Effects of a fizzy drink tax on obesity rates estima...

"A 20% tax on sugary drinks would reduce the number of obese adults in the UK by 180,000, say researchers," BBC News reports. The news is based on research published in the British Medical Journal (BMJ) estimating that taxing drinks would reduce the number of obese adults by around 1%. This would mean 180,000 fewer obese adults in the UK.

This valuable study used a large quantity of UK survey data to estimate the effects that a 20% tax on sugar-sweetened soft drinks would have on consumption habits.

Using a series of statistical models based on available data the researchers estimated that a 20% tax (which would raise the cost of a 70p can to 84p) would decrease sugary drink consumption by about 15%. This, they estimate, would lead to a 180,000 (1.3%) reduction in the number of obese adults in the UK, and a 285,000 (0.9%) reduction in the number of overweight and obese adults.

The estimated effect was greatest in the youngest age group in this study – those aged between 16 and 29.

However, this was a modelling study and can give estimates of change only.

It is likely that a new tax on sugary drinks would be politically unpopular but, seeing that obesity-related complaints cost the NHS £5 billion a year, a "sugar tax" may be an unpopular decision that has to be taken in the future.

Where did the story come from?

The study was carried out by researchers from the University of Reading and the University of Oxford and was published in the peer-reviewed British Medical Journal. The study has been published on an open access basis, so it is free to read online or download.

The study received no specific funding, though two authors have received funding from the British Heart Foundation and two from the National Institute for Health Research.

The media’s reporting of this research is generally accurate.

What kind of research was this?

This was an economic modelling study to estimate the effect that adding a 20% tax on sugar-sweetened drinks would have on the prevalence of overweight and obesity in the UK.

There is growing evidence of the negative effect of sugar-sweetened soft drinks on our short and longer term health. Possible adverse effects related to consumption include weight gain, tooth decay, type 2 diabetes and cardiovascular problems (problems affecting the heart and blood vessels). This has led to calls for action to limit their consumption, with options including controlling portion sizes, banning advertising to children and taxation.

As the researchers say, sweetened soft drinks are full of what are known as ‘empty calories’. They add extra calories to the diet but they provide little or no nutritional benefits.

While taxes on other unhealthy foods have led to concerns about unintended substitution effects (for example, taxing foods high in saturated fat may lead to a shift towards salty foods), they say that the potential substitutes for-sugar sweetened drinks (diet drinks, fruit juice, milk, water) are probably less harmful for health.

In January 2013 the non-governmental organisation Sustain called for a 20 pence per litre excise duty on sugar-sweetened drinks. In February, the Academy of Medical Royal Colleges called for a 20% tax on sugar-sweetened drinks as part of its enquiry into clinical and public health solutions to the obesity epidemic.

However, although there is growing support for a tax on sugar-sweetened soft drinks, the likely health effects remain uncertain. This research set out to estimate the effect of a 20% tax on sugar-sweetened drinks on obesity in the UK, and to understand the health effects on different income groups.

What did the research involve?

The researchers used household survey data to estimate the effect that a 20% tax on sugar-sweetened drinks would have.

For information on price and purchasing in the UK they used data from the Living Costs and Food Survey, 2010. This is said to be a representative sample of food and drink purchasing in the UK, which uses two-week food expenditure diaries. A total 5,263 households (including 12,196 people) completed the 2010 survey, giving information on their household food purchases. The survey divides purchases into 256 food and drink categories, and for the purposes of the current study, the researchers grouped 12 main drink categories. They put two survey groups – “soft drinks, concentrated, not low calorie” and “soft drinks, not concentrated, not low calorie” – into a category of sugar-sweetened drinks, or soft drinks with added sugar.

For information on drink consumption they used the National Diet and Nutrition Survey (2008–10) which is said to be representative of the diets of UK individuals, giving estimations of consumption according to age. In this survey 2,126 people completed a four-day food and drink diary.

To estimate the prevalence of overweight and obesity they used BMI (body mass index) data from the Health Survey for England, 2010, and the Scottish Health Survey, 2010. These large surveys involve objective rather than self-reported measures. The Welsh Health Survey was not used because it includes only self-reported measures, while the Northern Ireland Survey was too small. Population size estimates for England, Scotland, Wales and Northern Ireland came from the 2011 census.

They carried out three modelling steps:

estimating the effect that a tax increase would have on drink purchases
estimating the effect that changes in drink purchases would have on energy intake
estimating the effect that changes in energy intake would have on BMI

The main overall outcome they were interested in was to see the overall and income-specific changes that the 20% tax would have on the number of overweight (BMI ≥25kg/m2) and obese (BMI ≥30kg/m2) adults in the UK. Other outcomes were the effects according to age group (16–29, 30–49 and ≥50 years) and by UK country.

