Sugary drinks increase cancer risk? Maybe not in this study.
Chazelas and colleagues report a positive association between intake of sugary drinks and the risk of cancer. This conclusion is unsurprising, but a few observations should question the validity of the evidence.
The study is an online survey study with an average follow-up of 5 years. As is common in online studies, participants were younger, more highly educated, healthier, and more health conscious than the average citizen. The participants hardly drank sugary drinks and fruit juice, on average 92.9ml per day. Also the consumption in the high-intake group was moderate: 98.3ml of sugary drinks and 107.5ml of fruit juice per day, which is a can of coke in 3 days and a liter of juice in 9.
Are sugary drinks so harmful that even small amounts at a young age increase the risk of cancer within 5 years?
The crude data show otherwise. The high-intake group least often developed cancer: there were 414 patients in the high-intake group and 743 in the lowest intake group. The high-intake group also had a lower body mass index and a lower fat percentage; they were less likely to have diabetes and high blood pressure; and cancer was less common in their families.
The reason why the sign of their effect size has flipped is likely because of age. The average age in the high- and low-intake group was 34 and 53 years. In their analysis, the researchers correctly take into account person years at risk but they do not mention how many people in each group were excluded because they had cancer prior to inclusion. In the supplementary file we read that 6,735 participants were excluded because of prevalent cancer, which is a number that cannot be ignored given that “only” 2,193 participants developed cancer during the study.
If older participants are more likely to have had cancer prior to the study, then the cancer risk of the (older) low-intake group is underestimated. Then the risk for the high-intake group is not increased but seems to be increased. Then drinking small amounts of sugary drinks seems harmful to health, but may not be.
Competing interests: No competing interests