A study published in the journal of the British Medical Association, BMJ, today says a tax on sugary drinks could cut the number of obese adults in the United Kingdom by 180,000. Similar Australian projections predict an even bigger impact.

The BMJ study shows people aged from 16 to 29 years would be most affected, and the tax would raise up to £276 million (AU$442 million) annually, which could be invested in the health sector.

Lennert Veerman, senior research fellow in population health at the University of Queensland, is in the process of doing a similar projection with Australian data.

He has found that if such a tax were implemented here, the number of Australians with obesity would drop by 110,000, a 2.6% drop in the nation’s obesity rate. This is twice the impact expected in the United Kingdom according to the BMJ study.

Veerman said that one reason for the difference is “unique consumption” of sugary drinks in the two countries; people in the United Kingdom consume less sugary drinks per person than Australia.

Another reason is that we are reliant on very old data; Australia’s last national nutrition survey was conducted in 1995. The survey collects data on what and how much people consume.

Veerman also noted that such studies tend to underestimate true effects because people under-report how much they indulge in unhealthy habits.

Nutritional clinical affiliate at the University of Sydney, Suzie Ferrie said a tax on sugary drinks would greatly benefit society.

Ferrie said “patterns of consumption” show young people under the age of 30 drink more soft drinks than older people, so the potential for change would be great because this age group is when habits really become entrenched.

“Changing youth drinking habits to healthier options could carry on to future generations,” she said.

But the chances of having such a tax implemented are low.

“Not many countries have managed to successfully implement these taxes,” said research fellow at Deakin population health, Gary Sacks.

The problem is that all governments have faced opposition from the private sector. In France, Sacks pointed out, “the food industry threatened to close down some of their factories”.

Despite full-page ads in Mexican papers opposing the measure, the government has just implemented a tax of one peso (A$0.08) per litre of soft drink and a 5% excise on high-calorie packaged food.

It is also too early to understand the full effect of the tax on the obesity rates in these countries because most laws have only been in effect for a couple of years. So, there is still only a limited amount of data to analyse.

However, when Denmark put a tax on fatty foods, consumption dropped by 15%, which was consistent with the modelling, Sacks said.

In an accompanying editorial in the BMJ, assistant professor of population medicine at Harvard Pilgrim Healthcare, Jason P Block suggests the only way we can truly find out what the impact of such a tax would be is by implementing it and reviewing the results.

But Ferrie suggested a pilot study would be adequate to collected data.

A new national nutrition survey to update from the one in 1995 may be a better way help calculate the results of such measures. It would be very expensive, but useful for many public health initiatives.

“We’re really suffering now because we still use old data,” Ferrie said. “I think the investment is worth it.”