What were the basic results?

According to the National Diet and Nutrition Survey, we drink an average of 123ml of sugar-sweetened drinks per person per day, with an average energy intake of 206kJ per person per day. However, there is variation by age, with young people aged 16–29 years consuming an average 452kJ/person/day from sugar-sweetened drinks, and those aged over 50 years only consuming 96kJ per person per day. Each week we spend an average of 61.1p per person on sugar-sweetened drinks.

Prevalence of overweight and obesity was 26% in the Health Survey for England, and 28% for the Scottish Health Survey.

The 20% tax is estimated to reduce consumption of concentrated sugar-sweetened drinks by 15% and non-concentrated sugar-sweetened drinks by 16%, with compensatory increases in consumption of diet drinks, tea and coffee, milk and fruit juice. The effect that this would have on average energy intake is a reduction of 16.7kJ per person per day.

There was considerable variation according to age. The youngest age group, those aged 16–29 years, would reduce their energy intake by 56.3kJ per person per day, while those aged over 50 years would have no significant change in their energy intake.

The tax is estimated to reduce the number of people in the UK who are obese by 1.3% (95% credible interval – an estimate of the accuracy of a prediction – 0.8% to 1.7%) or by 180,000 people (95% credible interval 110,000 to 247,000).

The number who are overweight or obese would reduce by 0.9% (95% credible interval 0.6% to 1.1%) or by 285,000 people (95% credible interval 201 000 to 364 000). The percentage changes were similar across the four UK countries. There was predicted to be greater reduction in obesity in the highest income group (2.1% reduction) compared with the middle income group (0.9%) and lowest income group (1.3%), though the effects would be broadly similar.

Again, there was marked variation according to age group, with those aged 16–29 having an estimated average BMI reduction of 0.23kg/m2 while those aged 30 and above had an estimated reduction of only between 0.01 and 0.05kg/m2.

The tax would increase our weekly expenditure on sugar-sweetened drinks from 61.1p per person to 61.7p per person. The net increase in expenditure on all drinks would be 8.4p per person per week. Meanwhile, the 20% tax is estimated to generate £276 million revenue annually.

How did the researchers interpret the results?

The researchers conclude that a 20% sales tax on sugar-sweetened drinks would reduce the number of adults in the UK who are obese by 180,000 (1.3% reduction), and the number who are overweight or obese by 285,000 (0.9% reduction). They estimate that the tax would have the greatest effect on people under 30 years. The tax would have a similar effect in each country within the UK, and have health effects across income groups.

Conclusion

This valuable modelling study draws on a large quantity of UK survey data to estimate the effect that a 20% tax on sugar-sweetened soft drinks would have on overweight and obesity in the UK. Taxation has often been proposed as a way of reducing sugar and calorie consumption, while increasing government revenue.

The study estimates that taxation may result in a 1.3% reduction in obesity across the UK, with the greatest effects in the 16–29 age group, who are the biggest adult consumers of sugar-sweetened drinks in the UK.

There are a couple of points about this study to bear in mind. The various household surveys were completed by adults. Although adults are likely to be the main purchasers of food and drinks in a household, we have no information on the consumption by children and adolescents. While the study finds that the 16–29 year age group consume the most sugar-sweetened drinks, it may be that those under the age of 16 consume more.

Children and adolescents who have a high consumption of sugary drinks are also likely to have increased risk of adverse health effects from sugar, such as weight gain and dental caries, and risk associated health problems as they grow older, such as type 2 diabetes and cardiovascular disease.

Therefore, although this study shows us how adult sugary drink consumption, and adult weight problems, could change as a result of taxation of sugar-sweetened drinks, it would be interesting to know what effect taxation would have on the future adult generation.

It is also important to remember that, even for adults, these are estimates only.

The researchers used survey data that is representative of the UK population. However, as they say, no single dataset contains all relevant information.

Some data may be missing or inaccurate, particularly when it comes to household estimates of consumption and expenditure. Researchers may be able to give a fairly accurate estimate of how much overweight and obesity would change with a given reduction in calorie intake. But it is difficult to estimate how much the tax would influence purchasing habits – and the estimates of calorie change are based on the estimates of purchasing habits.

The researchers also add that their study has only covered the influence that price change could have on purchasing practice. They say they can’t account for other things that may have an influence, such as the “negative publicity surrounding sugar-sweetened drinks, resulting from either the continuing debate about legislation or the stigma attached to public awareness of a special tax on health grounds”.

Overall, while the research leaves some unanswered questions, it gives a valuable indication of the possible effects that a 20% tax on sugar-sweetened soft drinks may have on UK overweight and obesity.