Eating foods with a high glycemic index (GI) can make your blood sugar rise higher and faster after eating. Theoretically, that could cause unhealthy levels of hormones like insulin, which seem to promote development of some cancers, including breast.
However, research suggests that glycemic index by itself has little to no relation to breast cancer risk.
An analysis of 19 studies found no link between breast cancer risk and diets high in GI beyond what could occur by chance. Even glycemic load (GL), which takes portion size of foods into account, showed no significant link to breast cancer risk. The links were not consistent and could reflect other qualities of those diets. Another analysis that included only studies with a stronger design that follows people over time (called prospective cohort studies) found a weak five to six percent increase in breast cancer risk when comparing diets at the very highest to the very lowest glycemic index or glycemic load, respectively.
Foods low in glycemic index are often healthy choices because they’re high in fiber, nutrients and protective plant compounds. In contrast, many high-GI foods are high in sugar or refined grains, concentrated in calories, and lacking significant nutrient contributions.
If you use glycemic index as the defining criteria for choosing healthy foods, you may be shunning some relatively high GI foods that have valuable health benefits. Foods similar in glycemic index can also differ widely in cancer-protective potential.
Although books and websites list foods’ GI values, the food’s actual effect on blood sugar depends on how your food is prepared and whether you eat it alone or with sources of protein, fiber or fat. Moreover, a large portion of a “low GI food” could end up raising blood sugar as much as a small portion of a “high GI food”.
A recent Tufts University study shows that even among relatively similar circumstances, glycemic index for the same food varies by 25% among people without a chronic disease. In fact, how a food affected blood sugar in the same individual varied by 20% among 6 different tests over a 3-month period.
The Take-Home Message
Instead of making GI the focus, try targeting overall healthy choices and appropriate portions. AICR’s New American Plate and USDA’s MyPlate are great tools to help you do this.
Some people (those who are inactive or have type 2 diabetes, for example) may be especially likely to have blood sugar and insulin rise in response to poor quality or too much carbohydrate. But even then, GI shouldn’t be a starting point and can’t replace the importance of overall eating pattern and variety of healthful foods.