On Thursday, the consumer advocacy group, U.S. Right to Know (RTK), asked the Federal Trade Commission (FTC) and the Food and Drug Administration (FDA) to prohibit the Coca Cola Company and PepsiCo Inc. from using the allegedly “deceptive” term “diet” in advertising Diet Coke and Diet Pepsi.

According to RTK, “the use of the term ‘diet’ is false and misleading” because “scientific evidence suggests artificial sweeteners [used in these beverages] contribute to weight gain, not weight loss.” See letter to FTC and FDA.

FDA regulations provide that:

A soft drink that used the term diet as part of its brand name before October 25, 1989, and whose use of that term was in compliance with [the general regulation for use of the term diet] on that date, may continue to use that term as part of its brand name, provided that its use of the term is not false or misleading.

21 CFR 101.13(q)(2). Soft drinks marketed after October 1989[1], may use the term “diet” provided they are “labeled ‘low calorie’ or ‘reduced calorie’ or bear[] another comparative calorie claim,” and if the term is not false or misleading. 21 C.F.R. § 105.66(e)(1).

Both Diet Coke and Diet Pepsi are labeled as having 0 calories, 0 grams of total fat, 0 grams of total carbohydrates, 0 sugars and 0 grams of protein. Diet Coke derives its sweet taste from aspartame, a non-caloric artificial sweetener, whereas Diet Pepsi uses a combination of the artificial sweeteners aspartame and acesulfame potassium. Coca Cola and PepsiCo use the term “diet” to distinguish these beverages from their Coke and Pepsi products, which contain caloric sweeteners (high fructose corn syrup and/or sugar) and provide approximately 140 and 150 calories per 12-ounce can, respectively.

RTK does not challenge the accuracy of the Diet Coke or Diet Pepsi “0 calorie” “0 fat” etc. labeling. Instead, RTK claims the term “diet” refers to or implies weight loss and then asserts that the consumption of these “diet” beverages may actually contribute to weight gain. RTK cites four reviews published between 2009 and 2013, and four earlier epidemiological studies that first appeared between 2003 and 2008, as evidence that consumption of artificial sweetened sodas does not contribute to weight loss and instead may contribute to weight gain. The group acknowledged that while two industry-funded studies found no link between artificial sweeteners and weight gain, it dismissed those studies as purportedly “less trustworthy than those funded independently.”

The theoretical mechanism by which zero calorie sodas might contribute to weight gain is unknown. And all of the studies RTK cites are either epidemiological studies – which can identify associations but not causal connections – or animal studies, generally conducted in rodents. Furthermore, the FDA considers the artificial sweetners in diet soft drinks (e.g. aspertame) to be Generally Recognized as Safe (GRAS) and the American Diebetes Association includes diet soda among its “Healthy Food Choices.” However, some have at least hypothesized that that artificially sweetened sodas can increase cravings for sugary or starchy carbohydrates, leading to overeating, or somehow “trick” the body into producing more insulin, leading to greater fat storage in the body.

RTK’s letters follow actions by other consumer advocacy groups, including the Center for Science in the Public Interest, urging consumers to avoid most artificial sweeteners, and to limit their beverage choices to water, seltzer, seltzer mixed with a little fruit juice, low-fat or fat-free milk, or unsweetened tea.[2]See Recommendations. Neither the FTC or FDA has responded to RTK’s request and it is unclear when or how the agencies will respond.

[1] Diet Pepsi was first introduced in 1964 and Diet Coke followed in 1982 (replacing Tab). Different variations of these products, e.g., Diet Coke with Lemon, were introduced more recently.

[2] The CSPI had a different (decidedly more positive) view of diet soda in 2005, when it petitioned the FDA to require health messages on soft drinks containing high-fructose corn syrup and other caloric sweetners. See Citizens Petition.