Adding evidence to the weight gain causality of sugary drinks

A review of dozens of studies from the last decade, led by researchers at the University of Toronto and Harvard University, recently found that sugar-sweetened beverages promote weight gain in children and adults.

The review, published in the American Journal of Clinical Nutrition, is the most extensive analysis to date of research on sweetened drinks and overweight and obesity -; both of which heighten risks for diabetes, heart disease, some cancers, and other diseases.

Vasanti Malik led the study with colleagues in Toronto and Boston. Malik is an assistant professor of nutritional sciences at U of T's Temerty Faculty of Medicine and the Joannah & Brian Lawson Centre for Child Nutrition. She spoke with writer Jim Oldfield about the findings and take-aways for public policy and personal health.

Original Research Article: Sugar-sweetened beverage consumption and weight gain in children and adults: a systematic review and meta-analysis of prospective cohort studies and randomized controlled trials. Image Credit: Valerii__Dex / Shutterstock

Why do this study now?

Our last meta-analysis on this topic was in 2013. You want to update a meta-analysis every five to 10 years anyway, especially in this area because there has been so much new research in the last decade. Evidence has continued to accumulate, showing associations between sugar-sweetened beverages, or SSBs as we call them, and weight and chronic disease. And it's essential to have an updated synthesis of that evidence, especially for public policy. For example, the Canada Food Guide clearly states the need to limit SSBs and recommends water as the drink of choice. Due to public policies, added sugar intake, including SSBs, has declined in Canada. But levels here are still too high. The USDA Dietary Guidelines for Americans are arguably not as strong. With U.S. policymakers coming together soon to discuss the 2025 guidelines, our study will be an essential piece of evidence to inform their work. 

What did your study show?

In cohort studies and randomized clinical trials, we expected to find a positive association between SSBs and weight gain among adults and children. And that was precisely what we found. We analyzed 85 studies, which totaled over half a million participants. In cohort studies, which follow people over long periods, each serving-per-day increase in SSBs was associated with a 0.42-kg (almost one-pound) higher body weight in adults. In children, we saw a 0.07-unit higher body-mass index (just under one-twelfth of a BMI unit). Perhaps most striking, findings from our dose-response analysis showed that weight gain increases with increasing SSB intake levels in both children and adults. A dose-response relationship provides strong evidence for a cause-and-effect relationship. 

How much weight gain might one drink a day lead to over time?

Well, we estimated the associated change in body weight over one year. For adults, one additional 12-ounce serving daily was linked to a 0.20-kg higher body weight (about half a pound) in one year. Over ten years that could be about five pounds. In children, we observed a 0.03-unit higher BMI for each additional daily serving of SSBs over one year. Although these results may seem modest, weight gain is gradual, with adults averaging about one pound (0.45 kg) of weight gain per year. So, limiting SSB consumption could be an effective way to prevent age-related weight gain. Limiting SSB intake among children is also an important strategy to help them develop healthy lifestyle habits and weight trajectories.

How typical is excess consumption of these drinks?

It's widespread. Sugar-sweetened beverages include sodas, fruit, sports, and energy drinks, which are the largest source of added sugar in the North American diet. Moreover, the rise in consumption of these drinks has mirrored the epidemic of overweight and obesity. In 2016, almost two billion adults were estimated to having overweight, and 650 million had obesity. Even more worrying, the rate of increase in obesity in children and adolescents is now greater than in adults. The prevalence of childhood obesity has increased more than four-fold globally since the 1970s, which is truly alarming.

What are some of the health effects of SSBs?

A typical 12-ounce serving of an SSB contains over 140 calories and more than eight teaspoons of sugar. That nearly reaches the recommended daily limit for added sugar, which is no more than 10 percent of total calories, or about 200 calories for a 2000-calorie per day diet. These drinks are sugar in liquid form. They're usually made with table sugar, high-fructose corn syrup, or other sweeteners that provide calories and are digested rapidly, more so than sugar consumed as a solid. This bolus of glucose increases blood sugar levels, which triggers a glycemic response that, over time, can lead to insulin resistance and diabetes. The fructose component also floods the liver, which can cause lipogenesis (creation of fat), which puts a person on the path to fatty liver and metabolic disease. Fructose also increases uric acid, contributing to insulin resistance and risk for cardiovascular and other diseases. Insulin spikes from the glycemic response can result in an appetite cascade and over-eating, as can increase insulin in the blood over more extended periods. In addition, some evidence shows that SSBs activate the dopaminergic reward system in the brain and encourage addictive behavior and that they alter the gut microbiome. Still, we need more research on those effects.

Are you optimistic, given these effects and the research evidence? 

Intake levels of SSBs have come down in the developed world. We're still seeing increases in the developing world, but some countries taxes are working. For example, Thailand introduced tax-reducing consumption, as has Mexico and South Africa. At least 85 countries now have a tax on SSBs, partly reflecting the World Health Organization's stand on this issue. In Canada, Newfoundland introduced a tax recently, and several U.S. regions and cities have had a surcharge for years in response to public health efforts, more awareness, and advocacy. The general effect of these taxes is reduced intake, and the revenues can be put toward further public health measures and health care. Other changes will also help in Canada and elsewhere, such as limiting marketing to children and better front-of-pack and nutrition labels. All these efforts will push intake down, but it's important to remember that as that happens, people need access to clean, safe drinking water as an alternative. That's an ongoing challenge globally, and in many parts of Canada, that we need to address.