Kids are getting fatter, but what they eat may not be to blame, according to an Australian authored historical analysis of children's diets and activity patterns.
A report in the latest edition of Australasian Science magazine says a 12-year-old Australian boy in 2006 is, on average, seven kilograms heavier and 25 per cent fatter than his counterpart from 1970.
The changes are due to increasing overweight caused by an energy imbalance brought about by either an increase in energy intake or a decrease in energy expenditure, or both, according to the report.
The article brings together 1,600 studies on more than 257,000 children and their diets from 25 developed countries.
Children's diets have changed dramatically over the past century due to the effect of technologies such as improved transport, canning and refrigeration, the report says.
The fat content of diets appears to have reached a peak in about
1965, when it accounted for about 40 per cent of total daily kilojoules.
But the report says the contribution of fat has been falling since then and in most developed countries it is now about 35 per cent.
Between 1955 and 1985 reported energy intake fell by four per cent per decade and there was a further flattening out in energy intake between 1985 and 1995, according to Australasian Science.
"These results are very surprising given that these apparent decreases in energy intake were occurring at a time when childhood overweight and obesity were increasing rapidly," the report says.
"If energy intakes really were declining, there must have been drastic declines in energy expenditure."
The average Australian child now spends about four hours a day in front of a screen of some sort, a finding which the report says suggests massive energy expenditure decreases over the past decades are conceivable.
The report also found that recent data indicates that energy intake may be on the rise again.
"If these trends are confirmed, children face the double impact of reduced energy expenditure and increased energy intake."