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Almost 20% of the food made available to consumers is lost through over-eating or waste, a study suggests.

It says the world consumes about 10% more food than it needs, while almost 9% is thrown away or left to spoil.

Edinburgh scientists say efforts to reduce the billions of tonnes lost could improve global food security - ensuring everyone has access to a safe, affordable, nutritious diet.

The scientists looked at 10 stages in the global food system.

Using data collected primarily by the UN's Food and Agriculture Organization, the team found that more food was lost from the system than previously thought.

Almost half of harvested crops - or 2.1 billion tonnes - were lost through over-consumption, consumer waste and inefficiencies in production processes.

The researchers found that livestock production was the least efficient process, with losses of 78% or 840 million tonnes.

About 1.08 billion tonnes of harvested crops are used to produce 240 million tonnes of edible animal products including meat, milk and eggs.

Greenhouse gas emissions

This stage accounted for 40% of all losses of harvested crops, the researchers said.

They found that increased demand for some foods, particularly meat and dairy products, would decrease the efficiency of the food system and could make it difficult to feed the world's expanding population in sustainable ways.

Meeting the demand could cause environmental harm by increasing greenhouse gas emissions, depleting water supplies and causing loss of biodiversity.

The team said that encouraging people to eat fewer animal products, reduce waste and not exceed their nutritional needs could help to reverse these trends.

Dr Peter Alexander, of Edinburgh University's school of geosciences and Scotland's rural college, said: "Reducing losses from the global food system would improve food security and help prevent environmental harm.

"Until now, it was not known how over-eating impacts on the system. Not only is it harmful to health, we found that over-eating is bad for the environment and impairs food security."

Prof Dominic Moran, of the University of York, who was involved in the study, said: "This study highlights that food security has production and consumption dimensions that need to be considered when designing sustainable food systems.

"It also highlights that the definition of waste can mean different things to different people."

The study is published in the journal