Philadelphia, Pennsylvania - Glycemic index, a measure of carbohydrate intake and glucose levels, was associated with increased risk of lung cancer in non-Hispanic whites, with a significantly higher risk in those who had never smoked, according to a study published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.

“Although smoking is a major, well-characterized risk factor for lung cancer, it does not account for all the variations in lung cancer risk,” said the study’s senior author, Xifeng Wu, MD, PhD, professor and chair of Betty Marcus Cancer Prevention in the Department of Epidemiology at The University of Texas MD Anderson Cancer Center. “This study provides additional evidence that diet may independently, and jointly with other risk factors, impact lung cancer etiology.”

Glycemic index (GI) assigns foods an indexed value to show how quickly and how much carbohydrates in the food cause blood glucose levels to rise after eating. Glycemic load (GL) is a related measure that is calculated by multiplying GI by the amount of carbohydrates in grams, then dividing by 100. Previous studies have investigated the association of GI and GL with certain types of cancer, including colorectal, stomach, and pancreatic cancers, but there has been limited research into the association with lung cancer.

Wu and colleagues selected patients and control subjects from an ongoing case-control study of lung cancer conducted at MD Anderson. The patients were newly diagnosed and had not received treatment other than surgery. The healthy control subjects were selected from patient lists at Kelsey-Seybold Clinics, a large physician group in the Houston area. The study results encompass 1,905 cases and 2,413 controls. Researchers conducted in-person interviews to determine the subjects’ health histories and dietary behavior including food frequency and portion size. Foods were assigned GI values according to previously published research, and the study subjects were categorized into quintiles, based on GI and GL as determined by Wu and her colleagues. The researchers stratified the data by age, gender, education, smoking status, and body mass index to adjust for confounding factors.

Overall, those in the highest quintile of GI had a 49 percent greater risk for developing lung cancer than those in the lowest quintile. Compared with those in the lowest quintile of GI, those in the highest quintile had a 92 percent higher risk of developing squamous cell carcinoma, but less significant association was seen with developing adenocarcinoma.

Among subjects who had never smoked, those in the highest quintile of GI were more than twice as likely to develop lung cancer as those in the lowest quintile. Wu explained that because smoking is the strongest risk factor for lung cancer, the absence of smoking allowed for detection of other risk factors, such as GI.

According to Stephanie Melkonian, PhD, a postdoctoral fellow in Wu’s group and lead author of the paper, GL was not associated with increased lung cancer risk for most patients, with the exceptions of the never-smokers, where those in the highest quintile, compared with those in the lowest quintile, had an 81 percent higher risk. Among participants with less than 12 years of education, those in the highest quintile had a 55 percent higher risk of developing lung cancer than those in the lowest quintile.

Melkonian said one possible explanation for the connection between GI and lung cancer is that diets high in GI result in higher levels of blood glucose and insulin, which promote glucose intolerance and insulin resistance. In turn, insulin resistance has been associated with changes in the body’s insulin-like growth factors, which play a role in cell proliferation and differentiation in cancer, according to previous research.

Wu said further research could examine whether dietary changes have an effect on cancer-related biomarkers, and whether the association between GI and lung cancer carries over to other racial and ethnic groups. She said the study has some immediate implications for patients and health-care providers.  

“The results from this study suggest that, besides maintaining healthy lifestyles, reducing the consumption of foods and beverages with high glycemic index may serve as a means to lower the risk of lung cancer,” Wu said.     

“Traditionally, glycemic index is only emphasized for diabetes prevention and management.  However, if the results from this study are confirmed, health care providers should be made aware of the link between glycemic index and lung cancer so they can communicate with their patients and the public about dietary changes for lung cancer prevention,” Wu added.

Wu identified certain limitations of the study. First, as a retrospective study, it is subject to errors in a participant’s recall and reporting of his or her dietary intake, she noted. Also, this study did not include information on whether the subjects had diabetes, hypertension, or heart disease.

“Given the link between diets with high GI and diabetes, this association warrants further attention in future studies,” Wu said.

This study was supported in part by the National Institutes of Health, the Cancer Prevention & Research Institute of Texas, MD Anderson’s institutional support for the Center for Translational and Public Health Genomics, and the NCI R25T Postdoctoral Fellowship in Cancer Prevention. Wu declares no conflicts of interest.