Shuttle Mission Unearthed Surprises on Weightlessness : Research: Experiments on 1991 flight crew reveal that changes to the human body will make long stays in space difficult.

A space biology mission flown last year aboard the shuttle Columbia, which used astronauts as guinea pigs, has revealed unexpectedly dramatic effects of weightlessness on the human body, researchers reported Friday.

The effects included a severe loss of muscle tissue, blood-pressure irregularities and reduced ability to burn stored fat for energy.

The findings could mean that extended space travel will be more difficult for humans than previously thought, scientists said at a NASA briefing Friday, and only more biomedical research in orbit will resolve the questions.

Kenneth Baldwin, a physiologist from UC Irvine, reported “a significant and dramatic” reduction of 25% of the muscle mass in weight-bearing muscles, such as calves and thighs. Muscle loss had been predicted, but not that much.

He said the crew also took an unexpectedly long time to recover. They had restored only half of the muscle mass after being back on the ground for a length of time equal to the flight: nine days.

The muscles also showed a decline in their ability to burn fat for energy, Baldwin said. This forced them to rely on carbohydrates, which the body has a limited capacity to store. As a result, future crews “will need to increase their early intake of carbohydrates,” he said.

In another experiment, readings from a catheter inserted through a blood vessel just above the heart of Dr. Drew Gaffney before the June, 1991, flight showed that he experienced a much more rapid fall in central-body blood pressure than scientists had predicted, according to C. Gunnar Blomqvist, a cardiologist at the University of Texas Health Science Center in Dallas.

The catheter monitor tracked the redistribution of the body’s fluids as the force of gravity was suddenly lifted, Blomqvist said. The “highly surprising finding” shows that the processes involved are “much more complicated than we thought,” he said.

Scientists had expected the pressure to remain elevated early in the flight and gradually return to normal. The pressure increased as expected during launch but then “fell well below levels seen on the ground,” he said.

These and other findings, researchers said, will lay the foundation for further research aboard the National Aeronautics and Space Administration’s planned space station Freedom. The research will help shape plans to send humans on extended space voyages, and it also will influence medical research on Earth.

Columbia carried three women and four men, 29 rats and 2,400 jellyfish. It was the first U.S. biomedical flight since 1974, and the first dedicated to comprehensive research on all physiological changes believed to occur in weightlessness.

When gravity is lifted, past studies have shown, fluids that had pooled in the legs rise to the upper torso, fooling the body into thinking it is too “full.”