Despite spiralling concerns over the obesity crisis, consumers are in fact eating 600 fewer calories a day than they were 30 years ago, a new report set to be published in a leading medical journal will claim.

Researchers conducting a five-year study, part-funded by the government, claim there has been a 20% drop in calories consumed over the period, with the most stark reduction that of calories consumed at home - down by a quarter.

The study claims that while the average consumer in England has piled on 0.25kg per year, diets - particularly those at home - have become increasingly healthy.

“For food [the fall in calories] is almost all due to changes in choices towards a less calorie-dense diet,” said report author professor Rachel Griffith, director at the Institute of Fiscal Studies, whose research is funded by the European Research Council (ERC) and the Economic and Social Research Council (ESRC), which receives support from the Department for Business, Innovation & Skills.

Griffith said food at home typically contained less milk, fats and sugar less red meat but more chicken and fish and more fruit and vegetables and cereals.

Conversely, calories from eating out, takeaways and soft drinks and snacks consumed on the go had increased by around 15% over the period, claimed the report, while calories from alcohol fell by 25%.

Mars bar

600 calories equates to three pints fo Guiness, a burger and chips, 2.5 Mars bars or a takeaway pizza

Yet over the period of the study, from 1980 to 2010, there has been a steady increase in the weight of both women and men. The average man in his 20s weighs 7kg more than in 1980, while those aged 55 to 60 have piled on an extra 14kg compared with their predecessors in 1980.

The report, to be published in the next two months, suggests increasingly sedentary lifestyles account for some weight gain, but did not account for all of the disparity.

“The drop in calories consumed would have been expected to have caused a weight loss of 1kg per year over the period,” said Griffith.

The researchers are now looking at a possible link between the rise in obesity and certain food types such as sugar. “We are also looking at why certain age groups and people seem to be far more susceptible to weight gain,” she said.

The report said men burn off an average 3.6kg less per year than they did in 1980, linked to a switch from manual occupations to deskbound office jobs. Women have experienced similar falls in work strenuousness.