Scientists 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 including milk and eggs, in 95% of countries they analysed.

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

Diets that contain insects, small fish and molluscs, meanwhile, have as similarly small an environmental impact as plant-based vegan diets but are generally more nutritious, said researchers at the Johns Hopkins Center for a Livable Future.

They 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.

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.

Agriculture, forestry and other land use activities accounted for nearly a quarter of man-made greenhouse gas emissions from 2007-2016, the UN climate science panel said in a flagship report last month.

Read also: Can we eat Big Macs and still avoid climate chaos?

But there is no one-size-fits-all solution, said Keeve Nachman, assistant professor at the Baltimore-based Johns Hopkins Bloomberg School of Public Health, who led the study on diets.

In low- and middle-income countries such as Indonesia, citizens on average need to eat more animal protein for adequate nutrition, he told the Thomson Reuters Foundation.

That means diet-related heat-trapping emissions and water use in poorer countries would need to rise to reduce hunger and malnutrition, while high-income countries should reduce their consumption of meat, dairy and eggs, the study said.

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

According to the World Resources Institute, a US-based think-tank, diners in North and South America, Europe and the former Soviet Union make up only a quarter of the global population but ate more than half of the world's meat from ruminants - such as cattle, sheep and goats - in 2010.

The latest study also found that producing a pound of beef in Paraguay contributes nearly 17 times more greenhouse gases than in Denmark, partly because in Latin America, it often involves cutting down forests to clear land for cattle grazing.

A typical diet in Niger has the highest water footprint, researchers noted, mainly due to millet production and crop residues that cannot be consumed.

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