MANY of us will have started the New Year with a vow to lose weight and exercise more. But how many people think about the environmental impact of what we eat?

The issue was thrust into the spotlight last week by a landmark Lancet study which warned that the world's current patterns of food production and consumption are not only killing millions prematurely every year, through malnutrition in impoverished regions and obesity-related disease in the West, but "stretching Earth to its limits, and threatening human and other species' sustained existence".

It's not what they tell you at your average Weight Watchers weigh-in.

Read more: Proposed 'meat tax' would save lives, say scientists

The solution? The scientists - a 37-strong team of international experts in nutrition, climate research, food policy and population health - say we have to turn our diet upside down.

Globally, sugar and red meat intake should be at least halved by 2050. In practice, it means around 100g of red meat per week: think one Sirloin steak every two weeks, or one quarter-pounder beefburger per week.

Poultry and fish are allowed in slightly higher quantities - a 200g packet of prawns or one and a half salmon fillets per week for example, and one and a bit chicken breasts.

For dairy, they advise limits of one and a half eggs per week per person, and 250ml of whole milk or 250g of cheese or yoghurt per day.

The vast bulk of the human diet should instead consist of fruit, vegetables, nuts, and legumes - lentils, chickpeas, beans - with smaller amounts of wholegrains, such as brown rice.

Professor Tim Lang, a expert in food policy who helped devise the diet, admits that humanity "has never aimed to change the food system this radically at such speed or scale".

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But the stakes are high. Unhealthy diets now "pose a greater risk to morbidity and mortality than does unsafe sex, alcohol, drug and tobacco use combined", while food production is "the largest cause of global environmental change".

Currently, almost two thirds of all soybeans, maize, barley, and about a third of all grains are used as feed for animals.

Worldwide, 40% of land is used for agriculture and food production contributes up to 30% of global greenhouse-gas emissions and 70% of freshwater use.

The science is coinciding with a culture shift that has already seen a surge in the popularity of 'plant-based' diets, especially veganism.

Barbara Bolton, co-founder and volunteer with Go Vegan Scotland, welcomed the findings.

She said: "Time will tell if governments will finally acknowledge the critical role of agriculture in climate change and environmental destruction and make the necessary policy changes.

"They should shift subsidies away from meat, dairy and eggs, tax those products, and use the funds to support plant-based agriculture, including veganic growing. Farmers should be helped to make this critical transition."

The scientists behind the Lancet research accept that food prices will probably have to rise as farming subsidies for fertilisers, water, pesticides, fuel and electricity are cut or axed altogether, and agriculture shifts to "nutritionally important but lower yielding crops".

Read more: Scientists warn red meat and sugar consumption must halve by 2050 to save lives - and the planet

They envisage an end to advertising of junk foods and a reduction in the portion sizes available to consumers in high-income nations.

They also challenge widespread beliefs about the importance of red meat and dairy in human diets.

It is true, they acknowledge, that proteins from animal sources "are of higher quality than most plant sources" - this relates to their composition of amino acids, the building blocks of protein.

But they stress that while this is significant for rapidly growing young children and the elderly who are losing muscle mass, for most adults the benefits of consuming amino acids which "maximally stimulate cell replication" are overshadowed by the increased risk of cancer that brings.

As for dairy, while while high intakes have long been promoted for bone health in western countries they point to evidence gathered by the World Health Organisation (WHO) which found that regions with comparatively low dairy consumption actually had lower fracture rates.

Professor Jennie Macdiarmid, an expert in Sustainable Nutrition and Health at Aberdeen's Rowett Institute, said she is "in complete agreement" with the Lancet's recommendations.

She said: "We are going to have to change our diets, we can't get away from that. You can't dispute it - the evidence is there. The science is showing that we have to move to more plant-based diets. The biggest challenge is how we persuade people.

"Work we've done in Scotland shows there's a lot of barriers about reducing meat consumption. People described the fact that they like eating meat, that a meal wouldn't feel complete without meat, and there are a lot of obstacles around social norms.

"People are concerned about getting enough protein if they don't eat meat. That isn't the case. On average, people in the UK are consuming about twice as much protein as they need."

Prof Macdiarmid conceded that the outlook for farmers was "uncomfortable".

"We are going to have to look for other opportunities within agriculture if we are to reduce production of meat - which is going to have to happen if we're serious about tackling climate change," she said.

The task is formidable, however, especially in Scotland where dairy and meat dominate our agricultural output.

Andrew McCornick, president of the National Farmers Union Scotland, said the country's farmers and crofters were already among those hardest hit by changing weather patterns, but stressed that efforts to reduce greenhouse emissions from farming "must not be at the expense of producing food, cutting livestock numbers or exporting our emissions by relying on food imports"

He added: “Around 85% of Scotland’s farmland is defined as Less Favoured Area, and 43% is defined as High Nature Value land.

"Our unique landscape and climate means that much of Scotland’s farmland is unploughable and unsuitable for crops other than grass. However, that grass is readily converted by Scotland’s cattle and sheep into fresh, nutritious meat and milk.

“Scottish farmers and crofters should be praised for their environmental contribution and encouraged to join the climate change conversation, rather than being vilified by an overly simplified message of ‘don’t eat red meat’ from a report which incorrectly painted all red meat production to be the same.”