If it’s not fermented, don’t eat it.

That’s a rule from a best-selling diet book that a health guru—maybe you, or Gwyneth Paltrow—could write. The cover could be you and Gwyneth surrounded by honey and dirt, applying probiotic ointments, eating kimchi and smile-laughing over a cauldron of home-brewed kombucha.

Kombucha is a smart choice, because the drink has the fastest-growing segment of the “functional beverage” market in the U.S.—a category vaguely defined by one industry publication as “drinks with added functionality, such as ingredients and associated health benefits and functional positioning.” As in, water isn’t functional. Or, used in a sentence: “Kombucha now occupies about one-third of our refrigerated functional-beverage shelf.”

That’s according to Whole Foods. Overall, kombucha sales in the U.S. this year will be around $600 million, with projections for 25 percent annual growth. Last month, PepsiCo acquired the small kombucha company Kevita for around $200 million. At least in that way, the drink is functional.

The unique functional ingredient, meanwhile, is microbes. The recent spike in popular awareness that not all bacteria are evil—and that many are good and necessary to human health—has created a sort of fascination with live cultures and fermented products. That idea has brought the centuries-old drink roaring into upper-middle class consciousness at $5 per bottle in New York bodegas.

So chapter one of your fermented-health book would be “What is kombucha?”

It’s a short chapter; space can be filled with illustrations. Kombucha is a sugar-sweetened tea (black or green) that has been mixed with yeast and bacteria and then given time to ferment. The microbes are together known as a SCOBY (symbiotic colony of bacteria and yeast). The entity looks like a pallid slab of human subcutaneous tissue, or a shiny undercooked pancake. It is alive and self-perpetuating—new starter colonies (sometimes known as “daughters”) typically come from other kombucha (“big momma”).

The drink has been around in some form for between 2000 and 200 years—its history is shrouded in mystery, but it does involve transport out of Asia by German World War I POWs, and then seeding in California during the AIDS epidemic. The mass-market U.S. boom has really come in the last decade, increasing more rapidly year over year. As the importance of gut microbes in health has come to light, sales of live-bacterial products like kombucha suddenly have seemed to offer some explanation for long-held health beliefs.

Part of the idea is that ingesting life microorganisms is good for the balance of the ecosystems that live in our guts, commonly known as the gut microbiome. These beliefs are at the center of the marketing strategy by the largest producer of kombucha in the U.S., GT’s Kombucha (“Living Food for the Living Body”). The company’s mission is to “combine the wisdom of ancient medicinal foods with the resources of the modern day to create products that uplift and enlighten the health of all those who enjoy them.”

The ethereally young-looking founder, GT Dave, describes himself as “a lifelong practitioner of health” whose bottled microbial product “bolsters immunity.” In 1994, GT’s mother, Laraine (who inspired the brand’s logo when she conceived GT in the lotus position), was diagnosed with breast cancer. GT came to believe that kombucha “played a strong role in helping her to overcome the breast cancer.” Other kombucha producers—and consumers—have made claims about improved liver function, weight loss, improved complexion, and  reduced hair loss.

Such claims have not born out in clinical studies; while gut microbes are vital to health, maintaining them isn’t necessarily best approached by introducing random microbial colonies floating in fermented tea.

All of this I know, and yet I buy kombucha occasionally, because I started to like the flavor, which is sometimes worth the financial regret. And if there is one claim that seems to have merit, it’s GT Dave’s insistence that kombucha “makes your spirits fly. You feel on top of the world.”

In other words, kombucha contains alcohol. The yeast consumes the sugar, fermenting it into carbon dioxide and ethanol. Some of that is converted by bacteria into acetic acid, but the process isn’t perfectly predictable, so alcohol remains in various amounts.

I became acutely aware of that when I got carded for buying a bottle. The first time it happened, I laughed. But the young man behind the counter looked back at me dead seriously and pulled the bottle back toward his green smock. This was at a Whole Foods in Brooklyn. (I know, I know—an alternate headline here: “I Got Carded Buying Kombucha at the Whole Foods in Williamsburg And I’m Not Going to Take It Anymore.”)

