Veganism is touted as a solution to climate change, but is a plant based diet really the answer?

According to one of the biggest studies carried out to date, at Oxford University, meat and dairy production uses 83 per cent of farmland yet only provides 18% of the calories and 37% of the protein we consume.

But many experts say it isn't quite as simple as that. Vegan health claims may be overstated too.

People who cut out meat and dairy products can miss out on essential nutrients, and while some work hard to replace them, research regularly shows some are risking their health with deficient diets.

On Thursday, Dr Emma Derbyshire of the consultancy Nutritional Insight warned those switching to a plant-based diet that they risk worsening existing deficiencies of an important brain-nourishing chemical.

Writing in the British Medical Journal (BMJ), she said most people already have a low intake of choline – a nutrient found predominantly in animal products such as beef, eggs, dairy products, fish and chicken, and found in much lower levels in nuts, beans and vegetables.

“This is….concerning given that current trends appear to be towards meat reduction and plant-based diets,” says Dr Derbyshire. “If choline is not obtained in the levels needed from dietary sources... then supplementation strategies will be required, especially in relation to key stages of the life cycle, such as pregnancy, when choline intakes are critical to infant development.”

Not everyone agrees – the politics of food and farming are hugely polarised. The Vegan Society (VS) claims Dr Derbyshire has links to the meat industry, and is overstating the importance of choline.

The Society's "Plate up for the Planet" campaign is unequivocal. "Going vegan is one of the most significant things that you can do to help combat climate change and reduce further damage to the natural world," the Society's website claims.

But, as ever, one should be wary of grasping at seemingly simple answers.

Browse vegan recipe books or websites and you'll come across countless alternative meal ideas for those looking to cut out animal products from their diet. Delicious-sounding options such as linguine with avocado and lime, sweet potato and lentil curry, pomegranate quinoa salad.

However, when you get into the detail the taste can sour. Avocado production in Mexico is responsible for deforestation and demand for the hipster favourite has seen prices soar so that local people can no longer afford what was once a local staple.

The same has happened to indigenous communities in Peru and Bolivia where demand from Europe and America has driven up the price of "superfood" quinoa. Now the economic benefits of growing the grain for export has to be balanced against the spiralling cost of a staple for people in poverty.

Alternatives to dairy products such as almond milk and soya milk also have an environmental cost. In California, one of the world's major almond producing areas, despite the state's serious problems with water shortages, it takes five litres of water to grow a single almond. Depending on the quality of the end product – and which study you believe – it takes between 350 and 6,000 litres of water to produce one litre of almond milk.

Given that more than half of the world's almonds are produced in California, with other sources including Australia and the Middle East, the air miles might give climate-conscious consumers pause, and then there are those rarely-recycled Tetrapaks in which most nut milk is sold.

It is true that producing dairy milk is also water intensive, but cows' milk is more likely to be produced locally, and less likely – in the UK at least – that water will need to come from irrigation.

This helps explain why organisations like the Soil Association, which campaigns for a more organic and sustainable food system, are not advocates for a wholly plant-based diet.

"This is a debate which really needs to be had but with a bit more nuance about it," says Scottish director Aoife Behan. "Absolutely we should reduce the amount of meat we eat. Our diets need to change. The way we eat is unsustainable.

"But sometimes it is about the implications of different types of meat. Current production levels release too many greenhouse gases and the huge amount of land we need to grow food just for animals is part of the problem."

Without change in methods, though, a switch away from meat alone is of dubious value, she argues. "Rearing anything intensively – meat or plants – requires the addition of nitrogen fertiliser, which contributes to much of agricultures greenhouse gas emissions," she says.

The Scottish climate is sub-optimal for growing lots of food crops, she says, but we can grow trees and grass which can shelter and feed cattle and sheep.

In August the UN's Intergovernmental Panel on Climate Change published a report on food security which called for change in agricultural practices. "They found that while cows and other ruminants emit methane it is relatively short-lived," Behan says. "The deforestation taking place to produce food for pigs and poultry has a bigger climate impact.

"Understanding where your food comes from is really key. If you had a choice between an avocado grown through deforestation in the Amazon and grass-fed lamb from Scotland or the UK you might be better eating the meat."

We need to become less reliant on imports, Behan says, and more diverse in what we produce in our own islands.

"We import soya for animal feed and it is just crazy. We believe in a shift to grass-fed systems. That may take time but is entirely possible."

