The revelation comes from a professor at Bristol University. In a recent article in the academic journal, Nature Neuroscience, Professor Kwangwook Cho claims that five years of long-haul travel without adequate rest in between flights leads to shrivelling of the thinking and learning parts of the brain.
Cho postulates that disrupted circadian rhythms may be responsible for the shrinkage. These rhythms dictate when the body is sleepy. In a normal day, people experience two lows: one between 3am and 5am, the other between 3pm and 5pm. Flying across time zones breaks this cycle, leading to jet lag.
Charalambos Kyriacou, an expert on circadian rhythms at Leicester University, says much of the problem is due to changes at the molecular level. 'The brain resets very quickly in the new situation, but peripheral organs like the liver take a long time to catch up.'
Kyriacou says that while the body is catching up, travellers have a higher level of toxins in their bloodstream and poor digestion. Over time, hormones released in the catching-up process may also shrivel the brain.
Cho advises resting between flights. His research indicates that an intervening 14-day rest period helps to alleviate brain shrinkage. It seems unlikely, however, that banks will ever allow staff such lengthy downtime.
But other experts are less concerned about Cho's brain-shrinking allegations. 'Frequent business travellers should be more worried about deep vein thrombosis,' advises Professor Jo Arendt, a sleep hormone expert at the University of Surrey. Arendt says that while brain shrinking is a new idea, problems such as poor digestion, higher blood fat and thrombosis, are well established.
'Organisations should be conscious that employees are flying a lot and should provide medical checks and guidelines,' she says.
Surprisingly, perhaps, for an industry in which long-haul travel is so common, banks appear to provide little advice on coping with the problems of flying.
Of a sample comprising Deutsche Bank, Goldman Sachs, Morgan Stanley, and Salomon Smith Barney, none said it provided staff with advice on dealing with frequent flights.
A straw poll of bankers, meanwhile, has revealed that several use Melatonin, a herbal preparation designed to help the body reset its internal clock. David Soanes, a managing director at UBS Warburg, takes Melatonin and sleeps well on frequent long-haul flights.
He says, however, that he does find his spatial reasoning is affected by long flights and he is less able to visualise things.
Peter Lindgren of US fund management group Pimco, reports no adverse effects. He drinks plenty of water and does not sleep on flights.
Other frequent fliers report drinking generous amounts of alcohol - a tactic advised against by airline health experts.
British Airways has a number of in-house specialists, such as Dr David Flower. BA recommends eating low-fat food during flights in order to reduce the danger of high blood fat, which can lead to heart problems in the longer term.
To avoid jet lag, Flower advises that passengers are well rested before a flight and that they avoid alcohol, as this reduces dreaming when sleep does occur. Nap when necessary, he says, but not for more than 45 minutes at a time, or you will suffer from sleep inertia when you wake up.
Leicester University's Kyriacou says travellers should eat meals at times consistent with the new time zone as this helps reset the 'clocks' of your internal organs.
It is, however, thrombosis that presents the greatest danger. As banks look to cut costs, staff are increasingly likely to find themselves in economy class seats - Merrill Lynch apparently banned staff from travelling business class in 1998. Cramped economy seating may increase the risk of clotting.
But incidences of thrombosis occurring among passengers travelling in the more spacious business class seats, suggests that seating is not the only factor.
BA's Flower cites immobility as one of the main causes of thrombosis. He suggests that flyers do a range of gentle stretching exercises to improve circulation.
Simpler still is to avoid flying as much as possible. Deutsche Bank appears to have embarked on this route. Cost-cutting means that as of last month, staff in the bank's global equities division have been banned from travelling to internal meetings. Newly reliant on video conferencing, Deutsche global equities staff may find themselves healthier and more intelligent than their better-travelled counterparts.