People trying to shed pounds often cut calories by consuming diet drinks, artificially sweetened treats, and other products containing substitute sugars. But according to a new assessment from the World Health Organization, those artificial sweeteners don't appear effective for weight control—and worse, they seem to increase long-term risks of Type 2 diabetes, cardiovascular disease, and even death.

In a guidance released Monday, WHO recommended against using artificial sweeteners for weight control or attempting to boost health generally. The recommendation applies to healthy children and adults but is not intended for people with pre-existing diabetes, who may still find benefits from using artificial sweeteners.

"Replacing free sugars with NSS [non-sugar sweeteners] does not help with weight control in the long term. People need to consider other ways to reduce free sugars intake, such as consuming food with naturally occurring sugars, like fruit, or unsweetened food and beverages," Francesco Branca, WHO director for Nutrition and Food Safety, said in a statement.  "People should reduce the sweetness of the diet altogether, starting early in life, to improve their health," he added.

Bittersweet findings

WHO's conclusion is not surprising; data has mounted for years suggesting that synthetic sweeteners may be backfiring in efforts to improve health. In the US, as obesity rates have risen, so has the use of artificial sweeteners.

For WHO's assessment of the substitute sweeteners, experts systematically reviewed data from 283 unique studies, including 50 randomized controlled trials, 97 prospective cohort studies, and 47 case-control studies. The studies looked at a range of artificial sweeteners, including common ones used in the US: saccharin (Sweet'n Low), aspartame (Equal), acesulfame potassium (Ace-K), sucralose (Splenda), neotame, advantame, stevia, and stevia derivatives.

In the randomized controlled trials, WHO experts noted that artificial sweeteners lowered calorie intake, body weight, and body mass index (BMI)—at least at first. Most of these trials lasted just three months or less. In the trials that stretched six to 18 months, the findings were muddled but didn't seem to suggest an effect on body weight.

Part of the problem was how some of the trials compared the use of artificial sweeteners to plain sugar. For some, trial participants were given foods or beverages containing either sugar or artificial sweeteners in addition to their regular diets, resulting in direct comparisons. In these cases, participants who got the artificially sweetened products showed lower body weights than people who ate the sugary bonus treats.

But in real-world settings, many people choose to swap sugary foods and beverages in their current diet for artificially sweetened versions. Only four trials specifically tested this—subbing artificially sweetened beverages, like diet soda, into the diets of people who normally consumed sugary beverages. These studies reported drops in weight from the use of artificial sweeteners, but the drops were smaller than in the direct comparison trials and were statistically insignificant. The beneficial effect on BMI was lost entirely. Notably, some of these trials had groups where participants were switched from sugary drinks to water or nothing (placebo)—and water or placebos were as effective, if not more effective, at reducing weight. WHO experts note that this suggests that the weight loss seen in some trials of artificial sweeteners is likely just down to reducing calories, not an inherent property of artificial sweeteners.