If you've spent any time online, you've probably seen them: the ads for diet products made with the trendy berry açai. But you may want to think twice about signing up for one of these diets.

The consumer advocacy group, the Center for Science in the Public Interest, is warning that not only are the diets questionable, many are offered through an online scam.

There's no evidence whatsoever that diet pills made with açai (pronounced a-sigh-EE) will help flatten tummies, cleanse colons, enhance sexual desire, or perform any of the other commonly advertised functions, the CSPI warns in a news release Monday.

What's more, many of these pills are offered through "free, risk-free" 14-day trial offers that are really part of a scheme called "negative option" advertising.

The companies offer samples of the products but then charge the customer's credit card month after month unless the consumer cancels the order.

CSPI senior nutritionist David Schardt, who authored an exposé of the scam in the April issue of CSPI's Nutrition Action Healthletter, says oftentimes, there is little that police can do.

"If Bernard Madoff were in the food business, he'd be offering 'free' trials of açai-based weight-loss products," Schardt said in a statement Monday.

"Law enforcement has yet to catch up to these rogue operators. Until they do, consumers have to protect themselves."

The Better Business Bureau in the U.S. released a statement this January warning consumers to be wary of website offering acai berry-related weight loss products, saying it has received "thousands" of complaints from consumers.

In many cases, when customers try to contact the company and cancel their subscriptions, they are forced to sit for more than an hour on hold. Additionally, some consumers have complained of unauthorized charges on their credit card or bank accounts for products they did not order.

Others have had trouble cancelling their subscription using the email addresses provided. In some cases, the address did not work or the complainant continued to be billed despite multiple emails.

Several customers reported they were eventually forced to close bank accounts and cancel credit cards to stop the charges.

Connecticut Attorney General Richard Blumenthal has taken up the cause along with the CSPI to expose these scams, reminding consumers of the old adage, "If it looks too good to be true, it probably is."

"There are no magical berries from the Brazilian rainforest that cure obesity -- only painfully real credit card charges and empty weight loss promises," Blumenthal said in a statement.

"Aggressive acai berry pitches on the Internet entice countless consumers into free trials promising weight loss, energy and detoxification. These claims are based on folklore, traditional remedies and outright fabrications--unproven by real scientific evidence. In reality, consumers lose more money than weight after free trials transition into inescapable charges."

He has promised to continue to investigate misleading nutrition and health claims and take action under consumer protection laws when possible.

Acai began attracting attention in 2005 with word that the Brazilian fruit's juice was especially high in antioxidants, which are thought to slow or preventing the oxidative damage from oxygen from free radicals.

The CSPI says in fact, acai juice has no more antioxidants than grape, blueberry, or black cherry juices. What's more, there is no credible evidence that antioxidants in themselves can promote weight loss.

In early 2008, acai got a jolt of publicity when the berry was mentioned in a segment on the Oprah Winfrey show by Dr. Mehmet Oz, who wrote the "You" series of health books with Dr. Michael Roizen. A guest on Rachael Ray also discussed an acai beverage.

Since then, ads on Google, Facebook, and major websites have steered consumers to sites with names such as OprahsAmazingDiet.com, DrOzMiracle.com, rachaelray.drozdiet-acaiberry.com with such teasers as: "Lose weight with Oprah's favorite diet secret!" "Eat the berry that Dr. Oz calls the 'No. 1' superfood!"

Winfrey, Oz, and Ray have all publicly disassociated themselves from the sites that make unauthorized use of their names.

A disclaimer was added to the Oprah website last month that reads: "Consumers should be aware that Oprah Winfrey is not associated with nor does she endorse any açaí berry product or online solicitation of such products."