Sales of diet soft drinks have been fizzling out for several years running, and the industry has not been able to come up with a way to stop the decline. Soft drink makers have tried raising the price, lowering the price and leaving the price unchanged. Volume continued to slide, and the main culprit is diet soda, where sales are down much further than among the regular versions of the drinks.

The Coca-Cola Co. (NYSE: KO) saw sales of its brand (non-diet) Coke drop 0.5% in 2013 while sales of Diet Coke dropped 6.8%. Brand Coke’s total share of the carbonated beverage market was 17.4%, up 0.4% year-over-year, and nearly double brand Pepsi’s market share of 9%.

Sales of PepsiCo Inc.’s (NYSE: PEP) brand Pepsi fell 3.6% and sales of Diet Pepsi fell 6.9%. Of the diet soft drinks, only Coke Zero got a market share boost in 2013, up 0.1% to 1.9% of the total market for carbonated soft drinks. Of the company’s Mt. Dew drinks, sales of the brand beverage fell 2.2%, while sales of the diet version fell 3.1%.

Some of the sales decline in the diet drinks is down to consumers’ increasing wariness of the non-sugar sweeteners used in the diet beverages. This isn’t the first time the safety of sugar substitutes has been questioned, and the industry has always managed to come up with a solution. Saccharin, aspartame, stevia and sucralose are among the best known of the sugar substitutes, and although none has been declared unsafe by the U.S. Department of Agriculture, that has not stopped opponents of some of these substances from raising nagging questions that have dogged the soft drink makers for years.

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However, a recent study funded by the American Beverage Association and conducted at the University of Colorado compared the effect of drinking a diet soft drink while on a weight-loss program with drinking just water while on a weight-loss program. Perhaps non-intuitively, the people who drank just water lost less weight than the people who drank the diet soft-drinks.

The researchers concluded that it is easier to lose weight when a person does not have to also give up the pleasure of having a soft drink. As the American Psychological Association points out, an individual’s will power is a limited resource, and changing one’s diet and banishing all soft drinks in an effort to lose weight is simply asking too much of a person.

So, maybe that is the marketing message for the makers of diet drinks: our drinks make staying on your diet easier. The companies have already tried their usual assortment of tactics to boost sales and those have not worked. Time to think outside the pop can.

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