It seems that the skeletons within living cells may not form properly in zero gravity.
This means that it may be impossible to live in space over the long-term without creating a form of artificial gravity.
Most cells have skeletons made up of microtubules made from fibres of the protein tubulin.
New Scientist magazine reports that Dr James Tabony and his colleagues from the French Atomic Energy Commission mixed up cold solutions of mammalian tubulin with an energy-releasing compound.
When the mixture was warmed to body temperature for six minutes, microtubules began to form in distinct bands at right angles to gravity.
Next, the team sent up tubulin on a European Space Agency (Esa) rocket to expose it to the effect of weightlessness.
They found that when microtubules formed, they pointed in all directions.
Dr Tabony said: "This shows gravity triggers the pattern."
Previous work by Dr Marian Lewis of the University of Alabama at Huntsville produced similar results.
Dr Lewis's team tested the impact of weightlessness on human white blood cells that were flown on board the space shuttle.
After a day in orbit, the microtubules grew in random directions.
The findings might explain some of the health problems people living in space have, such as depressed immune systems.
Cancer drug effect
Professor Brian Anderton, an expert in cell structures at the UK Institute of Psychiatry, said microtubules played a vital role in the successful division of cells.
Malformation of microtubules would therefore hinder the process.
This could blunt the function of the immune system, which relies on rapid production of white blood cells to fight off invaders when the body is infected.
It could also cause problems with the renewal of epithelial tissues which line organs in the body. For instance, it could cause problems with the gut.
It might also lead to a thinning of the skin.
Similar side effects are produced by anti-cancer drugs, which work by blocking the uncontrolled division of cancer cells.
Professor Anderton said: "If it is really true that weightlessness interferes with microtubule function one could expect to see the same kind of adverse effects that are associated with quite a lot of anti-cancer drugs."