A "poorly planned" vegan diet could be potentially fatal, a Queen's University Belfast academic has warned.
Prof Chris Elliott said a lack of essential micronutrients could have severe consequences and lead to "hidden hunger" in the developed world.
Poor bone health, lower omega-3 and iodine levels and a vitamin B12 deficiency are among those cited.
Hidden hunger occurs when the quality of food people eat does not meet their nutrient requirements.
But a dietician from The Vegan Society has refuted these claims.
"Well-planned vegan diets contain all the nutrients that our bodies need," Heather Russell told BBC News NI.
There are four times as many vegans in the UK as there were four years ago - they do not eat meat, poultry, fish, or any products derived from animals, including eggs, dairy products, and gelatin.
In an article published on academic website The Conversation last month, Prof Elliott and QUB lecturers Claire McEvoy and Chen Situ suggest vegans can prevent micronutrient deficiency by consuming foods fortified with vitamins and minerals, as well as taking supplements.
"We work with the British Dietetic Association to share the message that they can support healthy living in people of all ages," said Ms Russell.
"If you're switching to a totally plant-based diet, set aside a bit of time to work out how you're going to hit your nutrition targets without animal products."
Bethany Kenny, from Dromore in County Down, developed a vitamin B12 deficiency after turning vegan almost two years ago.
"For sure there were some 'issues' but that was due to my inexperience," she told BBC News NI.
"When I was brushing my hair it was coming out a lot more than what was normal before."
But after a month on tablets, recommended by a pharmacist, the 23-year-old says she now feels healthier than ever.
"It's important to know that your body has become used to specific foods therefore to change all that will obviously induce some sort of change," said Miss Kenny, who teaches English in Spain.
"It took me about a year to understand what I needed to have a well-rounded vegan diet but once I'd figured it out I noted a change in my lifestyle.
"I had more energy, my skin cleared up, I started to really enjoy eating vegetables and most of all, when I did go to the gym, I found it a whole lot easier to run for 30 minutes on the treadmill than I ever had before and that's when I really appreciated the benefits.
"Now I continue to take a multivitamin everyday, but that's just me."
Business owner Clare Hickey set up a vegan coffee shop in south Belfast in a bid to change people's perceptions on veganism and make vegan food more accessible to the people of Northern Ireland.
But she told BBC News NI she was careful not to "plaster the word 'vegan' all over the place" when she opened 387 Ormeau Road last year.
"We wanted to attract a broad demographic of people and show them what we can do without being judged from the onset," said Ms Hickey.
"Once inside, people quickly realised it wasn't all quinoa and avocados.
"I grew up in the country and comfort food for me is big plates of stodgy food.
"I have the ability to recreate these dinners using meat alternatives on the market, so it never felt like a big dramatic change to life as I knew it."