"White bread, bagels and rice 'increase the risk of lung cancer by 49%','' the Mail Online reports after a US study found a link between lung cancer and eating a diet with a high glycaemic index (GI), a measure of carbohydrate content.
The study included more than 4,000 white people from Texas, both people newly diagnosed with lung cancer and healthy controls.
The researchers assessed their diet to look at whether there was any link between their diagnosis and their intake of high-GI foods – typically, high-carbohydrate foods such as white bread, potatoes and rice.
Researchers found people in the highest (fifth) intake group for high-GI foods were about 49% more likely to have lung cancer than someone in the lowest (first) intake group.
But drawing any firm conclusions from this is difficult. There was no increased risk for anyone in the intermediate intake groups, and there is no information we can use to give an equivalent food intake.
This study was a cross-sectional assessment of diet in people who had already been diagnosed with lung cancer. We don't know whether this is a reliable indication of lifetime dietary patterns, so it can't prove cause and effect.
Overall, the possible link between carbohydrates and lung cancer needs further investigation to see whether there is a direct link and, if there is, find out the possible cause.
On its own, this study does not provide strong evidence that "carbs are the new cigarettes".
Where did the story come from?
The study was carried out by researchers from the University of Texas.
It was funded by the National Institutes of Health, the Cancer Prevention & Research Institute of Texas, the University of Texas MD Anderson Cancer Center, and the National Cancer Institute.
It was published in the peer-reviewed journal, Cancer Epidemiological Biomarkers Prevention.
While the Mail's reporting was broadly accurate, it would have benefited from considering some of the limitations of this research, which cannot prove carbohydrates directly cause lung cancer.
What kind of research was this?
This case-control study included people who had been newly diagnosed with lung cancer and healthy controls.
The researchers aimed to compare these two groups, looking at the glycaemic index and glycaemic load of the foods they ate – in other words, how quickly these foods cause a rise in blood glucose after eating.
Smoking is a well-established risk factor for lung cancer, and is reportedly linked to 85% of cases. But evidence suggests certain dietary factors may also influence risk.
Carbohydrate is the main contributor to a surge in blood sugar levels after eating, and causes the release of insulin.
Previous studies have shown that insulin-like growth factors (IGFs) – proteins similar to insulin – can simulate abnormal cell growth and division. And some studies have suggested IGFs are raised in people with lung cancer.
Although this study included both people with and without cancer, it wasn't a case-control study in the traditional sense – it didn't look back at data collected previously on factors that may have influenced cancer risk. Dietary factors were assessed cross-sectionally, so cause and effect can't be proved.
What did the research involve?
The study involved 1,905 white adults newly diagnosed with lung cancer at the University of Texas MD Anderson Cancer Center. The comparison group were 2,413 healthy controls recruited from routine clinics, matched by age, gender and ethnicity.
All participants had their body mass index (BMI) measured, and were interviewed about their medical history and lifestyle factors, including detailed assessments of their smoking history, physical activity and dietary intake.
Diet assessment queried food preparation methods, portion size, grams per day consumption for each item, total intake of carbs, fibre and meat, as well as total energy intake.
Using a previously developed method, glycaemic index (GI) was calculated, taking into account the quantity and type of carbohydrate consumed each day.
The GI and glycaemic load were categorised into five groups (quintiles) and analysed for cases and controls, taking account of various potential confounding factors, including smoking, alcohol, physical activity, and BMI.
What were the basic results?
Overall, when comparing the groups, cases with lung cancer were more likely to smoke, take less physical activity, and be less educated. Cases also ate more high-GI foods, but had a lower total carb and fibre intake.
People who were in the highest (fifth) quintile for GI intake were calculated to have 49% increased odds of having lung cancer compared with those in the bottom (first) group (odds ratio [OR] 1.49, 95% confidence interval [CI] 1.21 to 1.83).
People in the fifth quintile for GI similarly had a 48% increased risk. Those in the lower GI quintiles (second to fourth) did not have an increased risk of lung cancer, and there was no association between lung cancer and glycaemic load.
The effect of a high GI was most pronounced for people who had never smoked. This suggests that for smokers, smoking had more of an influence on risk than GI.
How did the researchers interpret the results?
The researchers concluded: "This study suggests that dietary GI and other lung cancer risk factors may jointly and independently influence lung cancer aetiology."
They say understanding the role of GI in lung cancer could inform prevention strategies and help identify biological pathways related to lung cancer risk.
The researchers say this is only the second study to have looked at the association between glycaemic index (GI) and lung cancer risk.
The researchers found people with the greatest daily intake of high GI foods – in particular coming from carbs – were about 50% as likely to have lung cancer than someone with the lowest intake.
The study suggests a link between high GI foods and lung cancer, but drawing any definite conclusions is difficult.
Cause and effect not proven
This study cannot prove cause and effect. Although it is a case-control study, it only uses a cross-sectional dietary assessment performed when the person already had cancer.
We don't know whether the diet intake reported reflects someone's eating patterns over the course of their lifetime.
A more reliable method would be a cohort study that followed healthy people in the long term, regularly assessing their diet, activity and other lifestyle risk factors, and seeing whether this was linked to lung cancer.
Relied on self-reported data
The dietary questionnaire responses – and hence the categorisation of people according to the glycaemic load and index of the foods they eat – may contain inaccuracies.
People may not be able to reliably assess the dietary components, portion size and grams of each item they consume each day, particularly if they are trying to report on their intake over the course of their life.
No consistent pattern
A significant link with lung cancer was only found for those in the highest (fifth) quintile for GI when compared with the lowest (first) quintile. What this actually means is difficult to say.
There was no risk increase for anyone in quintiles two to four, and we can't give an equivalent food intake of what the fifth quintile actually meant – for example, how many slices of white bread or potatoes a day this would equate to.
Other confounders could be involved
The researchers made careful attempts to adjust for other important lifestyle factors that could be associated with lung cancer, such as smoking and physical activity. But we don't know that all of these lifestyle factors have been fully accounted for.
This is a specific population sample of white people from Texas, and their results may not be applicable to all people across the US, let alone those from other countries.
This study does suggest a link between GI and lung cancer that is worthy of further investigation. We now need to find out whether there is a direct link and the possible causes.
If there is a link, it would also be valuable to know whether the effect is specific to lung cancer or if it could also apply to other cancers.
Despite the media headlines, this study does not provide evidence that suggests carbohydrates are as much of a risk factor for lung cancer as smoking. Smoking remains the most well-established risk factor for lung cancer.
Carbohydrates should make up a third of your total energy intake. Read more diet advice about carbs.