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1 Department of Epidemiology, UCLA School of Public Health, University of California at Los Angeles (UCLA), Los Angeles, California; 2 Division of Preventive Medicine, Harvard Medical School and Brigham and Womens Hospital, Boston Massachusetts; and 3 Department of Epidemiology, Harvard School of Public Health, Boston Massachusetts
| Abstract |
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| Introduction |
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The glycemic potential of a diet high in refined-carbohydrate has long been of interest in the management of type 2 diabetes mellitus, but there is growing recognition that consumption of a diet with a high glycemic load (GL) may have adverse effects in the nondiabetic population as well. In 1992, Bruning et al. (7) found evidence of insulin resistance in breast cancer cases compared with controls and in the mid-1990s, Kazer (8) hypothesized that insulin-like growth factor 1 may be an important factor in this relationship. Giovannucci and McKeown-Eyssen (9 , 10) further developed the insulin resistance hypothesis, but in relation to colon cancer, suggesting that hyperinsulinemia and insulin resistance may act as tumor promoters. They noted that dietary and lifestyle risk factors for developing insulin resistance, such as physical inactivity, obesity, and positive energy balance, were risk factors for developing cancer, and suggested that chronic hyperinsulinemia may create an unfavorable metabolic environment that facilitates carcinogenesis. Both insulin and insulin-like growth factor 1 are anabolic, stimulating mitosis and proliferation and inhibiting apoptosis in both normal and cancer cells of the breast (11 , 12) . Also, insulin and insulin-like growth factor 1 stimulate the synthesis of sex steroids and decrease concentrations of their binding proteins, which may increase risk of breast cancer, especially in premenopausal women (11 , 13, 14, 15) , although not all studies have supported this finding (16) .
Many factors influence how rapidly carbohydrates are digested and absorbed, and hence what their glycemic and insulinemic effects will be (17) . The glycemic index (GI) was introduced in 1981 as a way to rank foods according to their measured effect on blood glucose response (18) . The GI of a food is based on portions that contain a fixed amount of carbohydrate (generally 50 g) rather than on portions that are typically consumed (19) . A related measure, the GL, is calculated using both the GI of a food as well as the actual amount of carbohydrate consumed in a portion (20) . We hypothesize that a diet with a high dietary GL and high overall GI increases a womans risk of developing breast cancer.
| Subjects and Methods |
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Assessment of Dietary Variables.
A 131-item semiquantitative food frequency questionnaire (SFFQ) was sent to participants at baseline. The SFFQ listed common serving sizes for each food and asked participants to report how often they consumed a serving of that food, on average, over the previous year. There were nine categories of response: never or less than once per month, 13 times per month, once per week, 24 times per week, 56 times per week, once per day, 23 times per day, 45 times per day, and
6 times per day. Nutrient values for items on the SFFQ were obtained from United States Department of Agriculture materials and information from manufacturers; GI values were obtained primarily from the tables compiled by Foster-Powell and Brand-Miller (22)
. Details of calculating dietary GL and overall GI have been described elsewhere (23)
. Briefly, the GL for each food item on the SFFQ was calculated by multiplying the GI of the food by the number of carbohydrate grams in a serving of that food. The dietary GL for each participant was estimated by multiplying the GL for each reported food item by the participants frequency of consumption, then summing overall foods. The overall GI for each participant was calculated by dividing the participants dietary GL by the total grams of carbohydrate consumed. This variable is a weighted average of the GI of the foods consumed (the weights are the carbohydrate grams) and is an indicator of the average GI of the carbohydrate consumed. Glucose was used as the standard in calculating GI and GL values.
