Alison Young

USA TODAY

Makers of dietary supplements are at it again, putting a new chemical stimulant – untested on humans — into workout, weight-loss and brain-function products sold widely through retailers including Amazon and GNC. Yet federal regulators have taken no action despite the urging of scientists and a supplement industry group.

The compound, often listed on labels as AMP Citrate, is a close chemical cousin of DMAA — a stimulant the U.S. Food and Drug Administration has warned is an illegal ingredient that carries risks of heart attacks, seizures and neurological conditions. Scientists in the United States and the Netherlands testing several products found that the amount of the new synthetic stimulant varies widely, the dosage isn't disclosed on labels, there's scant evidence the chemical is natural and none that it's safe in humans.

"We want the FDA and we want the stores to immediately remove these products from store shelves," said Pieter Cohen, an assistant professor at Harvard Medical School, and co-author of research published Wednesday in the scientific journal Drug Testing and Analysis. The researchers tested 14 products listing AMP Citrate, 4-amino-2-methylpentane citrate or some of the other chemical names used to describe the new stimulant. All of these are marketing names for 1,3-dimethylbutylamine, or DMBA, which was given to a few dogs and cats in two experiments published decades ago, Cohen said.

"They're taking something described in an obscure article in the 1940s, synthesizing it in pharmaceutical doses and selling it directly to consumers," Cohen said of the chemical. The research team shared their concerns with the FDA last week, he said.

The FDA, in a statement, said it "is aware of concerns regarding DMBA/AMP Citrate and will consider taking regulatory action, as appropriate, to protect consumers."

On Sept. 12, the Council for Responsible Nutrition, a dietary supplement industry group, separately sent a letter to the FDA urging regulatory action on AMP Citrate, pointing out that it is being marketed as the "next DMAA," that it has a similar chemical structure to the banned ingredient, and noting that none of those selling it have filed required "new dietary ingredient" paperwork with the agency to substantiate its safety. "CRN is concerned about the potential dangers to consumers who may be using possible unsafe supplements containing Amp Citrate," the letter said.

But the council has heard nothing from the FDA, said Steve Mister, CRN president. Mister said this is a clear-cut case where the agency has the authority to act swiftly because AMP Citrate does not appear to have been used as a dietary ingredient prior to a 1994 supplement law. If companies want to sell "new" dietary ingredients, they must file paperwork with the FDA in advance of selling their products.

But no such notifications have been filed with the agency, Mister said. "Which means the FDA has tools to conduct seizures, recalls, import alerts … and issue consumer advisories," Mister said. "The question really is: Why hasn't the agency moved on this ingredient?"

On Wednesday, U.S. Sen. Dick Durbin of Illinois, the Senate's No. 2 Democratic leader, and U.S. Sen. Richard Blumenthal, D-Conn., sent a letter to FDA Commissioner Margaret Hamburg calling on the agency to take action on products that contain the new stimulant. They expressed dismay about the growing number of studies in recent months showing new "designer stimulants" in supposedly all-natural supplements.

"The FDA has yet to take any effective action against companies manufacturing these products despite indications that the chemical may be mass-produced, has not been safety tested, and may be deceptively labeled," wrote the senators, who have sought passage of legislation to increase oversight of supplements.

Daniel Fabricant, executive director of the Natural Products Association who until recently was the FDA's top supplement regulator, said the agency has the authority to take action on AMP Citrate. "It's incredibly close to DMAA, and it's certainly being positioned as a replacement for DMAA," Fabricant said. "It's pretty easy to show something wasn't marketed pre-1994."

Since Fabricant announced his resignation as director of the agency's division of dietary supplement programs in April, the FDA has filled his position with a rotating series of acting directors.

Consumers purchasing supplements containing the new stimulant often have no way of knowing the dose they're consuming, the new research found. Supplement bottles often just list the ingredient among several in a "proprietary blend" without any indication of dosages.

The tests showed the amount of the stimulant varied greatly among the products. AMPilean, marketed as a fat burner by Lecheek Nutrition, contained the highest level per serving: 120 mg. Three products delivered 110 mg per serving: Frenzy, a preworkout from Driven Sports; AMPitropin, sold as a brain enhancer by Lecheek Nutrition; and MD2 Meltdown, a weight-loss product sold by VPX Sports.

