Eating a healthier diet rich in fruit and vegetables could actually be more harmful to the environment than consuming some meat, a US study has claimed.

Lettuce is “over three times worse in greenhouse gas emissions than eating bacon”, according to researchers from the Carnegie Mellon University who analysed the impact per calorie of different foods in terms of energy cost, water use and emissions.

Published in the Environment Systems and Decisions journal, the study goes against the grain of recent calls for humans to quit eating meat to curb climate change.

Researchers did not argue against the idea people should be eating less meat, or the fact that livestock contributes to an enormous proportion of global emissions – up to 51 per cent according to some studies.

But they found that eating only the recommended “healthier” foods prescribed in recent advice from the US Department of Agriculture increased a person’s impact on the environment across all three factors – even when overall calorie intake was reduced.

The experts examined how growing, processing and transporting food; sales and service; and household storage and use all take a toll on the environment for different foods.

Paul Fischbeck, study co-author and CMU’s professor of social and decisions sciences, said: “Lots of common vegetables require more resources per calorie than you would think.

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“Eggplant, celery and cucumbers look particularly bad when compared to pork or chicken.”

The initial findings of the study were "surprising", according to senior research fellow Anthony Froggatt at Chatham House, an independent think-tank which is currently running a project looking at the link between meat consumption and greenhouse gas emissions.

Mr Froggatt told the Independent it is "true lettuce can be incredibly water intensive and energy intensive to produce", but such comparative exercises vary hugely depending on how the foods are raised or grown.

"We usually look at proteins rather than calories, and as a general rule it is still the case that reducing meat consumption in favour of plant-based proteins can reduce emissions," he said.

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According to the authors, the study analysed the impact on the environment from changing the average US diet to three new “dietary scenarios”.

Simply reducing the number of calories consumed, without changing the proportion of meat and other food types, cut combined emissions, energy and water use by around 9 per cent.

Perhaps understandably, maintaining calorie intake but completely shifting to healthy foods increased energy use by 43 per cent, water use by 16 per cent and emissions by 11 per cent.

But surprisingly, even if people cut out meat and reduced their calories to USDA-recommended levels, their environmental impact would increase across energy use (38 per cent), water (10 per cent) and emissions (6 per cent).

Michelle Tom, another co-author, said the relationship between diet and environment was “complex”.

“What is good for us health-wise isn’t always what’s best for the environment,” she said. “That’s important for public officials to know and for them to be cognisant of these trade-offs as they develop or continue to develop dietary guidelines in the future.”

Chatham House's Mr Froggatt, who was not involved in the research, said it was important to look at production methods as well as the complex issue of how land use is "likely to be impacted by changing diets".

"The key point I would agree with here is that you need to look at both the environmental and health impacts at the same time," he said.