The SFFQ was validated for nutrients against multiple diet records in a similar population of women participating in the Nurses Health Study and was found to be reasonably well correlated with the diet records (mean energy-adjusted correlation for the nutrients measured was r = 0.62; Ref. 24 ). The validity and physiological relevance of dietary GL as assessed by food frequency questionnaires is supported by several studies. In a cross-sectional study of healthy postmenopausal women, dietary GL was positively associated with plasma triacylglycerol concentrations (geometric mean, 0.98 and 1.75 mmol/liter, for the lowest and highest quintiles, respectively, of GL, P for trend < 0.001) and negatively associated with high-density lipoprotein-cholesterol concentrations (geometric mean, 1.50 and 1.34 µmol/liter, for the lowest and highest quintiles, respectively, of GL, P for trend = 0.03; Ref. 23 ). The positive dose-response relations between dietary GL and plasma lipid profiles observed in these epidemiological data were consistent with findings from controlled metabolic experiments and, thus, provide objective evidence that our SFFQ provided sensitive measures of both the quality and quantity of carbohydrate intake, and that measurement error, although inevitably present, did not preclude our ability to detect important associations between dietary GL and subsequent occurrence of breast cancer. Dietary GL has also been found to be associated with increased concentrations of high-sensitivity C-reactive protein, a marker of systemic inflammation and risk factor for ischemic heart disease (25) . In prospective cohort studies, GL was associated with increased coronary heart disease risk in women and with increased diabetes mellitus risk in both men and women (20 , 26 , 27) .
Women were excluded from these analyses if their SFFQ contained more than 70 blanks or if their reported daily total energy intake was less than 2514 kJ or greater than 14,665 kJ. These exclusions left a cohort of 38,446 women. Study participants completed baseline and run-in questionnaires regarding demographic variables, health habits, physical activity, and medical history. Follow-up questionnaires were administered yearly.
Assessment of Physical Activity.
Energy (kJ) expended each week in physical activity was estimated from the reported amount of time spent during the past year in eight groups of recreational activities: walking or hiking; jogging; running; bicycling; aerobic exercise or aerobic dance; lower-intensity exercise; racquet sports; or lap swimming, and from how many flights of stairs climbed daily. Each group of activities, including stair climbing, was assigned a multiple of resting metabolic rate (MET score), which was multiplied by the womans estimated resting metabolic rate (4.2 kJ/kg body weight) and by the hours per week engaged in the activity. These values were summed to obtain an estimate of weekly energy expenditure from recreational activities and stair climbing for each woman (28)
. The validity of physical activity assessment from self-administered questionnaires has been demonstrated in a random sample from a large population study of nurses, using past-week recalls and 7-day diaries as the referents. The correlations obtained were 0.79 for the recalls and 0.62 for the diaries (29)
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Breast Cancer Ascertainment.
Women were asked to report new diagnoses of major illnesses, including breast cancer. Diagnoses were confirmed by an Endpoints Committee that reviewed relevant records, including pathology reports, and determined how to classify the cancer. Only the first occurrence of breast cancer was considered an end point. Each cancer was classified as either in situ or invasive, and its hormone receptor status was recorded as estrogen receptor positive or negative and/or progesterone receptor positive or negative
Data Analysis.
We categorized dietary GL and overall GI into quintiles of intake and used Cox proportional hazards models to estimate relative risks while controlling for age (years), body mass index, baseline alcohol use (never/rarely, 13 drinks/month, 16 drinks/week,
1 drink/day), baseline smoking status (never, current, past), age at menarche (
10, 11, 12, 13, 14,
15), age at first pregnancy (never, <20, 2024, 2520,
30), number of pregnancies lasting 6 months or longer (0, 1, 2, 3, 4,
5), history of using oral contraceptives for 2 months or more (yes, no), use of postmenopausal hormones (never, past, current), family history of breast cancer (mother or sister diagnosed at age
60), and physical activity (tertiles of kJ expended weekly in recreational activity and stair climbing). In addition to these variables, we controlled for total energy (kJ), total fat (g), total fiber (g), and folate (µg) in one set of models and for total energy (kJ), folate (µg), and intake of fruits and vegetables (servings/day), whole grains (servings/day), and red meat intake (servings/day) in another set of models. To test for trend, we assigned the dietary GL or overall GI quintile median value to each subject in that quintile. To determine whether the relationships between foods with a high GL and breast cancer risk were consistent with the dietary GL models, we examined quartiles of intake of refined grain, whole grain, and total grain, and servings per day of cold cereal, dark bread, potatoes, and soft drinks, in both age-adjusted and multivariable-adjusted models. We repeated the dietary GL and overall GI analyses restricting the outcome to invasive cases only and again restricting to cases that were either estrogen receptor positive and/or progesterone receptor positive.
