You've been watching your sugar intake lately, so you select a diet soft drink from the office pop machine for a cool, refreshing pick-me-up. It's sugar-free and has no calories. About as harmless as water, right?
Not so, says a new study published Monday in the Canadian Medical Association Journal. Nonnutritive sweeteners such as those used in diet pop were associated with weight gain, a higher risk of Type 2 diabetes and other health problems, according to researchers from the University of Manitoba's George and Fay Yee Centre for Healthcare Innovation.
The study involved a systematic review and meta-analysis of 37 studies dating back to the early 1980s and involving more than 400,000 participants.
Considering the potential risks, lead author Meghan Azad says it's not a bad idea to avoid products containing nonnutritive sweeteners – including artificial sweeteners such as aspartame and sucralose and stevioside, a natural sweetener derived from stevia plants – until researchers know more about their long-term effects. However, other experts say there's not enough evidence to recommend abstaining from these sugar substitutes, noting they're approved by Health Canada.
"I think a lot of people consuming them kind of assume they're harmless because they contain zero calories. But what the evidence is suggesting is maybe there's more to the story than that," said Dr. Azad, an assistant professor of pediatrics and child health at the University of Manitoba.
Interest in the health effects of nonnutritive sweetners has grown over the years as public consumption of them has increased. Jane Shearer, an associate professor of kinesiology at the University of Calgary who is studying these sweeteners, notes the number of products that contain sugar substitutes has grown significantly in Canada in the past five years, with energy drinks, no-sugar-added ice creams, yogurt and even some bread products.
Dr. Shearer, who was not involved in the CMAJ study, says the findings from the Manitoba researchers add to mounting evidence against nonnutritive sweeteners. Her own work on animals has shown these sweeteners can alter the composition of gut microbiota, which she says could play a role in long-term changes in metabolism.
Other hypotheses suggest they promote a preference for sweetness, leading to further consumption of sweet foods and beverages, or may lead people to indulge in other ways. If you choose a no-sugar-added ice cream, for instance, you may eat more of it.
Nonnutritive sweeteners may also alter the body's response to sweetness over time, changing the way it metabolizes actual sugar, says Susan Swithers, a professor in the department of psychological sciences at Purdue University in Indiana. Dr. Swithers, who studies the health impact of nonnutritive sweeteners, suggests that for this reason, all sweeteners are problematic.
But surely a diet pop is a wiser choice than regular pop? Do the potential risks of sweeteners outweigh the risks of sugar itself?
It's a tough question, Dr. Swithers says, in part because people who report they drink diet pop tend to be heavier and are more likely to have a family history of health problems than those who drink regular pop.
These differences also make it hard to say definitively whether nonnutritive sweeteners are bad for you. Dr. Swithers notes that those who are skeptical of the potential harms of nonnutritive sweeteners tend to point to the lack of causal evidence.
Dr. Shearer adds there's a large industry of diet and sugar-free products that has an interest in discrediting any findings of possible health risks.
In the CMAJ study, the researchers examined seven randomized control trials, which followed participants for an average of six months. The rest were larger observational studies that examined participants over an average of 10 years.
The researchers found the randomized control trials suggest sugar substitutes don't help with weight management. Furthermore, over the long term, the observational studies indicated the sweeteners were not only associated with weight gain, they were also linked to diabetes, obesity, high blood pressure and heart disease.
In nine studies, participants who consumed the greatest amount of nonnutritive sweeteners had a 14-per-cent higher risk of Type 2 diabetes than those who consumed the least. And in five studies, those who consumed the greatest amount of nonnutritive sweeteners had a 31-per-cent higher risk of having metabolic syndrome – a cluster of conditions that includes higher blood pressure and cholesterol levels – than those who consumed the least.
"The message is there isn't strong evidence for a benefit from these products – and there's potential evidence for harm," Dr. Azad said.
Diabetes Canada includes nonnutritive sweeteners in its guidelines, citing Health Canada's acceptable daily intake values. (The organization has received funding from companies that use nonnutritive sweeteners, though it says no company or representative has any influence in the development of its recommendations.)
Seema Nagpal, an epidemiologist and a senior leader on Diabetes Canada's government relations and public policy team, said the CMAJ study points to the need for more research, but the "data aren't sufficient at this point to cause alarm or to recommend avoidance. … When used in moderation, these products can be used as part of a healthy and well-balanced diet."
Dr. Azad, however, says it's likely safer to steer clear of nonnutritive sweeteners in general. She offered this advice: "I'm going to say that you should drink water."