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Diet sodas are not actually good for your diet, WHO guidance suggests

Packets of Equal and Splenda in a coffee bar in New York in 2016. (credit: )

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.

Long-term risks

For a longer look at the effects of artificial sweetener use, the experts turned to prospective cohort studies, which tracked health outcomes for at least two years to more than 30 years. Collectively, these studies found higher use of artificial sweeteners was linked to higher BMIs and a 76 percent higher incidence of obesity. Regarding Type 2 diabetes, people had a 23 percent higher risk if they consumed the sweeteners in beverages and a 34 percent higher risk if they had a packaged version of artificial sweeteners that they added to foods and beverages. People with higher use of sweeteners also had a 32 percent higher risk of cardiovascular disease, including stroke (19 percent higher risk) and hypertension (13 percent higher risk).

Last, higher intakes of artificial sweeteners were linked to a 10 percent increased risk of death from any cause and a 19 percent increased risk of death from cardiovascular diseases (but there was no increased risk associated with deaths from cancers).

With all of the findings, the experts recommended against the use of sweeteners. "[T]he lack of evidence to suggest that NSS use is beneficial for body weight or other measures of body fatness over the long term, together with possible long-term undesirable effects in the form of increased risk of [non-communicable disease] and death, outweighed any potential short-term health effects resulting from the small reductions in body weight and BMI observed in RCTs," WHO concluded.

The review and meta-analyses are not the last word on artificial sweeteners, though. As the experts noted, the randomized controlled trials were short and hard to compare, and the prospective studies could have included biases. Overall, the experts found the certainty of their recommendation to be low, which led them to label their recommendation as "conditional." This tells policymakers that they should take their own deep look into the matter before adopting the recommendations.

One of the key points of uncertainty is whether there is "reverse causation" at play in the harmful long-term health effects. That is, the people who are more likely to use artificial sweeteners may already have elevated risks for developing conditions such as obesity, Type 2 diabetes, and cardiovascular disease. Thus, perhaps the use of artificial sweeteners was not the cause of the increased risk, rather the increased risk led to the use of artificial sweeteners.


Researchers are well aware of this pitfall and try to circumvent it using several strategies. These can include controlling for known confounders (like BMI and diet quality), stratifying results by body weight, and conducting sensitivity analyses (like doing a subset analysis of only the healthiest participants at baseline to see if results hold up). Researchers used these methods in the studies reviewed by the WHO experts. The results of sensitivity analyses were mixed. In some cases, they weakened findings, in others, they strengthened them. And confounders like diet quality also did not consistently link to the use of artificial sweeteners and their outcomes.

The experts concluded that "although reverse causation and residual confounding may be contributing factors, the available evidence suggests that the associations observed between NSS use and health outcomes in observational studies cannot be dismissed as being solely a result of reverse causation or residual confounding."

There's also the issue of multiple kinds of artificial sweeteners, each with their own structures, sweetness levels, and processing in humans. For the review, with the limited data available, the WHO guidance considered the findings as a single class, but the researchers note that individual sweeteners may have unique properties. In addition, the different sweeteners may have differing effects on different people based on sex, ethnicity, health, and other factors. Experts will need more data to assess the risks and benefits of individual sweeteners in various populations.

The possible mechanisms for artificial sweeteners to cause harmful health effects are also in need of more research. But, there's been plenty of speculation on the matter, as the WHO experts summarize in their guidance:

In brief, potential mechanisms include effects on taste perception (e.g. sweet taste preference, thresholds of sweet-taste sensitivity), eating behaviour (e.g. hunger, appetite) and other neural responses (e.g. hedonic response to sweet taste, memory and reward pathways in the brain); pathways that link the sensing of sweet taste in the oral cavity with the expectation of subsequent energy delivery to the digestive tract; release of metabolic hormones and other biological molecules; and alterations to the bacteria colonizing the small and large intestines (i.e. gut microbiota). Proposed mechanisms are not mutually exclusive and may ultimately differ between individual NSS.

For now, experts at WHO and elsewhere suggest finding other ways of cutting back on sugar than swapping to sweeter artificial sweeteners, such as eating fruits and simply adjusting to a less sweet diet overall. They also caution consumers to remain vigilant about understanding what's in their foods and to check ingredient lists to see if foods and beverages contain artificial sweeteners even if they're not overtly labeled.

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