Katharina Wirnitzer was in the midst of training for the Bike Transalp race, one of the world’s toughest endurance events, when she began investigating whether a vegan diet was suitable for athletes.

The year was 2003 and veganism was a long way from the current boom, which has established it as one of the most in-vogue dietary trends. But Wirnitzer, a sports scientist at the University of Innsbruck, had become intrigued by the resurgence of ancient theories linking plant-based diets with improved athletic performance.

“The first athletes on strict plant-based diets were gladiators,” she says. “Roman scripts report that all fighters adhered to gladiatoriam saginam, which was based on plant foods, including large amounts of legumes, pulses and grains, and contained little or no animal protein.”

Now, almost two millennia later, Wirnitzer is one of a handful of researchers trying to get to the bottom of whether veganism could enhance an athlete’s chances of sporting success. Over the past decade, she has led the NURMI study, the broadest initiative so far investigating the effects of a vegan diet in high-performance, ultra-endurance sports.

NURMI is particularly timely because veganism’s association with various health benefits – from weight loss to decreased risk of inflammatory disease – has seen the diet soar in popularity in recent years, both amongst the general public and elite sportsmen. The most recent survey by the Vegan Society estimates that there are around 600,000 vegans in the UK – a fourfold increase over the past five years – while high-profile athletes from Lewis Hamilton to Jermain Defoe have begun experimenting with veganism.

However, despite the boom in veganism, even the most optimistic scientists caution that there is still much we do not understand about the diet. In particular, little is known about the long-term consequences of veganism and whether it does hold significant advantages over an omnivorous or vegetarian diet.

Portrayals of the diet can be partisan: the recent blockbuster Netflix documentary The Game Changers has since been tainted by revelations that the executive producers are cofounders of a vegan food company and that much of the evidence presented in the film is selective, low-quality and anecdotal. Moreover, as with so many dietary interventions, the search for the truth about veganism is often clouded by the potential financial gains – with predictions that the global vegan food market will be worth $24.3bn by 2026.

This is perhaps unsurprising. Whether it be the trendy city bars offering vegan wine, or the array of new products launching in supermarkets and health food stores, veganism is the wellness industry’s new cash cow. Market-research experts have already predicted that the value of the global vegan food market will reach $24.3bn by 2026. Vegan cheese alone is expected to develop into an industry worth nearly $4bn within the next five years.

So what do we really know about veganism and what it can do for our health?

The quest to reduce cardiovascular disease

At Sheffield Hallam University, David Rogerson has spent the past decade studying the effects of dietary interventions on physical health. He says that one reason veganism could be good for you is because it can protect against cardiosvascular diseases, by reducing obesity and lowering cholesterol. These chronic illnesses cost the UK around £9bn a year; veganism may be the solution.

“There’s growing evidence that reduced consumption of animal products, coupled with an increase in plant-based foods, seems to be good for our health,” says Rogerson. “This is perhaps due to these foods containing lot of antioxidant phytonutrients and nitrates, while some animal products contain lots of pro-inflammatory fats and lead to the production of a metabolite called TMAO, which has been linked to cardiovascular problems.”

The anti-inflammatory effect of plant-based foods is thought to be the reason why vegan diets appear to relieve symptoms of some auto-immune diseases such as rheumatoid arthritis. The tennis player Venus Williams, who suffers from Sjögren’s syndrome, credits turning vegan with mitigating the extreme fatigue associated with the condition, and with enabling her to continue competing at the highest level.

The full picture is rather more complex than it first seems. Scientists have found that a combined group of vegetarians and vegans appeared to have a higher risk of haemorrhagic stroke than did meat-eaters. But owing to the small number of vegans in the study, it is hard to draw firm conclusions. “Possible reasons might be related to lower cholesterol levels or a deficiency of some nutrients, such as vitamin B12,” says Tammy Tong, a researcher in the University of Oxford’s Nuffield Department of Population Health. “Vegans are also at a higher risk of B12-deficiency, since the nutrient is only naturally available from animal foods. Low B12 levels may be linked to raised blood levels of homocysteine, which may be linked to higher risk of stroke.”

While vegan lobby groups have claimed that the diet results in a healthier gut microbiome and reduces the risk of some cancers, compared to meat-based diets, experts say there is little concrete evidence to back this up. “There was one US study which looked at all gastrointestinal-tract cancers combined and found no difference in vegans compared with non-vegetarians,” says Tong. “Two studies have looked at colorectal cancer risk in vegans and both reported no significant difference compared to non-vegans.”

The reason we still know relatively little is because while the term “vegan” was coined in 1962, for a long time scientific studies classed vegans and vegetarians together. But with increasing amounts of sports-science funding going into studying veganism, it may actually be through athletes, and their endless quest for “faster, higher, stronger”, that we learn most about the diet in the years to come.