But it made me curious. I thought there was only, as GT’s label says,a trace amount” of alcohol? What am I actually drinking?

Asking that question lead me into the world of kombucha production that took me all the way to Washington, sharing a bottle on the steps of Congress with a U.S. representative, hoping we didn’t get nailed for an open container.

Hamblin and Polis discuss kombucha regulation.

How much alcohol is in kombucha?

This is the key question. Like most health questions, it’s actually about money.

If you’ve kept up with the kombucha wars in the United States over the past few years, then you know Jared Polis. Polis doesn’t come from kombucha money. He founded Blue Mountain e-greeting cards and sold it for $780 million at age 23. Since 2009, he’s been a U.S. Representative from Colorado’s second district. In his office in DC he showed me a picture of him having a kombucha with Cindy Lauper.

Polis took up the mission when he learned that some small kombucha breweries— actually, that’s a loaded term … kombucheries?—were feeling the impact of federal regulation. In 2010, there was a federal crackdown over some hyper-fermented bottles that made news. According to Kombucha Brewers International President Hannah Crum, “the kombucha crisis” began when with a routine audit of a Whole Foods in Portland, Maine. There a consumer-protection inspector with Maine’s department of agriculture found that some of the kombucha bottles were leaking, which got him thinking about what happens when you combine sugar and yeast: “Kids could get hold of this and get a buzz.”

Samples of several brands were sent to the University of Maine, Crum recounts, where tests showed alcohol levels of 0.5 percent to 2.5 percent by volume. (Beer tends to be around 5.0 percent.) None of that is legal to be sold to minors; the U.S. government’s Alcohol and Tobacco Tax and Trade Bureau (TTB) requires anything above 0.5 percent to be regulated as an alcoholic beverage.

Whole Foods pulled kombucha from its shelves until it could figure out how to proceed. Crum describes this as a devastating blow to the industry just as it was taking off. GT’s and other brands tweaked their formulas and made it back to shelves, and Whole Foods implemented testing requirements. Though that added costs for kombucha producers, things seemed to be clearing up for the industry. But then, again, in 2015, a series of kombucheries received warning letters.

Polis came to the rescue, issuing a stern letter back to the TTB. The crackdown threatens small businesses, he writes, which are unfairly punished by standards that hold producers accountable for alcohol levels that may be the result of improper storage. He also wants a more accurate, less expensive testing process for alcohol content, which the kombucha industry is currently working with scientists to develop and vet.

The alcohol-level testing process is difficult for several reasons— including the caveat that according to the TTB, “Regardless of the alcohol content of the finished beverage, when kombucha reaches 0.5 percent alcohol or more by volume at any time during the production process, it must be produced on a TTB-qualified premises and is subject to TTB regulation.”

Emphasis mine; essentially this means that a product is being regulated based on what it was, not what it is. And because fermentation continues after the product leaves the kombuchery, some variables are beyond the control of the producers.

The FDA has suggested that kombucha be pasteurized, killing the microbes before delivery to the consumer. While that would help to standardize the alcohol content, it would ruin the whole idea of the drink and its purported benefits.

Without pasteurization, though, any bottle of kombucha that sits too long unrefrigerated, or simply too long in a refrigerator, or simply rots for another reason, could have a significant alcohol content. (In addition to overgrowth of bacteria that can sicken people—the very thing that pasteurization was invented to prevent.)

In light of the recent crackdowns, GT’s now sells different formulations of kombucha, one of which is only intended for people over 21. It turns out that was the kind I picked up, and the people at Whole Foods were right to card me. I apologize for my outburst.

For smaller producers, though, creating multiple product lines isn’t an easy option. As kombucha and other live bacterial products become ever more common, the issue of regulation only stands to grow. In a rare alliance, free-spirited believers will side with ardent conservatives in demanding that big government stay out of their lives. Paired with Donald Trump’s staunchly anti-regulation cabinet picks so far, the years ahead could be a time of great prosperity for sellers of microbes and actively fermenting products—and the people who want to imply these products will prevent and cure disease. The challenge could fall to consumers to discern what’s true and what they’re actually drinking.