Image caption Fruit has risen in price by 34% since 2007, according to the Office for National Statistics

Many young families cut back on fresh fruit and vegetables and switched to less healthy processed food as the recession squeezed budgets, a UK study of 15,000 households' data suggests.

It showed rising food prices and stagnating wages had led people to buy less food and choose cheaper products.

The Institute for Fiscal Studies said pensioners, single parent households and families had the biggest drop in the nutritional quality of their diets.

Food campaigners expressed concern.

The Children's Food Trust said the move to processed food was a "huge worry".

The report's authors used food purchasing data from 15,850 British households from 2005 to 2012, enabling them to analyse the impact on spending of the recession.

They found that households spent 8.5% less on food in real terms across the period as disposable incomes failed to keep pace with rising food prices.

Over the recession households have responded to higher food prices and the squeezes on their incomes by switching to cheaper calories Kate Smith, Economist

People also swapped the type of food they bought, shifting from fresh fruit and vegetables to "calorie dense" processed food, with a resulting increase in saturated fat and sugar content, the Food Expenditure and Nutritional Quality over the Great Recession report said.

Food prices rose by 33% between 2007 and 2013, official figures show. Butter, meat and fruit prices all increased by more than average while processed food rose by 28%.

The IFS researchers found that on every measure, pensioner households, single parents and families with young children experienced a worse-than-average decline in nutritional quality.

Pensioners tended to increase their purchases of fatty foods while households with young children chose more sugary products.

IFS research economist Kate Smith, one of the authors of the report, said: "Over the recession households have responded to higher food prices and the squeezes on their incomes by switching to cheaper calories.

"This has coincided with a fall in the nutritional quality of foods purchased, with moves away from fresh fruit and vegetables and towards processed foods. As a result, the average saturated fat and sugar content of food purchases has increased over this period."

Children's Food Trust chief executive Linda Cregan said: "Feeding children well is absolutely crucial for their future health - these figures are an indication of just how tough this has become for many families in recent years.

"Some of the trends in this report are a huge worry - we need to see the foods children eat containing less saturated fat, salt and sugar, not more."

Long-term calorie fall

A second report from the IFS, looked at longer term trends.

Between 1980 and 2009, households bought 15% to 30% fewer calories, but average weight continued to climb.

During this time there was a big rise in snacking and eating out, but an even bigger fall in calories bought for the home during the 30-year period.

"We were surprised to find that there has been a substantial decline in total calories purchased at a time when obesity has increased," said one of this study's authors Melanie Luhrmann.

"This does not mean that poor diet plays no part in rising obesity. But understanding the interaction between diet and physical activity is clearly crucial."

Both reports are being presented as part of the Economic and Social Research Council's Festival of Social Science in London.