A group of women students who said they were on weight-reducing diets performed less well than non-dieters when tested on reaction times, rapid information processing and a memory task.
Even women not on diets but controlling their eating with weight in mind did less well than those who ate what they wanted when they wanted.
The London meeting of the British Psychological Society was told yesterday that the 'impaired cognitive performance' experienced by dieters could affect their working lives if they had to operate complicated machinery or, for example, perform effectively as junior doctors.
Dr Michael Green and Dr Peter Rogers, psychologists at the Institute of Food Research in Reading, Berkshire, said that until now the cognitive effects of diet had been almost completely ignored.
'Dieting in Western society is treated as some kind of inconsequential leisure activity but it definitely isn't. It should not be taken lightly,' Dr Green said.
About 30 per cent of women are on diets at any one time.
Researchers studied 55 women, 13 of them on diets. From records of what the group had eaten over the previous 24 hours they estimated that the dieters were 30 per cent undernourished given average calorie requirements.
They found that the women who did least well in the tests were also those who had been on diets longest and who had lost the most weight.
Dr Green said it was not easy to explain their findings but the dieters and weight contollers might be displaying a 'distraction' effect - their anxiety about dieting and shape kept their minds off the tasks they were given.
Another explanation could be that 'heart rate immediately before and after testing was lowest in those on diets, perhaps implicating lower arousal or alertness as a factor mediating the poorer cognitive
Or perhaps the dieters were simply hungry. The researchers did not make a meal of that.
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