A vegan woman who was accidentally served a pork sausage roll said the experience traumatized her and poisoned her for life.

Nutritionally, adding meat to your diet isn't harmful unless you have an allergy, which is very rare.

Still, some people may have a sensitivity to certain foods or even anxiety related to what they eat, which can cause symptoms like heart palpitations and indigestion.

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Sharleen Ndungu just wanted to enjoy a vegan sausage roll, a hugely popular meatless version of the beloved classic, at a Greggs bakery in the UK.

But a few bites in, she had heart palpitations and an upset stomach. As it turned out, she had been accidentally served the meat pastry, she told Vice this week.

Having been vegan for two years, Ndungu said she had a strong reaction to the meaty mistake. She told Vice that she refused a refund but was "traumatized" and "poisoned for life."

Read more: Researchers may have figured out why vegan diets lead to weight and fat loss even if dieters don't cut calories

People may be vegan for many reasons, including for their health, religious values, or ethical and environmental concerns. As a result, accidentally eating meat can be upsetting on several levels.

But an unfortunate bite of meat won't hurt you if you're vegan — let alone cause lasting damage — unless you're one of the rare people with a severe meat allergy, a nutritionist told Insider.

Here's what actually happens when you stop being vegan, involuntarily or otherwise.

A stomachache after eating could be due to any number of things. Sarah Schmalbruch/INSIDER

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 representative 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.

But there's no reason to believe that a food sensitivity would cause any long-term side effects.

"The foods you eat don't stay in your body forever," Foroutan said.

Read more: A 'MasterChef' winner called a vegan customer a d--- after they complained about his restaurant's lack of meatless options

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.

Robin Foroutan, a registered dietitian, encourages people who eat animal products to get the highest quality possible. iStock

For most people, meat can be a part of a healthy diet

The bottom line is that a balanced diet with lots of fruits and vegetables is best for health, Foroutan said. Whether that includes some animal products is a matter of personal preference.

"There is no one dietary solution that works for everyone," she 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.

"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."