He claimed that the average man burned 1380 calories per day in the 1980s and continues to do so today. The average woman has burned 950 calories a day during the same period.
What has changed is that calorie intake has increased by at least a third to on average 3,500 calories a day, he said.
Prof Speakman, who is a member of the Energetics Research Group at the University of Aberdeen, said that his research showed that small changes in lifestyle were not enough to fight the obesity crisis.
“Over the past 25 years, during which time obesity levels have increased enormously, there has actually been no change in our levels of physical activity,” he said at the British Science Festival in Birmingham.
“We seem to have homeostatic mechanism that regulates our calorie expenditure. The more exercise we do during the day, the less active we are during the evening.
“The idea that small changes in life style are enough to offset obesity is wrong. In fact enormous changes in energy balance are needed and that can only realistically be achieved through changes in diet.”
His team calculated energy expenditure using the “doubly labelled water technique” which tracks the progress of a water isoptopes through the body, the speed if which in turn indicates rate of metabolism.
“We should be promoting exercise exercise for health reasons but weight is not realistically going to benefit from exercise,” he said.
“Exercise is not enough to reverse the epidemic.
“Dieting on the other hand does have an effect on energy intake and this is a more realistic approach.”
To illustrate the point he said that an hour of intensive exercise will only use up on average 300 calories – the equivalent of 10 per cent of food consumption or a small sandwich.
“Since the 1980s when the obesity epidemic really took off there has been any change in energy expenditure.
“We may watch televison at night but that has always been a time when we were inactive. Before television there was sitting around reading or listening to the radio and now there is looking at a computer.
“There may be differences in what we are doing but we use the same amount of energy.”