High hopes but little evidence

The NURMI study follows 8,000 runners from across Europe, including meat eaters, vegans and vegetarians and aims to see whether following a vegan diet over time leads to greater endurance over the half-marathon and marathon distances. In the next few years, NURMI will publish one of the first analyses of how vegan runners compare to their meat-eating equivalents and, according to Wirnitzer, we are still in the infancy of understanding how our nutritional intake can boost athletic ability.

“There is huge potential that is still untapped, both in terms of health and performance in sporting competition,” she says.

One of the reasons athletes across such a range of sports are interested in the vegan diet is because it may boost immunity as well as aiding recovery and rehabilitation from injury. Plant-based foods such as beetroot are known to contain dietary nitrates that aid blood-flow, and oxygen and nutrient transport through the body.

“Elite athletes are looking at all available legal options to enhance their performance,” says Richard Brennan, managing director of Sports Science Consultants, who is studying athletes who have been meat-eaters all their lives, and are now moving towards a vegan diet. “What we’re focusing on are the benefits to overall health which could enhance the training responses in terms of conditioning different energy systems, adapting more effectively to strength and power training programs, and having less time off sick to train.”

These are the hopes for veganism, but scientists warn that, so far, there have been so few studies of athletes that there is very little evidence to support them. Wirnitzer published a landmark 2014 paper that showed that a well-planned vegan diet meets the nutritional requirements of endurance athletes, but we still know virtually nothing about whether it is the optimum diet.

Scientists have raised concerns that the diet is too restrictive for athletes who are travelling the world competing in sporting competitions. Athletes could become malnourished, be unable to maintain muscle mass and suffer deficiencies in B12 (which would lead to fatigue and poor oxygen transport), calcium and vitamin D.

“There’s the potential for lower intakes of these minerals which play a role in bone health,” says Rogerson. “There is evidence to say that vegans experience greater bone turnover and reduced bone-mineral density, so this could mean that vegans are at an increased risk of bone injury. We also know that female athletes might be at an increased risk of such injuries if they don’t eat enough, so this is potentially a double-whammy.”

How practical is a vegan lifestyle?

Concerns about the practicality of veganism extend to the general population. One question is whether vegans can plan their diet well enough over many years to avoid developing deficiencies. There have been two population studies that have monitored vegans over time, one following Seventh Day Adventists in the US and Canada, and the EPIC-Oxford study, which tracked the health of nearly 50,000 meat-eaters, vegetarians and vegans across the UK. Scientists involved in the latter have found that while consuming vegetables rich in calcium, such as kale and broccoli, can protect bones, in reality many vegans don’t actually meet their calcium requirements. As a result, they have found a 30% increased risk of fracture in vegans compared to vegetarians and meat eaters.

“More research is still needed to understand possible differences in fracture risks and whether any differences are related to diet or other factors,” says Tong. “For example, low BMI has also been linked to higher risks of some fractures and in some studies vegans exhibit lower BMI and bone-mineral density than do vegetarians.”

Because of these concerns, some research groups have begun comparing veganism to other diets rich in plant-based foods, which are associated with many of the same benefits, such as the Mediterranean and New Nordic diets. Earlier this year, researchers at Sheffield Hallam University conducted a pilot study comparing a Mediterranean and vegan diet over a short-term period, with intriguing results. While both diets appeared to offer similar positives in terms of weight-loss and reduced cholesterol, evidence was much stronger for a Mediterranean diet when it came to improving blood-vessel health.

“Our findings suggested that the Mediterranean diet improved the way that the endothelium of the small veins work,” says Markos Klonizakis, one of the scientists who ran the study. “This might not sound important, but it is. This becomes dysfunctional over time so it is crucial for cardiovascular health. The magic of the family of Mediterranean diets is that they are tested and proved over a very long period of time, in a relatively large area of the globe. For example, we know that traditionally people in Crete lived long and had low rates of diabetes and cancer.”

So what next for veganism? Scientists across the board agree that we don’t yet know enough to decide conclusively one way or another, but as many point out, the success of any diet ultimately comes down to the eating habits of the individual.

“The success of a vegan diet will rest on the conscientiousness of the individual undertaking it,” says Rogerson. “It’s restrictive and unless we pay attention to the elements of the diet that it excludes, then we might be putting ourselves at risk of developing deficiency-related problems. It has become easier to follow with vegan-friendly food products in supermarkets, which are fortified with nutrients that can be absent from the diet.

“Another point is that people who choose to adopt a vegan diet might be more inclined to adopt health-related behaviours than the norm. Such groups might be more inclined to exercise and be aware of the nutritional adequacy of the foods they eat. We need to look at this further.”