Is it more difficult for vegans to push themselves to physical extremes?

Maria Strydom set out for Everest with a mission.

The 34-year-old, a South Africa-born lecturer at a business school in Melbourne, was determined to climb the world's highest mountain with her husband - and to set an example.

In an interview published on her university's website in March, Ms Strydom said she and her husband Robert, both vegans, hoped to show the way by ascending Everest and the highest mountains on other continents.

"It seems that people have this warped idea of vegans being malnourished and weak," she said. "By climbing the seven summits we want to prove that vegans can do anything and more."

Sadly, Maria did not make it to the summit. She was one of three people to die on Everest over the past three days.

All the signs are that she died from the effects of altitude sickness before she was able to reach the summit on Saturday. Robert reportedly also fell ill, but survived.

Many of the headlines since then have inevitably focused on the couple's lifestyle, However, there is no suggestion it played a part in what happened on Everest.

But were Maria's concerns over people's perceptions of vegans fair? And what challenges do vegan adventurers in particular face?

By committing to a diet that omits meat, fish and dairy, vegans risk missing out on certain nutrients that are crucial to building strength. Seeking alternative sources for those nutrients is the hard work.

"You can do it and you can be a serious elite athlete," Dr Frankie Phillips, a registered dietician with the British Dietetic Association, told the BBC. "You can do it, but it's more difficult."

Among the possible consequences of a poor vegan diet, she said, would be increased fatigue due to lower iron levels; weaker muscles because of a lower intake of amino acids that generate muscle mass; and the risk of bones fracturing because of less calcium in the diet.

All the required elements can be obtained from sources that fit a vegan diet or from supplements, Dr Phillips said. For example, two good vegan-friendly sources of amino acids can be combined to be more effective - a peanut butter sandwich does this job particularly well.

But, Dr Phillips said, it was crucial to seek a dietician's advice if you were a vegan looking to take part in sports that involve extreme physical endurance.

"Another issue is the sheer quantity you would need to eat," she added, an issue especially pertinent on Everest, where every kilogram counts as kits are carried up the side of a mountain. "Lots of animal-based foods are more densely-packed with energy, protein, vitamins and minerals.

"Because there's so much fibre in a vegan diet, you could be spending much more time eating. Look at the Tour de France - the time cyclists have to take in enough nutrition is minimal."

There is one big advantage vegans have over their meat-eating counterparts, though - and that is their lack of bulk. "Vegans tend to be a lot leaner," Dr Phillips said. "They carry much less body fat."

Two days before Maria Strydom and her husband attempted their ascent of Everest, another vegan climber made it.

Kuntal Joisher, a Mumbai-born computer programmer, climber and adventure photographer now living in Los Angeles, wrote on his Instagram account that he had reached the summit.

It was not his first attempt to reach the top of the world. In 2014, a large avalanche killed 16 sherpas near Base Camp in what was then the largest accident on Everest. The climbing season was cancelled and Mr Joisher's attempt that year ended.

Then, in April last year, a large earthquake struck Nepal and swept away much of Everest's base camp while Mr Joisher was there.

In 2014, after climbing Mount Elbrus in Russia in strong winds and whiteout conditions, Mr Joisher wrote an article for the Huffington Post website describing how he adapted to the challenge as a vegan.

"I was able to adjust to all of these severe conditions while eating healthy vegan food such as raw vegetables and fruits, buckwheat, rice porridges, wheat breads, and dried fruits and nuts," he wrote.

"There are plenty of vegetarian foods that are part of a core diet during high-altitude climbing expeditions. As such, veganising the menu isn't that difficult."

The main issue, he said, was finding clothing and equipment that was suitable for vegans - he said he was unable to find an alternative to the down jacket needed at the highest altitudes.

Venus and Serena Williams

The tennis-playing sisters admit to sticking to a "Cheagan" diet - one that is almost all vegan, but with the occasional 'cheat' ingredient.

Venus Williams suffers from Sjogren's syndrome, an autoimmune disease, and has to be careful about all she eats. She settled on a vegan diet as the best way to do so.

"You also have to look at everything else in your regimen, what you're putting into your body, like supplements," she told Shape magazine. "I'm always learning and I'm hoping to perfect my system."

David Haye

The British former heavyweight boxing champion became vegan two years ago for ethical reasons but has credited the change with an improvement in his health, and in helping him lose body fat.

"A lot of the meat that people eat has been genetically modified, or if it hasn't then the food the animal's been fed has been," he said in an interview with the Telegraph this month. "That's tough for a human being to process, so cutting it out made me feel immediately better and stronger than ever."

He also said his eczema and dandruff problems had stopped as soon as he changed his diet.

Meagan Duhamel

The Canadian figure skating world champion became vegan in 2008 after picking up a book on the subject at an airport.

"When I'm making my breakfast, I'm thinking, 'What am I going to eat that's going to fuel me to train better, focus for a longer period of time?'," she told CBC Sports. "And at night it's more about, 'What can I eat that's going to help me recover faster before I go back to training tomorrow?'"