Is it time we all went vegetarian? Even Arnold Schwarzenegger is now telling us to cut down on meat, so how serious should we be?

There is a scene in the 2013 film Escape Plan where Arnold Schwarzenegger and Sylvester Stallone face off against one another in the canteen of a maximum security prison.

Schwarzenegger throws the first punch, flooring Stallone in front of the baying crowd of fellow prisoners. Stallone, when he gets to his feet, responds with a left hook fierce enough to knock out most puny mortals.

But for the old slugger who once played the Terminator, it prompts only a chuckle.

"You punch like a vegetarian," he says, in that unmistakable Austrian drawl.

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Schwarzenegger continues to comment on the subject of eating meat. Speaking at the UN Climate Conference in Paris, the views of the 68-year-old bodybuilder-turned-actor-turned-politician were rather more cerebral - and surprising.

During an interview after a speech to the conference, the former Governor of California claimed that 28 per cent of all greenhouse gases are caused by intensive farming.

As a result, he said, it was a "good idea" for people to stop eating meat altogether and, at the very least, to stop for one or two days a week. "Luckily, we know you can get protein in many different ways," he said.

Schwarzenegger's comments come at a time when an increasing number of experts are clamouring for us to turn vegetarian or, better yet, leave that butter dish in the fridge and go all-out vegan.

One celebrity advocate is designer Stella McCartney, whose parents Paul and Linda McCartney flew the flag for vegetarianism long before it was fashionable.

There is growing evidence of the strain our meat consumption is placing on the environment. Every 1kg of meat protein requires the same amount of energy to produce as 3kg to 10kg of vegetable protein.

Last year, the UN Food and Agricultural Organisation released a report that claimed emissions from farming, forestry and fisheries had nearly doubled over the past half-century and may increase another 30 per cent by 2050.

Most of those emissions emanate from nitrogen fertilisers and cows releasing methane gas, but are exacerbated by the forests cut down to accommodate the animals.

But the call to go meat-free stems not just from concerns over climate change.

A new government review on antibiotic resistance warns that a prevalence of superbugs in rare steak and undercooked meats is now putting lives at risk.

The review found the use of antibiotics in agriculture represents a "critical threat to public health", with increasing numbers of drug-resistant strains that could be passed from animals to humans.

Lord O'Neill, former chief economist at Goldman Sachs, said the level of antibiotics being prescribed in modern-day farming was "staggering".

The persistence - and severity - of such warnings seem to be making a difference.

Some 12 per cent of Britons now follow a meat-free diet. According to the Vegan Society, the number of vegans has doubled over the past nine years, from 150,000 to around 300,000.

Bolstering their ranks is Barny du Plessis, the 41-year-old bodybuilder from Norwich who last year won Mr Universe.

He turned vegan 10 months ago - with his fiancee, Josie, who is also a bodybuilder - partly for the environment and partly because meat-eating aggravated his hernias. "All those problems disappeared," he says. "I was suddenly getting leaner, training better and putting on more muscle than ever. I never feel tired."

To maintain his current off-season weight of 101kg, du Plessis eats 5000 calories of brown rice, lentils, chickpeas, beetroot juice, coconut oil and seeds a day.

He also scoffs 500g of oats. "The environment is more important to me than my taste buds," he says.

Du Plessis is one of a number of vegan athletes filmed for a documentary produced by Terminator director James Cameron, whom Schwarzenegger namechecked in Paris as a close friend and a fellow meat-denier.

The documentary aims "to dispel the world's most-dangerous myth - that men need protein to be a real man," says director Louie Psihoyos.

Du Plessis agrees. "In the bodybuilding world, there is this whole link between eating meat and being a man.

"A lot of people have messaged me saying they are really behind what I'm doing. But there has been a lot of resistance and hatred as well."

Of course, your average vegetarian doesn't give two hoots about all that. They are more concerned about whether or not a meat-free diet can provide all the nutrients humans need.

So it was for health journalist Anna Magee, editor of healthista.com, when earlier this year she decided to embark on a 60-day trial of veganism and measure the effect on her health.

The 46-year-old from East London says she lost 6kg, her cholesterol level plunged and her muscle mass went up. She has remained a vegan.

"By the end of 60 days, I thought I would be craving a steak, but I just didn't want it," she says.

"I really don't want to be this vegan bore environmentalist-type person, but the arguments [against eating meat] are so strong."

She says the most difficult thing to counter at first was the "psychological deprivation" of not eating meat, "the smells and rituals" of a shared meal out with friends.

Her husband of 16 years, Kevin, remains a meat-eater but fortunately one willing to take part in her new supper menus of nut burgers, tofu stir-fry and bean casseroles.

This Christmas, Magee will roast a turkey and ham but she will also prepare a nut roast to eat on the side.

"I would never stop cooking meat for people and I would never make people go to a vegan restaurant," she says. "I try to work it around life."

Yet some medical experts are not convinced of the health benefits that come from giving up meat entirely. Professor David Haslam, a former GP and president of the British Medical Association, insists that he will never turn vegetarian.

"I love eating meat," he says. "The important thing is, you have to be very careful what you eat, where it comes from and how it is prepared.

"Meat is part of a normal mixed and balanced diet. There are certain things you can only get from animal products.

"Humans have been eating it for tens of thousands of years. It can't suddenly be bad for us."

Laura Piddock, professor of microbiology at the University of Birmingham and director of Antibiotic Action, which helped with the latest government report on antibiotic resistance, similarly denies that giving up meat is the answer to solving the growing crisis within our environment and the food chain.

"That has been suggested by some, but we are omnivores," she says. "We have to recognise that eating meat is a part of our diet and that we farm animals to eat them. Instead, we need to start realising that if we are going to eat these animals then we need to look after them."

For the likes of du Plessis, the time for that has already passed.

Schwarzenegger's intervention has, he says, put a "full stop" to this particular debate, meaning he could look forward to his evening meal of oats, mixed seeds, frozen berries and "vegan protein" with more than customary relish.

Vegetarians are taking over, and watch out - they still can land a mean right hook.