So are the claims for veganism as an environmental panacea overstated? Last month the Veganuary campaign issued a press release calling for cafes and coffee shops to stop charging customers who don't want to drink cows' milk extra for substitutes.

Calling for "free plant milk to protect the planet", the organisation characterised the extra expense as a "tax" on climate-conscious customers. Whether or not coffee chains and their dairy-drinking customers should subsidise the more expensive dietary preferences of those who prefer soya, or nut milks, there are serious questions about the core claim.

Dominika Piasecka, spokeswoman for the Vegan Society, admits almond milk is not particularly environmentally friendly. But she says anyone can make their own oat milk at home with a handful of oats, water and a blender.

Meanwhile, she insists there is no nutritional need to eat animal products. "You can get everything your body needs on a vegan diet." Research shows vegans have lower risks for a number of common health problems, she adds.

“What we eat has a huge impact on climate change and the natural environment. The global animal farming industry creates more greenhouse gases than the whole transport sector combined, while going vegan can reduce our food-related carbon footprint by up to 50%."

She cites recent reports from the UN, Greenpeace, WWF and Chatham House as all calling for a global shift towards a plant-based diet to save the world from hunger, fuel poverty and the worst impacts of climate change.

In fact, only the Greenpeace report recommended plant-based diets while the other three all call only for a reduction in meat consumption and towards greater consumption of plant products and less destructive farming methods.

That is less controversial. Even the meat industry concurs with the need to reduce the harmful environmental impact of farming.

Douglas Bell, Quality Meat Scotland’s Director of Strategic Engagement, says there is good evidence that meat is an important part of a healthy balanced diet. "[The] IPCC report ... acknowledged the role that sustainably-produced meat has in a healthy balanced diet, alongside coarse grains, legumes, fruits and vegetables. This diet, according to the report, presents major opportunities to help limit climate change."

He says that, in 17 years, Scottish agriculture has cut its greenhouse gas emissions by almost a third.

“Scotland has a very strong message to convey given our industry produces quality beef and lamb from the grass and rough grazing which make up around 80% of Scotland’s agricultural land which is not suitable for cereal, fruit or vegetable production.

“It’s also vital to note that Scotland’s production systems differ to others in other parts of the world. Scotland’s grassland acts as a carbon sink and grazing animals provide habitats for wildlife and help to maintain the landscape."

So what kind of shift would help? And how can those reluctant to grind their own nut milk or subsist on fava beans and pulses be persuaded to change their diets?

Aoife Behan says there are two imminent opportunities. As a side-effect of Brexit, leaving the Common Agricultural Policy gives the UK a chance to rethink the kind of food production it wants, while the Scottish Government is to developing a Good Food Nation policy.

Rather than pressing the public to change their diets, public procurement should be used to help boost demand and cut the cost of sustainable food, she says. "You have to normalise it. But we need to start now."

Clear problems - cloudy solutions

• Farming animals for meat is directly environmentally damaging, through overgrazing and the methane emissions it produces. But importing fruit and vegetables from around the world – superfoods such as quinoa from Peru or pomegranates from India, goji berries from China or avocados from Mexico – can result in deforestation and intensive farming techniques, as well as racking up the food miles.
• The movement against single-use plastic has seen drinking straws and cups replaced by alternatives including "sustainable" bamboo. But research on 12 different brands by a German consumer group found they are often made of bamboo fibres held together by a resin binding agent made from formaldehyde and melamine. Claims that they are readily biodegradable are overstated, but more worryingly, when exposed to hot liquid they leaked formaldehyde – an irritant suspected of being a possible carcinogen.
• The campaign against plastic has also seen concerns at the level of cling film we use, but alternatives such as beeswax wrapping are more expensive and the benefits of preventing food waste by wrapping up that left over sandwich effectively may be more important. Meanwhile, while the plastic bag charge has been effective, make sure you do re-use those bags for life. Otherwise you're just replacing your stash of once-used shopping bags with a new collection in heavier-duty plastic.
• Tofu is a key element of the diet of many vegans and vegetarians. It's a concentrated source of protein and versatile on the plate. But in many parts of the world soy farming is hugely destructive – in Brazil, for example, where it has led to deforestation and environmental degradation. One study found tofu from Brazilian plantations had twice the carbon footprint of chicken. Anyone concerned about the climate should be looking for Tofu which they know to be sustainably produced.