A vegan said he got sick after Domino's served him pizza with real ham. Here's what really happens when vegans eat meat.

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• A Domino's restaurant in Australia accidentally served real ham to a vegan man who had ordered a new plant-based pizza.
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• The man said he felt shocked, betrayed, and ill after finishing the meal, which employees had repeatedly reassured him was meat-free.
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• Accidentally eating meat may be upsetting, but it's unlikely to cause serious harm unless you have a rare allergy. However, there are other good reasons to stick to a plant-based diet.
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• Visit INSIDER's homepage for more.

A vegan man in Australia has spoken out after a local Domino's restaurant seemed to have accidentally served ham on what he thought was a plant-based pizza, according to Daily Mail Australia.

Patrick Hukins, who has been vegan for four years, said he felt sick to his stomach and betrayed after the supposed mix-up. He had finished his meal after double-checking with employees that it was indeed plant-based, and became ill shortly after.

After Hukins complained via social media with photo evidence of what he said was real meat, Domino's apologized and offered the man a voucher, though it was only enough to pay for a traditional, non-vegan, pie, according to the Daily Mail.

A vegan said she was 'poisoned for life' after accidentally eating meat. Here's the reality of what happens when you stop being vegan.

"I felt shame and pain for the animal. And felt guilt for having unknowingly consumed a product that goes against my endeavors to reduce my environmental footprint as well as harm to other animals ... I was shocked," Hukins told the Daily Mail.

There's a wealth of evidence that a mostly plant-based diet is best for your health, as well as the environment. But even if you've been vegan for many years, becoming an accidental carnivore for one meal is unlikely to do lasting damage or even make you sick, although it may be very upsetting.

Here's what actually happens when you stop being vegan, involuntarily or otherwise, according to a registered dietitian.

Eating meat after abstaining for years is rarely dangerous but could cause mental and physical reactions

Most of the time, when a vegan or vegetarian starts eating meat after a long period of abstaining, what happens is ... nothing, according to Robin Foroutan, a registered dietitian nutritionist and spokesperson for the Academy of Nutrition and Dietetics.

Some people may feel as though they have a harder time digesting meat if they aren't used to it, Foroutan said, but there's no scientific evidence for this.

There could also be a psychological element at play if people become anxious or even have a panic attack when they realize what they're eating, Foroutan said. "Distress could be one cause of that kind of response," she said.

It's also possible that some vegans have a food sensitivity to meat, causing symptoms like headaches, bloating or gassiness, heartburn, or irritability. While food sensitivities are vague and poorly understood, they're believed to be a result of the wide variation in people's bodies and digestive systems. Eating specific foods may trigger inflammation or antibodies in some people.

Allergies to any type of meat are possible but uncommon, according to the American College of Allergy, Asthma & Immunology. Unlike a food sensitivity, a food allergy can be consistently diagnosed with a type of test to detect the presence of immunoglobulin, which is created by the immune system reacting to the allergen.

A particularly dangerous type of meat allergy can develop from an insect bite. People bitten by the Lone Star tick can become allergic to red meat and animal products, which could cause a life-threatening anaphylactic reaction. This tick is found in the US, mostly in southeastern states.

Donald Bowers/GettyImages

Evidence shows a plant-based diet has lots of benefits, but meat can be perfectly healthy in moderation

There is some evidence that people on a vegan diet tend to have healthier gut microbiomes, or the collection of good bacteria that live in the digestive tract.

This is in part due to the extra fiber vegan eaters can get from a veggie- and whole-grains-rich diet; fiber helps cultivate healthy gut bacteria.

Studies have also suggested that a vegan diet cultivates different types of bacteria than an animal-based diet. Vegans' bacteria, the findings suggest, can help lower inflammation in the gut and may have other benefits like boosting metabolism.

Read more: New dietary guidelines suggest people should keep eating red meat and processed meat, but nutrition experts says the report is irresponsible

A vegan diet is also a positive move for the environment, since meat production has been proven to be a significant source of greenhouse gasses and contributes to other environmental damage by using a huge amount of natural resources like land and water.

Still, it's unlikely that a single meal can override an otherwise healthy diet - research shows that even a mostly vegetarian or vegan diet, with occasional animal products, has health benefits.

"There is no one dietary solution that works for everyone," Foroutan said. "I encourage people to eat mostly plants, foods high in antioxidants - and if you do eat animal products, get the highest quality available to you."

If you're considering a major shift in your diet, however, such as reintroducing certain foods, it's best to do so gradually if possible, Foroutan added.

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"It's really important to pay attention to your body, what it needs, and how you're feeling," she said. "But as long as the bulk of the diet is plant-based, you'll be in good shape."

A 25-year-old may go blind after eating nothing but pizza, pasta, fries, and chicken nuggets for 22 years,