It isn't flying across five time zones itself that seems to make elite athletes prone to falling ill, but the changes they face at the destination, an international study suggests.

The incidence of illness and injuries among athletes participating at the Games in Vancouver, FIFA Confederations Cup in 2009, aquatic events, and athletics have been reported in medical literature.

Illness risk among elite athletes doesn't seem to be directly related to air travel. (Nathan Denette/Canadian Press)

In Wednesday's online issue of the British Journal of Sports Medicine, researchers set out to look at the effect of international travel on the incidence of illness among rugby players participating in a 16-week tournament.

Players travelled between South Africa, New Zealand and Australia, where time zone differences varied between two hours and 11 hours, for competitions, as well as home games.

"This study shows for the first time that elite athletes travelling to international destinations greater than five time zone differences from their home country is associated with two to three times increased risk of illness, respiratory tract illness, gastrointestinal and all infective illness," Prof. Martin Schwellnus, of the department of human biology at the Institute of South Africa in Cape Town, and his co-authors concluded.

"Identification of this period where athletes are at higher risk allows the team physician to plan certain preventive measures and have increased vigilance during this time."

For the study, eight team physicians filled in daily logs of any illness that required medical attention for each team member, regardless of where they were playing. Data for each player and team was coded anonymously for the analysis.

For all teams, the average incidence of illness was higher after international travel, 32.6, compared with before leaving, 15.4, or after returning to their home country, 10.6, the researchers reported.

The reasons behind the higher incidence aren't clear but the investigators said it could be attributed to factors like:

• Prolonged and strenuous nature of the competition.
• Exposure to different environmental conditions such as temperature, humidity and air pollutants.
• Changes in diet.
• Exposure to different strains of germs.

If factors during air travel, such as close contact with fellow passengers and exposure to re-circulated air, predisposed players to illness, then the researchers said they'd expect a higher incidence of respiratory illness after athletes returned home.

But that wasn't the case — the incidence afterwards was similar to before leaving.

The case for fist bumps

"The results from our study indicate that the illness risk is not directly related to the travel itself, but rather the arrival and location of the team at a distant destination," Schwellnus said.

The findings don't necessarily apply to other sports, recreational or business travellers or north-south travel, the study's authors cautioned.

They also couldn't take into account possible illness prevention measures that doctors might have used for their teams.

At major sporting events, people bring germs from different climates, they may not be fully vaccinated and they could be having more unprotected sex that spreads sexually transmitted infections, said Jason Tetro, a microbiologist at the University of Ottawa.

From previous Olympics, researchers have learned about how to stop spread of infections, such as using fist bumps instead of handshakes, Tetro said in an interview with CBC Radio's Metro Morning.

The study was funded by the IOC Research Center.