August 2017
S. Sieri, C. Agnoli, V. Pala, S. Grioni, F. Brighenti, N. Pellegrini, G. Masala, D. Palli, A. Mattiello, S. Panico, F. Ricceri, F. Fasanelli, G. Frasca, R. Tumino, and V. Krogh
Abstract
Factors linked to glucose metabolism are involved in the etiology of several cancers. High glycemic index (GI) or high glycemic load (GL) diets, which chronically raise postprandial blood glucose, may increase cancer risk by affecting insulin-like growth factor. We prospectively investigated cancer risk and dietary GI/GL in the EPIC-Italy cohort.
After a median 14.9 years, 5112 incident cancers and 2460 deaths were identified among 45,148 recruited adults. High GI was associated with increased risk of colon and bladder cancer. High GL was associated with: increased risk of colon cancer; increased risk of diabetes-related cancers; and decreased risk of rectal cancer. High intake of carbohydrate from high GI foods was significantly associated with increased risk of colon and diabetes-related cancers, but decreased risk of stomach cancer; whereas high intake of carbohydrates from low GI foods was associated with reduced colon cancer risk.
In a Mediterranean population with high and varied carbohydrate intake, carbohydrates that strongly raise postprandial blood glucose may increase colon and bladder cancer risk, while the quantity of carbohydrate consumed may be involved in diabetes-related cancers. Further studies are needed to confirm the opposing effects of high dietary GL on risks of colon and rectal cancers.
Introduction
Factors linked to glucose metabolism seem to be involved in the etiology of several cancers1–4. Consumption of most carbohydrates increases blood glucose and blood insulin, but to varying extents, depending on carbohydrate type and processing, amount consumed, and presence of other nutrients. These variations are captured by the glycemic index (GI)5, which ranks carbohydrate foods according to their ability to raise blood glucose levels.
High GI foods, like white bread, are rapidly digested and cause a rapid peak in blood glucose. Low GI foods like pulses and pasta, are digested more slowly, prompting a more gradual rise in blood glucose. Glycemic load (GL), the product of a food’s GI and its available carbohydrate content, was introduced to incorporate the effect of the total amount of carbohydrate consumed: it is a measure of total glycemic effect, and is hence an indicator of the insulin demand of the diet.
Several observational studies have investigated associations between dietary GI/GL and risk of different types of cancer, but have produced mixed results. Three meta-analyses − one that investigated only cohort studies6, and others that considered both case-control and cohort studies7, 8 – found that high GI was associated with increased risk of colorectal cancer.
Meta-analyses also found that high GI and GL were weakly associated with increased risk of breast cancer9 and diabetes-related cancers6, while high GL was associated with increased risk of endometrial cancer6. The risks of developing other cancers do not appear to be influenced by dietary GI or GL6, 7, 10, 11.
Associations of dietary GI/GL with colorectal and breast cancer have been investigated previously in persons recruited to the Italian section of the European Prospective Investigation into Cancer and Nutrition (EPIC-Italy)12, 13. It was found that high GI was significantly associated with increased risk of colorectal cancer12, and high dietary GL was significantly associated with increased risk of breast cancer13. In the present study we updated the follow-up of the EPIC-Italy cohort in order to assess associations of dietary GI and GL with various types of cancer.
Discussion
The main findings of our study are that high dietary GI was significantly associated with increased risk of colon and bladder cancer; whereas high dietary GL was significantly associated with increased risk of colon cancer and DRCs (which include colon cancer), but decreased risk of rectal cancer.
Furthermore, high carbohydrate intake from high GI foods was significantly associated with increased risk of colon cancer and DRCs, but decreased risk of stomach cancer, whereas high carbohydrate intake from low GI foods was significantly associated with decreased risk of colon cancer.
Our finding that high dietary GI, high dietary GL, and high carbohydrate intake from high GI foods, are associated with increased colon cancer risk is in line with the previous EPIC-Italy study12 which found that high dietary GI (but not GL) and high consumption of carbohydrates from high GI foods, were associated with significantly increased colon cancer risk.
Like the present study, which considered 122 more colon cancer cases than the previous study, our previous study12 also found that high consumption of carbohydrates from low GI foods was associated with lowered colon cancer risk: thus taken together the results of both studies suggest that colon cancer risk depends more on the ability of the carbohydrate foods consumed to raise postprandial blood glucose than the overall quantity of carbohydrate consumed.
An alternative interpretation would be that high consumption of highly refined carbohydrates reduces consumption of carbohydrates from low GI foods and hence also reduces consumption of polyphenols and other antioxidants which may protect against colon cancer14.