Other products contained a fraction of those dosages per serving: Contraband, from Iron Forged Nutrition, 50 mg; OxyTHERMPro from deNOVOLABS, 39 mg; and Evol from Genomyx had just 13 mg, the tests found.

In two products labeled as containing the stimulant — Preamp by Hybrid and Genomyx's AMP Citrate — tests were unable to detect it, according to the study.

Labels can also be misleading in not disclosing that the new stimulant is in the product, Cohen said. The researchers note in their article that after some users of the pre-workout supplement Unstoppable in the Netherlands reported adverse events such as a feeling of rushing and difficulty sitting still, tests of the product found it contained the new stimulant — even though the chemical wasn't listed on the label. The company that makes Unstoppable, Dedicated Nutrition, as well as the other supplement firms selling products in the tests, did not respond to USA TODAY's interview requests; some could not be reached.

The research team included scientists from NSF International, a Michigan-based product testing organization, and the National Institute for Public Health and the Environment in the Netherlands, which also separately tested Unstoppable after adverse effects were reported to the Netherlands Anti-Doping Authority, which oversees athletes.

It is unclear whether adverse events have been reported in the United States. The FDA said USA TODAY would need to file a Freedom of Information Act request to find out, a process that can take months.

The health risks from DMBA are not known because it has never been studied in humans, Cohen said. The only two studies of the chemical's effects that the research team found involved a few cats and dogs and suggest that DMBA behaves similarly to DMAA, but may be less potent.

One of the products containing the new stimulant is Frenzy, a pre-workout sold by a company run by Matt Cahill, a convicted felon featured in USA TODAY's "Supplement Shell Game" series. Cahill's company, Driven Sports, launched Frenzy earlier this year through a distributor in the United Kingdom after halting production and sales of its blockbuster pre-workout Craze in the United States. Craze was featured in USA TODAY's 2013 investigation, and tests by Cohen and his co-researchers later showed the supplement contained an undisclosed methamphetamine-like compound. While Driven Sports has prohibited sales of Frenzy in the USA, in response to U.S. customers clamoring for "Craze 2.0," the company posted on its Facebook page Sept. 25: "We'll be revealing details super soon!"

The continued introduction of new products by Driven Sports comes as Cahill has had a federal felony criminal charge pending against him — without any public action — since August 2012. The case accuses Cahill of illegally putting a supplement called Rebound XT on the market around March 2008 that contained a drug that's used to reduce estrogen. The case remains open, court records show, despite nothing being filed publicly in the case since the initial charging documents more than two years ago. The U.S. Attorney's Office in San Jose, which is prosecuting the case, did not respond to questions. Cahill and his attorney, Marc Ullman, did not respond to interview requests.

On its label, Frenzy indicates that the new stimulant comes from "Pouchung Tea." Cohen said the research team could find only two obscure Chinese research papers that indicate the compound may be present in tea at trace levels, but neither paper authenticated the finding. Even if the compound were present in the amount indicated in one of the papers, supplement makers would need at least 2,200 pounds of tea to extract just 12 mg of the compound — showing the substance has never been in human diets at the dosages being put in the supplements, he said.

Ikhlas Khan, of the FDA Center of Excellence and National Center for Natural Products Research at the University of Mississippi, said there's little evidence the compound is found in tea, but his center is currently testing tea samples to see if it can be found. If it can't and DMBA is banned from supplements by regulators, Khan said, another synthetic stimulant is likely to replace it. "It never stops," he said. "It's a vicious cycle."

Several products containing the new stimulant are sold by Amazon.com and supplement retailer GNC. Amazon officials declined to comment but sent a link to the marketplace's rules against selling illegal, unsafe and restricted products. GNC, in an e-mailed statement, said: "Like all retailers, GNC relies upon the contractual representations and warranties made by the manufacturer of the product regarding its legality and safety. GNC is not aware of any regulatory commentary about, or actions against, these products or the ingredients they contain."

To report an adverse event involving a dietary supplement to the FDA, call 800-332-1088 or file an online form.

Read the full "Supplement Shell Game" series at: supplements.usatoday.com

Follow USA TODAY investigative reporter Alison Young on Twitter: @alisonannyoung