Going vegetarian for the sake of the planet may actually do more harm than good.

That's the surprise finding from a new study by US researchers at the Johns Hopkins Centre for a Livable Future.

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The boffins calculated greenhouse gas emissions and freshwater use for nine different diets - ranging from one meatless day a week and no red meat, to pescatarian and vegan - in 140 countries.

They found that diets in which meat, fish or dairy products were consumed only once a day would leave less of a footprint on climate change and water supplies than a vegetarian diet with more milk and eggs.

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That is partly because raising dairy cows for milk, butter and cheese requires large amounts of energy and land, as well as fertilisers and pesticides to grow fodder, emitting greenhouse gases that are heating up the planet, the study said.

Many climate activists and scientists have called for a shift to plant-based diets to keep climate change in check and reduce deforestation, since producing red meat requires a lot of land for grazing and growing feed.

On average, producing a serving of beef emits 316 times more greenhouse gases than pulses, 115 times more than nuts, and 40 times more than soy, it added.

And while vegan diets do have a minimal environmental impact, diets including insects, small fish and molluscs are just as good for the environment but are more nutritious for the consumer, the study found.