“Cereals here in the United States contain a packaging ingredient called—God, I’m paranoid." The natural-food advocate Vani Hari paused, laughing, looking at a man standing a few feet from our table in a Union Square coffee shop. He was huddled over his phone, just waiting for his coffee—or so it seemed. She lowered her voice, continuing, barely audible: "… called BHT."

Hari looked in my blank eyes. I asked, "In the plastic bags?"

She nodded as if I'd just been let in on the secret to end all secrets. "And in the U.K., they can't use it," Hari, who is better known through her blogging, speaking, and TV appearances as "The Food Babe," continued. "The purpose of it is to leach into the cereal, so it keeps it fresh. And, how many millions of kids are eating this every single day?"

"Why did the U.K. take it out?" I asked.

"They don't allow it," Hari said.

"They must have a reason."

"There are studies that suggest it's linked to cancer, tumors," she said. "It's an endocrine-disrupting chemical."

Such is the gist of many of the food-additive campaigns that Hari has undertaken: A chemical in the U.S. food supply is not allowed in other countries, so why is it being used here? Petition the food companies to take it out. Over the past three years, Hari has rapidly become one of the most popular voices on nutrition in mainstream media. She has lived the American dream: monetizing a lifestyle blog and quitting her job to write about what she's eating and why.

Hari is now working on developing a TV show, and her first book, released yesterday, is bound to lead bestseller lists. The title, a mouthful, leaves little to the imagination: The Food Babe Way: Break Free from the Hidden Toxins in Your Food and Lose Weight, Look Years Younger, and Get Healthy in Just 21 Days! It is more than just another ultra-simple diet plan, or a compendium of claims intended to provoke, devoid of nuance, though it is also those things. ("Could an apple be more fattening than a hot fudge sundae? Quite possibly, especially if you consider the exposure and accumulation of pesticides over time in the body.")

The book also offers the origin story of The Food Babe—how she left her job as a financial consultant and, despite no training in human metabolism, toxicology, or environmental science, became an unintentionally influential figure in public health. The book does little to address that she has also drawn the ire of many scientists who believe her claims are inaccurate or even dangerous. But Vani Hari did not intend to attract attention on the scale that she has. Her crusade began simply enough, with her own health issues, and the recovery that ensued after she discovered an all-natural approach to life. "Everything I had been putting in my body," she writes in the book, "was either made from something out of a chemical factory, sprayed with chemicals, or genetically modified to make companies richer and me sicker."

Hari's secrecy when we met in New York was not because the story of butylated hydroxytoluene (BHT) was a particularly hot one. The additive has been widely used in cereal packaging for many years. BHT has to be listed as an ingredient on food labels, and some consumer-protection advocates like the Environmental Working Group have advised people to avoid it when possible. BHT is not a listed carcinogen by the International Agency for Research on Cancer, but at high levels of exposure, rats have been found to develop lung and liver tumors, as well as problems with motor skills. These issues have not proven themselves to be relevant to humans, so the Food and Drug Administration classifies the chemical "Generally Recognized as Safe."