Alex Jamieson’s veganism became world famous when she appeared in the 2004 film Super Size Me, the seminal Oscar-nominated documentary that probed our relationship with fast food.

Jamieson was working as a vegan, health-focused private chef in New York when a conversation between her and then boyfriend Morgan Spurlock – the film’s star and director – led to the idea. Its success took them to 20 countries, won Jamieson deals for three books, including Vegan Cooking for Dummies, 25,000 subscribers to her vegan recipe and mindset newsletter and $7,000-an-hour speaking engagements across the US.

Then, in 2012, she began craving burgers. “I started having dreams about salmon and waking up shocked, thinking I’d eaten it,” she says. “I went out for dinner and ordered tofu; a girlfriend ordered a burger and my mouth was watering.” The irony was not lost on her, having gained followers and built a career after helping Spurlock back to health though veganism after his 30-day McDonald’s diet for the documentary. More seriously, though, more than a decade of veganism had made her ill.

“I initially went vegan around 2000 to help manage major health issues. I was a sugar addict with migraines most days.” A doctor suggested she change her diet. “There was a wall of health books in the library,” she says. “I’m a jump-in-with-both-feet person so I read everything I could. I’d never heard of veganism but after 10 days cutting out animal products, sugar and gluten, I felt clear-headed.”

She quit her job in an entertainment law firm and retrained as a specialist chef. “I didn’t know how to cook for a vegan life and couldn’t just eat quinoa and blueberries,” she says. What she cooked in Super Size Me (with recipe inserts included in the DVD format) became “my whole identity”, she says. “Instagram and the word influencer didn’t exist then, but it did the same thing, it launched my personal brand.” The speaking circuit and coaching followed.

But by 2012, by then divorced from Spurlock and with a six-year-old son, her energy was so depleted that she couldn’t get off the couch. “The doctor said I was so anaemic she didn’t know how I was standing up,” she says. Knowing she needed iron, Jamieson started craving animal protein but was afraid of the public reaction. “The vegan world was like any other community: people can be shitty. I’d often hear rumours of: ‘I saw so-and-so author eating eggs.’ I was worried that would be me.”

She gave in on a trip to Costa Rica, with non-vegan friends who preferred she get well than fear judgment. “I still remember the first bite of white fish, cooked lightly in olive oil with a little salt,” she says. “I felt simultaneously fantastic and guilty. I was ashamed that I wasn’t good enough.” The next year was very stressful. “I’d buy fish at the market and hide it under the kale. My acupuncturist recommended I eat liver, so I bought paté. I thought maybe I could just have animal products now and again to feel well.”

The commercial impact of telling her followers was a consideration, but the bigger dilemma was the moral one. “I felt like a liar. I stopped calling myself vegan and started using ‘plant-based’. I expanded my repertoire to healthy eating.”

Eventually, in March 2013, she put up a blog post entitled “I’m not vegan any more”. It went viral. “I woke up to an avalanche of horrible emails. People not just wishing me dead but making vague death threats. Friends in the vegan world uploaded whole posts of their own calling me the devil. That was the worst part – people I actually knew turning their back.”

She lost thousands of newsletter subscribers and speaking gigs dried up; there was a wave of negative book reviews, too. But she also became “a vegan confessional booth”. “People whose entire business was about veganism messaged privately or sent a text asking to go for a drink and said they’d been eating meat, too.”

Now 49, she says: “I definitely lost out. I also grappled with whether I’d set unattainable standards and made anyone feel worse about themselves. I had to be honest so I could move forward.”

The virality of the blog and its backlash created new opportunities. She was asked to write another book, Women, Food and Desire, and now works as a creative leadership coach, with 15,000 Instagram followers. She is also an abortion rights activist.

“I eat meat almost every day,” she says. “Breakfast is three eggs and bread, and I still really like liver. There’s something on a cellular level that is so satisfying.”

What she went through made her bolder. “I’ve done the best I can,” she says. “Now that I’ve gone through it once, spectacularly, I’m much more courageous about the changes I make in my life.”