Advertising campaigns behind diet drinks from Coca-Cola, Pepsi and Dr. Pepper have long promoted the idea that consumers are taking the healthier, more weight-conscious option when it comes to choosing their favorite sodas. Diet Coke emphasized its drink has "no sugar, no calories." Diet Pepsi tried launching its slender "skinny" can only a handful of years ago. And Diet Dr. Pepper's "Lil Sweet" mascot is no subtle nod to the product’s supposed ability to shrink those who drink it.

But this past October, six lawsuits were filed in federal courts in New York and California arguing that the makers of Coca-Cola, Pepsi and Dr. Pepper are engaging in misleading and unlawful marketing practices of their diet beverages, as these drinks contain ingredients that raise the risk not only of weight gain, but of developing serious health problems.

"Some great injunctive relief would be if they would just remove the letter 't' from the word diet," said Abraham Melamed, one of the attorneys working on the three suits filed in New York, who added that branding these drinks as "diet" is "fraudulent, illegal, improper and needs to stop."

AlterNet contacted all three beverage companies involved in the lawsuits. While none of the companies responded directly, Lauren Kane, a spokeswoman for the American Beverage Association, which represents the U.S. non-alcoholic beverage industry, wrote in an email that the "diet beverages that contain zero or barely any calories at all have repeatedly been shown to help people manage their diets. That is why we proudly stand by our products against these meritless legal claims."

The similarly worded lawsuits focus primarily on aspartame, a low-calorie sweetener used in all three diet drinks. And while the additive—commonly appearing under the names NutraSweet, Equal, Sugar Twin and AminoSweet—is frequently called one of the "most researched food additives in the world," there is growing scientific evidence that artificial sweeteners can disrupt the body's metabolism, causing weight gain, as well as an increased risk of health problems such as diabetes, cardiovascular disease, and metabolic syndrome (a cluster of conditions which together elevate the risk of heart disease and strokes, among other issues).

Each lawsuit claims that the three companies are, "or reasonably should have been," aware that promoting their products as "diet" was false and misleading, and cite a number of studies published over the past decade to help explain why the plaintiffs in the suits, who all drink large quantities of diet sodas, have struggled with obesity for years.

The San Antonia Heart Study, for example, found that the consumption of more than 21 artificially sweetened beverages a week almost doubled the risk of weight gain or obesity compared to those who drink none. A 2013 review of 30 studies involving some 450,000 participants found a link between artificial sweeteners and obesity, type 2 diabetes and heart disease. So, how exactly could the consumption of aspartame lead to weight gain?

Sweet-tasting foods and drinks send a signal to the brain, and the body responds to metabolize the corresponding amount of calories. But, as one recent study from Yale University suggests, when a product is sweetened with artificial additives that are either too high or too low for the number of calories present, the brain gets confused, prompting a metabolic response out of tune with the amount of sugars present. Therefore, the researchers concluded, an artificially sweet tasting product with very few calories can trigger a greater metabolic response than needed, which can bring on health issues like type 2 diabetes.

"In other words, the assumption that more calories trigger greater metabolic and brain response is wrong," Dana Small, a professor of psychiatry at Yale and a senior author of the study, told YaleNews in August. "Calories are only half of the equation; sweet taste perception is the other half."

But according to Vasanti Malik, a nutrition research scientist at the Harvard T.H. Chan School of Public Health, while the results from the recent Yale study are "really interesting," the science is still unclear whether diet sodas trigger a metabolic response. "There’s a handful of studies that say they do. There's a handful of studies that say they don't," she said. "We don't know enough either way to say."

Malik has been involved with a number of long-term studies into the way diet drinks affect the human body. The findings of her work suggest that, while weight gain is commonplace over time, drinking diet sodas can lead to less weight gain compared to drinking regular sugared beverages. And she hasn't observed positive associations between diet sodas and diabetes.

However, there are still many holes in the scientific literature surrounding all artificial sweeteners, she said. "The question that hasn't been answered is what is the lifetime impact of consuming these artificial sweeteners." That explains why she doesn't agree with the way in which diet sodas are marketed currently to consumers. "Drinking water is definitely going to be your best option because it doesn't have this intense sweet flavor that we're worrying about," she said. "I wouldn't consider [diet beverages] the healthy option."

The lawsuits aren't the first time beverage companies have come under fire for their labeling of diet sodas. In 2015, food industry watchdog U.S. Right to Know (USRTK) filed a petition with the U.S. Food and Drug Administration asking the department to stop Pepsi and Coca-Cola from using the word "diet" to label and market their products, and to conduct a "sweeping investigation of products containing artificial sweeteners" to determine whether any brand labels are false or misleading. The FDA recently denied the petition on grounds that the requests "are outside the scope of our citizen petition regulations."

The agency's reasoning for the denial is "so weak," said Gary Ruskin, founder and co-director of USRTK. "It was plainly just an effort to avoid protecting public health." That's because the weight of the scientific literature contradicts the way "diet" sodas are widely marketed as healthy alternatives to regular sugary drinks, he said. "Evidence only keeps getting stronger and stronger. We think this could be one of the greatest consumer frauds of modern times."

The lawsuits are still in the early stages of litigation, but Melamed is hopeful that the cases, if successful, could set an important new benchmark in how the food industry, especially bottled beverage companies, market their products. "Our expectations are that this will lead to more significant changes across the industry," he said.

Others are more circumspect about the prospect of going up against three behemoths of the bottled beverage industry. While the lawsuits are "strong" on the law and on the facts, said Ruskin, "sadly in America right now, that doesn’t necessarily get you too far."