More evidence that sugary drinks cause weight gain: Study

A review of dozens of studies from the last decade recently found that sugar-sweetened beverages promote weight gain in children and adults.

The review, led by researchers at the University of Toronto and Harvard University, is the largest and most thorough 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 – published in the American Journal of Clinical Nutrition – 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 recently spoke with writer Jim Oldfield about the findings and what they mean for public policy and personal health.

Why do this study now?

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

What did your study show?

We expected to find a positive association between SSBs and weight gain among adults and children, and in cohort studies and randomized clinical trials. And that was exactly what we found. We analyzed 85 studies, which totaled over half a million participants. In cohort studies, which follow people over long periods of time, each serving-per-day increase in SSBs was associated with a 0.42-kilogram (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 levels of SSB intake 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 per day lead to over time?

We estimated the associated change in body weight over a one-year period. For adults, one additional 12-ounce serving per day was linked to a 0.20-kg higher body weight (about half a pound) in one year. Over 10 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 a one-year period. Although these results may seem modest, weight gain is a gradual process, 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 common is excess consumption of these drinks?

It’s very common. Sugar-sweetened beverages include sodas, fruit drinks, sports and energy drinks, and they 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 as 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 per cent 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), that puts a person on the path to fatty liver and metabolic disease. Fructose also increases uric acid, which contributes 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 excess insulin in the blood over longer periods. Some evidence shows that SSBs activate the dopaminergic reward system in the brain and encourage addictive behaviour and that they alter the gut microbiome – but 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 taxes in some of those countries are working. Thailand introduced a tax that has reduced consumption, as has Mexico and South Africa. At least 85 countries now have a tax on SSBs, which in part reflects 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 tax 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 help in Canada and elsewhere as well, 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 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 really need to address.

Malik holds a Canada Research Chair in Nutrition and Chronic Disease Prevention at U of T and an adjunct faculty position in nutrition at the Harvard T.H. Chan School of Public Health. Michelle Nguyen, a doctoral student at U of T, conducted the study analysis and wrote the paper.