New studies are adding to the picture of how our eating habits, including the carbohydrate-containing foods we choose, affect our cancer risk. Although the amount of data is building, the picture is still anything but clear when we stack the studies next to one another.
This week I’m in Indianapolis, where the American Association of Diabetes Educators asked me to speak at their annual meeting. My topic: At the Crossroads: How do Diabetes and Cancer Intersect? As I’ve talked with colleagues here, the question of whether glycemic index is part of this intersection has come up several times.
Let’s step back from the relatively large volume of studies focused on glycemic index and cancer, and look at how we can weave together the common threads into a picture that makes sense.
Glycemic Index vs. Glycemic Load
Carbohydrate that is digested and absorbed quickly, raising blood sugar rapidly and leading to a prompt rise in insulin to handle it, is referred to as having a high glycemic index (GI). Such foods include not only sweets, but potatoes, white rice and refined cereals and breads. Foods with a low GI are digested and absorbed more slowly. These include whole grain breads, pasta, beans, and most fruits and vegetables.
The concept is simple, but, as they say, “the devil is in the details”….
- Although you can find lists of foods with their GI value in books and on websites, actual blood sugar-raising effects of foods vary with how they are prepared. So depending on the reference used, the GI listed for a food may or may not represent the effect of the food as you are eating it.
- What’s more, the actual blood sugar effects of any food change substantially based on whether you eat it on its own or along with sources of protein, fiber or fat, all of which blunt the rise in blood sugar.
- Furthermore, 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”. This is where Glycemic Load (GL) comes in, which is calculated based on the glycemic index of a food and its portion. In other words, GL is an expression of both type and amount of carbohydrate in a food or in an overall diet.
High levels of insulin circulating through the body seem to create metabolic conditions that favor cancer development. Insulin is not just a blood sugar control hormone. It also activates pathways that lead to growth and reproduction of cells, including any cancer or pre-cancerous cells present. In addition, high levels of insulin and a related growth factor (IGF-1) seem to inhibit the self-destruction of abnormal cells that is part of our body’s normal defense against cancer. What’s more, high levels of insulin lead to changes in proteins that tie up estrogen, resulting in greater availability of estrogen to promote estrogen-sensitive cancers.
The Latest Studies
The large EPIC study, a population study based on 23 centers throughout Europe, recently released analysis looking at breast cancer risk’s potential link to GI and GL. It showed no link of GI or GL to overall breast cancer risk or to estrogen-sensitive (ER-positive) postmenopausal breast cancer, which is the most common form. However, women with overall eating patterns with high GL did show 36 percent greater risk of ER-negative postmenopausal breast cancer compared to women with the very lowest GL. Furthermore, women with the very highest total carbohydrate consumption showed a similar 41 percent greater risk of this form of breast cancer compared to women eating the least. This is interesting follow-up to the very mixed bag of results from numerous previous studies of breast cancer. The most recent analysis combining multiple studies together does link the very highest GI with a statistically significant but modest 8 percent increased risk of breast cancer compared to those with lowest GI, but individual studies of GI and GL have shown inconsistent results. So this recent study may be the beginning of finding the specifics of which forms of this cancer and in whom GI/GL might be relevant.
The other hot-off-the-press study, a Canadian population study, continues the picture of mixed results. Here there was no link to GI or GL seen for pre- or postmenopausal breast cancer (no analysis by estrogen receptor type) nor for 11 other cancers. However, prostate cancer incidence was 26 percent higher in men with diets highest in GI. And both colorectal and pancreatic cancer incidence was higher (28 and 41 percent, respectively) in people with high-GL diets. Surprisingly, since the theory would be that increased cancer risk reflects the effects of elevated insulin levels, the effect of GL on cancer was no greater among those with most overweight and least physical activity – which is the group of people most likely to be “insulin resistant” and respond to high-GL diets with increased insulin levels. Also surprisingly, looking at breast cancer results the trend was not statistically significant (which means it can just be a matter of chance), but if anything, the trend was for diets highest in GL to be linked to lower incidence of postmenopausal breast cancer.
