Sedentary lifestyles and fatty food will make it almost normal for adults to be overweight, putting their health at risk, Alex Johnstone of the Rowett Research Institute in Aberdeen told the conference.

“By 2010, it will not be the norm to be normal weight and that is worrying,” Dr Johnstone warned. Estimates suggested that within six years 25 per cent of Scots would be obese and that another 15 per cent would be classed as overweight.

In 1979, she said, 5 per cent of boys were classed as obese and that figure has now risen to 9 per cent. Findings from a recent movement survey among toddlers in Glasgow carried out by Rowett staff showed that three-year-olds spend 79 per cent of their day inactive and at five that falls marginally to 76 per cent.

Most were failing to meet daily targets for moderate, vigorous physical activity.

Dr Johnstone believed prevention of obesity was the most sensible approach since dieting almost always failed. The majority of people who dieted eventually put weight back on after an initial loss.