But whatever the environmental pros and cons, the booming meat-free food market has prompted some doctors and scientists to question whether some of these products can be considered part of a healthy diet.
Meat substitutes
From "meatballs" and "burgers" to "goujons" and "bacon", imitation meats are everywhere. Some have been around for years. Seitan, traditionally used in Chinese cookery, is a form of wheat gluten. Many people are familiar with Quorn, although perhaps not what it's made from: mycoprotein, a protein derived from fermented fungi, bound with egg albumen or potato protein. And soy products like tofu and tempeh have long been used in Asian cuisine as a plant-based substitute for meat.
All these products are good sources of protein and are nutritious to varying degrees. But some, like tofu and seitan, are not "complete" sources of protein, that is, they don't contain all the essential amino acids our bodies need. And seitan and Quorn are also highly processed. Seitan would not be suitable for anyone with gluten or wheat sensitivity.
In recent years, a new generation of high-tech products made from plants has been developed to recreate the exact taste, texture and appearance of meat. Most are also highly processed, made with a long list of unfamiliar ingredients and sometimes new production methods. The Vegan Butcher range, for example, lists "soy structure" as the main ingredient in its Chickened Out Burger and Good Karma Shawarma. According to Unilever, which owns the brand, this is an amalgam of water, soy protein, wheat starch and wheat protein.
Beyond Burgers, which are sold in over 25,000 food outlets worldwide and found in the meat section of some British supermarkets, are among the new fake meats made with pea protein isolate. Impossible Burgers, widely available in the US but not yet approved for sale in the UK, are made with soy leghemoglobin. This is a protein that carries heme, an iron-rich molecule that gives the futuristic patties their realistic colour, aroma, and flavour of meat.
Last year, Harvard's School of Public Health researched these novel meats to determine whether they could be considered part of a healthy diet. They concluded that the answer was "far from clear" as studies are currently inconclusive.
However, chair of the Department of Nutrition, Dr Frank Hu, said it couldn't be assumed that the health benefits of fruits, vegetables, wholegrains and nuts were the same as meat alternatives made with highly processed plants.
"Food processing can lead to the loss of some nutrients and phytochemicals naturally present in minimally processed plant foods," he said.
Hu added that a recent study by the National Institute of Diabetes and Digestive Kidney Diseases found a link between highly processed foods and weight gain, although the study did not focus on meat substitutes.
Salt and sugar
Other ingredients going into industrially processed vegan food are also causing concern. To make these products taste as similar as possible to their non-vegan counterparts, manufacturers include many additives, notably salt and sugar. Recent research by content agency JBH revealed some vegan fast food contained much more salt and sugar than their non-vegan equivalents. Subway's Meatless Meatball Marina, for example, clocked in with 3.6g of salt (more than half the recommended daily intake of 6g) and 19.3g of sugar. Its Meatball Sub contained much less of both, with 1.9g and 13.5g of salt and sugar respectively.
Many popular brands of meat-free burgers, sausages and bacon sold in supermarkets also contain high levels of salt, according to Mhairi Brown, a nutritionist and policy co-ordinator with campaigning group Action Against Salt. She says the main problem with these products is the perception – encouraged by food manufacturers – that vegan food is healthy simply because it's made from plants. "They often use green or orange packaging, and also the term 'plant-based', to create a 'health halo'," she says. "People think these products are healthy when that might not be the case at all."
Registered dietician Sophie Medlin agrees. "Many people think that if a food is vegan it's healthier," she says. "The truth is there are some really great vegan alternatives to meat and dairy but there are plenty of food manufacturers simply chasing the vegan pound. Fast food outlets that have questionable animal welfare standards and poor environmental practices are selling vegan alternatives that are often deep fried carbohydrates in a bun."
Deficiencies
Although vegan advocates insist it's perfectly possible to eat a well-balanced plant-based diet, nutritionists are concerned that many people simply don't manage it. Medlin reports a rise in cases of anaemia at her clinic caused by vitamin B12 deficiency. Essential for brain and nervous system function, B12 is naturally found in animal products but generally not in plant foods unless they're fortified, putting vegetarians and vegans at particular risk of deficiency. Untreated, vitamin B12 deficiency can cause irreversible nerve damage.
It's not surprising that demand for B12 injections and intravenous drips at high street vitamin salons is rising. "We administer 20% more B12 shots now than we did two years ago, and around 30% of our customers are vegetarian or vegan" says Richard Chambers, founder of Get A Drip. "In December alone we administered 528 B12 products." (Medlin strongly advises against going to high-street providers for injections or IVs).
Another cause for concern is the risk to bone health caused by calcium and vitamin D deficiency, says Professor Ian Givens, director of the Institute of Food, Nutrition and Health at Reading University. Research shows vegans have lower bone mineral density and fracture rates nearly a third higher than the general population, with teenagers and post-menopausal women were particularly at risk.
"Vegetarian and vegan diets can increase the risk of reduced bone strength and special care is needed to ensure adequate intake of the key nutrients," Givens says.
"We also think there may be a lot of new food allergies issues emerging due to the ingredients being used in some vegan foods, " adds Professor Chris Elliott, from the Institute of Global Food Security at Queen's University Belfast.
"It is too early to say this for sure for we are watching this closely. We doubt very much about how well nutritionally balanced many of these are and will only add to the issues we're already concerned about."
He says long-term studies into these foods are needed.
Heather Russell, a dietician for the Vegan Society, says anyone considering opting for a plant-based diet needs to educate themselves about good nutrition and healthy protein sources (nutrition information is available on their website).
"Whether you're vegan or not, it's a good idea to use food labels to keep an eye on added fat, salt and sugar and limit highly processed foods," she says.
Experts do agree that the healthiest diet includes an abundance of minimally processed plant foods, and limited amounts of the highly processed stuff. But just because food is made from plants doesn't mean it's good for you - that bag of crisps might be vegan, but it's not health food.
