Jury still out on fizzy drinks and pancreatic cancer

This week, the headlines have made bold claims about a possible link between drinking too many fizzy drinks and an increased risk of pancreatic cancer.

The claims were based on a new study, led by Mark Pereira from the University of Minnesota and colleagues in Singapore. It was published in the journal Cancer Epidemiology Biomarkers and Prevention.

The new research is interesting but, on its own, it’s not enough to conclude that soft drinks cause pancreatic cancer. In fact, the existing evidence in this area is inconsistent – some studies have linked soft drinks to a higher risk of pancreatic cancer, but others disagree.

But the new study does raise some unanswered questions.

What is this new research?

The researchers recruited a ‘cohort’ of more than 60,500 people, as part of the Singapore Chinese Health study, and asked them about their lifestyles, including how many soft drinks they usually had. Then they followed up their recruits for up to 14 years, noting when someone in the group developed pancreatic cancer.

Once the team had collected all this information, they used it to find out whether people who drank more soft drinks were any more or less likely to develop pancreatic cancer than those who drank fewer, or if they had no effect on risk.

By the end of the study, 140 of the participants had developed pancreatic cancer.

Crunching the numbers, the researchers concluded that people on the study who drank more than two soft drinks a week (in fact, people in this group drank an average of five per week) had an 87 per cent higher risk of developing pancreatic cancer compared to people who avoided them. That’s almost double the risk.

It’s also worth noting that the in this study, drinking fruit juice didn’t affect the risk of pancreatic cancer – we’ll come to why this matters later.

How could soft drinks affect the risk of pancreatic cancer?

The pancreas helps us control our blood sugar levels by secreting the hormone insulin. Soft drinks contain lots of sugar – so drinking lots of them can encourage the pancreas to churn out insulin. But as well as regulating blood sugar, insulin can also encourage pancreatic cells to grow and divide. So Pereira’s team suggest that, by boosting insulin levels, soft drinks could in theory increase the risk of cancer.

A surplus of insulin also increases the amounts of another group of hormones called IGFs (insulin-like growth factors). These could also influence the growth of pancreatic cancer cells.

However, if these explanations are correct, it’s not clear why fruit juices, which can also be very sugary, didn’t affect the risk of pancreatic cancer in this study.

Of course, it’s possible that people who drink a lot of soft drinks are different in other ways from those who avoid them. Indeed, Pereira found that they were more likely to smoke, drink more alcohol and eat more calories, carbohydrates, fat, added sugar and red meat than those who shun soft drinks.

Could these differences explain their higher risk of pancreatic cancer? It’s possible, and we’ve blogged before about how lifestyle factors add up to affect pancreatic cancer risk. But Pereira recorded whether the participants smoked, how active they were, how much alcohol they drank, how much sugar, candy and calories they ate, their body weight, whether they were diabetic, and their educational level. His team adjusted for these things in their results, and still found that soft drinks affected cancer risk.

As with all studies like this, there are pitfalls to avoid when we’re drawing conclusions. The more people are included in a study like this, the more reliable the results will be. 60,000 people sounds like a lot, but pancreatic cancer is not very common, and only 140 people in the study actually developed the disease. So although the researchers recruited a lot of people, there is a chance the results could be a statistical ‘blip’ rather than a genuine relationship.

Finally, the researchers only asked people about their soft drink consumption once – right at the start of the study. So if they changed the amount they drank, it wouldn’t have been counted. But the study ran for fourteen years, so it’s quite possible that people did in fact change their habits.

The evidence is already inconsistent

Studying the links between diet and cancer is a tricky business. Our diet consists of so many foods, drinks and nutrients that it is difficult to isolate the influence of any specific ones. And even results from studies like this can sometimes be due to chance, or be biased  on things that haven’t been accounted for. This is why it’s important to consider any new study in the light of existing evidence.

So far, four similar cohort studies have looked for a possible link between soft drinks and pancreatic cancer. One agreed with Pereira’s study, finding that people who drank the most soft drinks had higher pancreatic cancer risks. A second study found a link in women but not men. But the other two studies found no association between soft drinks and pancreatic cancer.

One of these contradictory reports is worth paying attention to because it comes from the American AARP study, one of the largest studies on diet and cancer ever carried out. It included over 487,000 people, of whom 1,258 developed pancreatic cancer, far more than in the new Singaporean research.

All in all, this study adds some new evidence to the bigger picture, but it doesn’t make that picture much clearer.

There is a health risk…

Even if soft drinks don’t directly increase the risk of cancer, it’s worth remembering that they are high in calories. Drink too many and you could gradually pile on the pounds, and we know that an unhealthy bodyweight is one of the most important preventable causes of cancer.

A huge amount of consistent evidence shows that being overweight and obese can increase the risk of a wide range of cancers. These include breast cancer (after the menopause) as well as bowel, pancreatic, kidney, oesophageal, womb and gallbladder cancers and potentially many more. So cutting back on soft drinks could indirectly help to reduce the risk of cancer by making it easier to keep a healthy body weight.

What’s next?

Although the results are somewhat inconclusive, it’s still important that studies like these are carried out. For reasons that Henry’s discussed before, we’re still largely in the dark about what causes pancreatic cancer. As with many other cancers, tobacco is known to be a cause of the disease, but it still only accounts for around a fifth of cases. Being overweight, having diabetes or drinking a lot of alcohol have all been linked to a higher risk of pancreatic cancer too.

Pancreatic cancer also remains one of the most difficult types to treat. Around 7,600 people are diagnosed with pancreatic cancer every year, and only two to three per cent survive the disease for five years or more after their diagnosis. This is why Cancer Research UK’s 5-year strategy includes commitments to invest in research on pancreatic cancer, alongside other hard-to-treat cancer types.

Ed and Jess

Image from Wikimedia Commons

Reference:

Mueller, N., Odegaard, A., Anderson, K., Yuan, J., Gross, M., Koh, W., & Pereira, M. (2010). Soft Drink and Juice Consumption and Risk of Pancreatic Cancer: The Singapore Chinese Health Study Cancer Epidemiology Biomarkers & Prevention, 19 (2), 447-455 DOI: 10.1158/1055-9965.EPI-09-0862