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Smoking has topped the list of preventable causes of cancer for decades. But it might not be there forever.

While smoking rates have been falling, the proportion of the UK population who are overweight or obese has risen. And if these trends continue, obesity looks set to overtake smoking as the biggest preventable cause of cancer at some point in the future. But when?

Today, we released a report that tried to estimate this, in the hope that raising awareness will mean that day never comes.

“With smoking rates falling, we know that obesity will become the biggest preventable cause of cancer one day, if we don’t do anything about it,” says Dr Katrina Brown, Cancer Research UK. “What we wanted to know is, when? Would it be 5 years down the line or 50 years? And by putting a figure on it, we wanted to open a conversation that could help to change the trend.”

Here’s how our statisticians did it.

Crunching the numbers

Making projections like these isn’t straightforward.

“The International Agency for Research on Cancer, which is part of the World Health Organisation, reviews the evidence as to which cancers have a link with smoking or overweight/obesity, and where there is enough evidence to show a definite cause,” says Brown.

“It’s about combining this information with how many people are exposed to the risk factor in the UK and how many cases of cancer we’re expecting to see.”

To calculate the figures for smoking, the team combined data on how many people smoke or have smoked with the impact smoking has on a person’s risk of certain cancers. They could then calculate how many cancer cases were caused by smoking using UK cancer registry data.

This is the approach we used earlier this year to confirm around 4 in 10 cancer cases are preventable.

And to take the data into the future, they used projections that estimate what smoking and cancer rates might look like in years to come.

“Whilst the figures are based on projections, we’re using peer-reviewed methods, so we’re confident in our approach.”

These calculations can give us estimates for how many cancer cases will be caused by smoking or obesity in the next 20 years.

“We can’t project cancer rates too far in the future because there are just too many uncertainties, especially with new research in cancer prevention and diagnosis,” says Brown. “Generally speaking we don’t project cancer figures more than 20 years into the future. For these analyses, we used cancer incidence projections up to 2035.”

How did things change between 2015 and 2035? The proportion of cancer cases caused by smoking dropped from 15% in 2015 to 11.5% in 2035 in our estimates. And the proportion of cases caused by overweight and obesity rose from 5% in 2015 to almost 8% in 2035.

So, while the gap is set to close over the next 20 years, it looks like smoking will still top the list as the biggest preventable cause of cancer in 2035, if current trends continue.

Gender differences

The data got more interesting when the team looked at men and women separately.

The projections suggest that by 2035, smoking and obesity could cause almost equal numbers of cancers in women.

Graphic showing figures projecting when obesity could top smoking as the biggest preventable cause of cancer.

Copy this link to share our graphic. Credit: Cancer Research UK

And if these trends in the number of cases caused by these risk factors continue, overweight and obesity could overtake smoking as the biggest preventable cause of cancer in women by 2043.

While the gap between obesity and tobacco as causes of cancer in men is also expected to narrow in the next 20 years, there’s still a way to go before they cross over. And it’s too soon to estimate when this might happen.

The crossover is likely to happen earlier in women for two reasons.

First, more men smoke than women. And that means there are more smoking-related cancers in men. In 2015, 18% of cancer cases in men were caused by smoking, compared with just 12% in women.

And while men are also more likely to be overweight or obese than women, obesity has a bigger effect on women in terms of cancer. Some of the most common types of cancer caused by obesity are breast and womb cancer, which predominantly affect women.

Replicating ‘stop smoking’ success

These figures highlight a worrying trend around excess weight and cancer, especially as many people aren’t aware of the link. Last year, figures showed that only 1 in 7 people in the UK knew obesity was a cause of cancer.

But our latest estimates are also a cause for celebration. The fall in smoking rates in the UK is a massive win for cancer prevention and tobacco control policy.

In the first half of the 20th century, it’s estimated that up to 80% of men smoked. Today, 17% of UK males are smokers. And smoking rates in women peaked in the late 1960s and have been falling since.

This success is down to a combination of key policy changes – including advertising restrictions and increased tobacco taxes – and increased awareness of the risks.

There is still more to do though as we aim to reduce smoking prevalence further and try and achieve a smoke-free UK by 2035.

Tobacco control policy

Copy this link and share our graphic. Credit: Cancer Research UK

It’s this success in reducing smoking rates we hope to replicate with our obesity campaign.

We’re calling on the Government to ban junk food adverts on TV before the 9pm watershed to help protect children, with similar protections online, and to restrict unhealthy price promotions in supermarkets. And we’ve launched a UK-wide campaign to increase awareness of the link between obesity and cancer.

We’re starting to see promising movement. Earlier this year the Government released their updated childhood obesity plan, where they committed to consulting with the public, health bodies and industry on the 9pm junk food advert watershed. It’s not a done deal yet, but it’s a step in the right direction. And we’ll be keeping the pressure on to make sure they follow through.

“When it comes to these projections, our ideal scenario is that we’re wrong and the prevalence of overweight/obesity reduces,” says Brown. “And the work we do – our policy work, raising awareness and the research we fund – is to try and make sure we never reach this point.”