Ingrid asked a very good question about my participation in the UK Biobank study. How typical of the general population are the people who choose to take part in medical studies?
The subject popped up again this week. The headlines were full of how fruit and vegetables make little difference to your risk of getting cancer. Yet our whole ‘5-a-day’ strategy was based on the belief that eating more fruit and veg could make a big difference to your cancer risk, reducing it by around half. (I covered this in more detail for the BMJ)
What changed? Why do studies from 20 years ago show a halving of cancer risk, while a study published this week says, at best, eating plenty of fruit and veg reduces your risk of cancer by three percent?
It’s down to the type of study, and who takes part. Studies done in the 1980s compared the fruit and veg consumption of people who’d been diagnosed with cancer, with the diets of volunteers in a ‘control group’. The important issue here is, who were the volunteers?
People who agree to talk through their diet in detail with researchers tend to be interested in their health, perhaps proud of their healthy diet. Those who don’t want to admit to a diet of pasties and pot noodle may decide they’re too busy to take part. The result is that people in the ‘control’ group eat lots of fruit and vegetables, while the people with cancer have a diet more representative of the average. It looks like fruit and vegetables can stop you getting cancer.
That’s why we need prospective studies, like the one published this week. These studies follow vast numbers of people, ask them to fill in dietary questionnaires, and see how many get cancer in the years to come. Even if everyone in the study is healthier than average, you should be able to see whether there’s a real link between diet and cancer.
So should we abandon 5-a-day? Well, the researchers say that good, prospective studies show fruit and veg helps protect against heart disease, even if it doesn’t prevent cancer. So stick with the carrots – until the advice changes again.