  what other measures have been tried or even considered to prevent spaceflight vascular disease in the past 50 years? The answer may lie in a new plan, just published, involving gene therapy. 
William J Rowe FBIS
Virginia, United States
1. RH Fitts, SW Trappe, DL Costill et al., "Prolonged spaceflight-induced alterations in the structure and
function of human skeletal muscle fibres",
2. WJ Rowe, "Interplanetary travel and permanent injury to normal heart",Acta Astronautica, 40, pp.719-722, 1997.
3. WJ Rowe, "The case for a subcutaneous magnesium product and delivery device for space missions",J Am Coll Nutr, 23,pp.525S-528S, 2004.
4. WJ Rowe, "Extraordinary unremitting endurance exercise and permanent injury to normal heart,Lancet, 340, pp.712-714, 1992.
5. PM Vanhoutte, LP Perrault and JP Vilaine, "Endothelial dysfunction and vascular disease", in "The endothelium in clinical practice", GM Rubanyi, VJ Dzau eds and Marcel Dekker, New York, 1997, pp.265-289.
6. PM Vanhoutte, "Ageing and endothelial dysfunction",European Heart J. Suppl. 4, pp.A8-A17, 2002.
7. WJ Rowe, "The Reservoir",Spaceflight, 45, pp.88-89, 2003.
8. WJ Rowe, "A better way to exercise in space",Spaceflight, 45, p.480, 2003.
9. WJ Rowe, "Long space missions, gene therapy and the vital role of magnesium: a three-pronged plan for the next 50 years",J Nephrol and Renovasc. Dis, 2010 (in press).
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