Obesity has become a global public health problem. Children who are obese are more likely to suffer from serious health problems – both in childhood and later in life – ranging from high blood pressure to type 2 diabetes or depression. It also creates a large burden on healthcare systems.
In the US, around 1 in 5 children and adolescents have obesity according to the CDC. Obesity is more common in Hispanic children and non-Hispanic Black children. It is also linked to family income: obesity prevalence increases and family income decreases.
Sugar taxes (which generally apply specifically to beverages) are often justified by governments and public health officials by citing high levels of childhood obesity – and the ability of sugar taxes to cut sugary drink consumption among children. That’s certainly been the case in the UK, in Seattle, and many others. But what does the data show?
New data from Seattle
Published this week, a new study from the University of Washington has tackled the question directly: looking at whether the BMI of children in Seattle changed after the implementation of the Seattle Sweetened Beverage Tax in 2018.
The study assessed 6,313 children living in Seattle or in a nearby comparison area not covered by a sugar tax. They found a 'statistically significant reduction' in BMI in children in Seattle.
Seattle's sugar tax covers soda, energy drinks, sports drinks, sweetened ice teas and coffees, and juices or waters that contain added caloric sweeteners (100% juice, low calorie and diet drinks are exempt). The sweetened beverage tax is set at 1.75c per ounce.
The primary synthetic difference-in-difference (SDID) analysis included 6313 children (48% female and 52% male).
A third of children were aged 2 to 5 years; their mean age was 7.7 years.
With regard to race and ethnicity, 13% were Asian, 10% were Black, 10% were Hispanic, and 50% were White.
"The primary model results suggested that the Seattle tax was associated with a larger decrease in BMIp95 for children living in Seattle compared with those living in the comparison area (SDID: −0.90 percentage points [95% CI, −1.20 to −0.60]; P < .001)," notes the study. "Results from secondary models were similar."
The University of Washington researchers looked at children living in three adjacent counties in Washington State: King (which includes but is not limited to Seattle), Pierce and Snohomish. The children were aged between 2 and 18 years old (during the period 2014 to 2019) and lived in an urban area or cluster.
They assessed BMIp95 (BMI expressed as a percentage of the 95th percentile, a newly recommended metric for assessing BMI change). They found a ‘modest decrease’ in BMIp95 among children living in Seattle, compared with children living in nearby non-taxed areas (The children all received care within the same health care systems).
Previous data shows that Seattle's soda tax saw soda taxes rise 59%, while the volume of sugary drinks sold to the overall population fell by 22%.
One key limitation of the University of Washington study, however, is that data on sugary beverage consumption was not available: and researchers could not rule out that an unknown confounder might account for the differences between the pre-tax and post-tax periods.
Sugar taxes: Tackling obesity in multiple ways
Sugar taxes seek to discourage the consumption of sugary beverages: reducing calorie intake among children. But they also can also address childhood obesity in other ways.
Revenue from sugar taxes is often directed at other initiatives to improve children’s wellbeing. In Seattle, food access and child development programs and services are supported by the sugar tax. There is a particular focus on children from birth-to-three: because children’s experiences during this time can have profound effects on their academic, social and physical potential throughout their lives.
A 2022 study looked at high school aged children in Philadelphia, San Fransico and Oakland, which have all implemented sugar taxes. This found reductions in soda consumption and average BMI: noting that improvements were most concentrated among female and non-white teenagers.
Further afield, a study from Mexico – one of the first countries to introduce a sugar tax back in 2014 – found an association between the tax and decreased BMI among girls.
In the UK, researchers at the University of Cambridge have found a drop in the number of cases of obesity among older primary school children following the introduction of the UK Soft Drinks Industry Levy in 2018. They suggested that around 5,000 cases of obesity per year may have been prevented in year 6 girls alone. However, researchers did not find any significant association between the levy and obesity levels in year 6 boys, or younger children in reception class (aged 4-5 years old).
Another study links the sugar tax to a fall in child hospital admissions for teeth extraction: with the most reductions found in children aged up to nine years old.
Dr Jessica C. Jones-Smith, lead author of the University of Washington study on Seattle children and the soda tax, says her team's research had been carefully designed to see if BMI changes are linked to the tax, rather than other factors.
Reductions in BMI in Seattle have also seen among adults, according to research led by Jones-Smith last year.
Health records of nearly a million people aged 18-65 showed that BMI increased in both Seattle and comparison areas: however, the tax was associated with 'modestly lower increases in BMI' in Seattle than elsewhere.
In particular, the study used longitudinal data from the same children over time, rather than cross-sectional designs as used by other studies on the same subject.
"Our findings [of reduced BMI] are consistent with our expectations given the modest scale of the tax, and the complex social and behavioral mechanisms hypothesized to underlie current obesity trends, and with outcomes suggested by modeling studies," she said.
The findings are also in line with a 22% reduction in volume of taxed beverages sold in Seattle, which would be expected to result in a better BMI.
The University of Washington study adds to the body of research showing that sugar taxes may be associated with a ‘small but reasonable reduction’ in BMI among children, she continues.
“Taken together with existing US studies, our results suggest sweetened beverage taxes may be an effective policy for improving BMI,” she said, “Future research should test this association using longitudinal data in other US cities with sweetened beverage taxes."