These results showing a link to colorectal cancer add to the picture, but they are not the whole picture. An analysis of 14 population cohort studies (the strongest type of population research) published just a few months ago showed no link to colorectal cancer risk related to total carbohydrate consumption or GL. There was a trend for a seven percent increase in risk among those with the very highest GI, but this was not statistically significant, which means it could be found simply by chance.
It’s Got to be the Details
The wide variation in results of the many studies that have now looked at the potential relationship of GI and GL to cancer risk show that this relationship is not clear-cut.
- Some variation can come from differences in the studies themselves, for example differences in the forms used to ask about people’s diets and the tools used to analyze GI. Even when the GI score of a food is based on detailed description of how it is prepared, scores can’t reflect the different impact of other foods consumed at the same time.
- Studies differ in what other influences they adjust for in analysis of GI/GL and cancer risk. For example, the latest Canadian study reported above did not control for dietary fiber consumption, whereas the EPIC study of breast cancer risk did. Dietary fiber in a food or in other foods eaten at the same time reduces the blood-sugar rise it produces. In part, dietary fiber of a particular food is already reflected in its GI, but overall dietary fiber consumption could still affect its impact. Analysis by the American Institute for Cancer Research/World Cancer Research Fund in the continuous update project looking at all available research worldwide links higher dietary fiber with lower risk of both breast and colorectal cancers.
- Individual genetic and metabolic differences may make some people respond to similar food choices with varying jumps in insulin levels.
- Since insulin does not act alone in promoting cancer, other individual differences may make similar increases in insulin weigh more or less heavily among the many influences promoting and inhibiting cancer development.
Bottom line: We eat food, not an index
Evidence is now quite strong that high levels of insulin lead to metabolic conditions favoring cancer development. Awareness of glycemic index can help, but it can also lead you astray. Just because two foods are similar in glycemic index does not mean their total impact on health is the same.
Rather than focus specifically on the glycemic index of your diet, aim for an overall strategy to avoid elevated insulin and create a pattern of eating choices that supply nutrients and phytochemicals that reduce cancer risk.
♦ Accumulate at least 30 minutes of moderate physical activity a day to decrease insulin resistance. Don’t let more than a day go by without being active.
♦ Control portion sizes even of “healthy” food. This has immediate benefits on GL, but even more importantly is key to reach and maintain a healthy level of body fat, which acts in multiple ways to reduce cancer risk.
♦ Make vegetables, fruits, whole grains and beans the largest part of your plate. They’re more than just low GI: they provide dietary fiber that seems to have protective effects beyond its impact on blood sugar and thousands of phytochemicals that act as antioxidants and apparently with more direct effects on cells to decrease cancer development.
For the opposite of the complexities of glycemic index, check the very simple approach to balanced eating and sensible portions illustrated in the New American Plate developed by the American Institute for Cancer Research (AICR).
Romieu I et al. Dietary glycemic index and glycemic load and breast cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). Am J Clin Nutr 2012. 96:345–55.
Dong JY, Qin LQ. Dietary glycemic index, glycemic load, and risk of breast cancer: meta-analysis of prospective cohort studies. Breast Cancer Res Treat 2011;126:287–94.
Hu J et al. Glycemic index, glycemic load and cancer risk. Annals of Oncology. Published online July 25, 2012, ahead of print.
Aune D et al. Carbohydrates, glycemic index, glycemic load, and colorectal cancer risk: a systematic review and meta-analysis of cohort studies. Cancer Causes Control. 2012. 23(4):521-35.
Aune D et al. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies. Ann Oncol. 2012. 23 (6): 1394-1402.
Continuous Update Project Report Summary. Food, Nutrition, Physical Activity and the Prevention of Colorectal Cancer. World Cancer Research Fund / American Institute for Cancer Research. 2011.