"But then I started reading about the transformative effects of giving up meat and sugar, then carbohydrates, and it went from there."
The results promised on health forums included feeling more alert and energetic. In her pursuit of "wellness", Armstrong went vegan then switched to raw veganism, eschewing all animal-based food products and anything that had been cooked.
Over 18 months she dropped from 70kg to 40kg, stopped menstruating, and became "completely obsessed" with "detoxing and cleansing".
"I even went through a phase of eating only organic melon (because it was water-based and I couldn't see how it would poison me).
"Every time I cut something out, someone else - a nutritionist, well-meaning blogger or someone on a health forum - would tell me I was doing the wrong thing. I was not motivated by weight loss - it was about healing and being pure. You get an adrenalin rush from sticking to it and I would get stressed if I ate anything I considered 'bad'."
Armstrong's case is a textbook example of orthorexia nervosa. The term was coined in 1997 when Dr Steven Bratman wrote of his personal experience of evangelical eating; he wouldn't eat vegetables picked more than 15 minutes earlier and insisted on chewing every mouthful 50 times. He defined the condition as "a pathological fixation on eating proper food".
A bit like obsessive-compulsive disorder and closely connected to anorexia nervosa, although sufferers are concerned with quality rather than quantity of food, orthorexia has yet to be officially recognised as an eating disorder by the Diagnostic and Statistical Manual of Mental Disorders.
As a result it is hard to quantify the rise in cases in recent years, although experts and charities say it has been marked, encouraged by social media, targeting marketing and bad science.
On Instagram, the hashtag orthorexia has almost 70,000 posts, with users swapping clean-eating tips and dietary advice. The charity Beat, which works with eating disorders, blames a shift on emphasis from losing weight to seeking physical perfection.
"The increasing emphasis on body muscle and tone over and above size and shape may well be affecting the incidence of orthorexia, along with more imagery available on social media and the increase in marketed products for 'fit' bodies," says Beat's chief operating officer, Lorna Garner.
Armstrong realised the extent of her orthorexia after moving in with a "glamorous" friend whom she idolised.
"She cooked and went to the theatre and did normal things," says Armstrong. "One day, after I had come back with yet another detox kit, she said to me, 'What are you doing? That sounds like a waste of a life.' I realised that there was nothing healthy about what I was doing."
It is not just women who succumb. Author and wellness blogger Kevin Gianni, 36, whose health and fitness videos have had more than 10 million YouTube views, suffered orthorexia on and off for almost six years. He became a raw-food vegan after being introduced to the concept by a friend and becoming preoccupied with his family's history of cancer.
"I was eating purely, straight from the earth, as nature intended," says Gianni, who is married with two children and lives in California. "I ate kale salad, raw nut berries, goji berries, raw chocolate and dehydrated flax crackers. I drank green smoothies, wheatgrass and hemp milk. I was striving for dietary excellence and trying to be the best I could be. It felt great."
But after 12 months Gianni was struggling to get out of bed, suffering debilitating cramps, anxiety and his sex drive plummeted. A blood test with a doctor confirmed he was dangerously low on key hormones.
"I was at rock bottom. I had low cholesterol, vitamin D and B12. My pregnenolone [a hormone relating to energy, memory function, stress and immunity] was in single digits and the same level as you would find in an 85-year-old man."
But the pull to "eat clean" was hard to resist. "It is brainwashing. I had created and bought into this belief system that if I ate meat, I would eventually die. So many things in our lives are out of our control. For me, orthorexia was about control."
Gianni began to reintroduce food such as yogurt into his diet and slowly reaped the benefits, including improved energy and better sleep. Now he gets his blood tested every year, studies the results and determines what to do from there. Indeed, using annual blood tests to tailor a diet regime is a growing trend among disillusioned foodies.
Markers gauge cholesterol, cortisol, pregnenolone, vitamin B12, vitamin D and iron to determine which diet - from gluten-free to paleo and Atkins to the 5:2 - might work best.
Sport nutritionist James Collins, of the Harley Street Centre for Health and Human Performance, says that diets outlawing whole food groups result in deficiencies. Context, he says, is essential.
"The core principles of a diet might be good, but if you're applying them to someone who is training at the gym daily, working a 10-hour day, or has two children, energy levels are going to flatline. The result can be reduced concentration, irritability and depression."
He conducts blood tests for clients to better understand their metabolism and spot any alarming deficiencies.
It's good to want to be healthy, but the overriding advice from health professionals is in agreement with Oscar Wilde: "Everything in moderation, including moderation."