We hypothesized a priori that the potential effect of a high dietary GL or high overall GI diet may be modified by factors that are associated with hormone status or insulin resistance. To explore this possibility, we conducted analyses stratified by baseline measurements of menopausal status (postmenopausal, premenopausal/uncertain), physical activity (tertiles of kJ expended in recreational activity and stair climbing), body mass index (<25,
25), smoking status (never, past/current), alcohol use (
3 drinks/month, 16 drinks/week,
1 drink/day), and baseline medical history that may influence dietary changes (history of diabetes mellitus, hypertension, or hypercholesterolemia versus no history). Because the effect of physical activity may be influenced by menopausal status, we examined dietary GL and overall GI intake while stratifying on physical activity and menopausal status simultaneously. We used SAS (version 8, SAS Institute Inc., Cary, NC) to analyze these data.
| Results |
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Estimated risk was somewhat increased in premenopausal women (GL multivariable-adjusted RR, 1.27; CI, 0.792.03, comparing extreme quintiles) but not in postmenopausal women (Table 3)
. Risk was also weakly elevated in women in the lowest tertile of physical activity and the highest quintile of GL intake (multivariable-adjusted RR, 1.24; CI, 0.752.04; Table 4
). Examining strata of menopausal status and exercise simultaneously, risk was increased in women who were premenopausal at baseline and who were in the lowest tertile of physical activity (multivariable-adjusted RR, 2.35; CI, 1.035.37; Table 4
). For overall GI, the multivariable-adjusted RR was 1.56 (CI, 0.882.78; P for trend = 0.02, comparing extreme quintiles).
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We tested for the presence of multiplicative interaction between dietary GL and each of the stratification variables; none of the Ps were significant at the conventional
= 0.05 level, perhaps because of a lack of statistical power.
| Discussion |
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7 years. Results from previous studies have been mixed. A prospective cohort study of postmenopausal American women reported an RR of 0.90 (CI, 0.761.08, comparing extreme quintiles of GL; P for trend = 0.68), and a corresponding RR of 1.03 (CI, 0.871.22; P for trend = 0.71) for GI (30)
. Two case-control studies, however, reported positive associations between GI or dietary GL measures and breast cancer risk. Augustin et al. (in 2001; Ref. 31
) report an odds ratio of 1.34 (CI, 1.101.61, comparing extreme quintiles of GL; P for trend = <0.01) in a large study of Italian women. Levi et al. (in 2002; Ref. 32
) examined the association between overall GI and breast cancer in a hospital-based casecontrol study and found an odds ratio of 1.25 (CI, 0.831.87, comparing extreme tertiles; P for trend = 0.39), although they found no association between dietary GL and breast cancer. The dietary and lifestyle factors that we examined are highly interrelated and difficult to measure accurately. Our findings may be biased by unmeasured confounders as well as by residual confounding from poorly measured dietary and lifestyle variables. Dietary information was collected only at baseline, reflecting the previous years intake. It is uncertain whether changes in diet during the follow-up period influenced breast cancer risk. Another potential source of bias is mammogram use in the study population. Women in the lowest quintile of dietary GL were least likely to have had a screening mammogram before enrollment in the study. If women with undiagnosed breast cancer are more likely to have a low dietary GL diet, risk estimates could be biased away from the null.
High dietary GL and overall GI were weakly associated with increased breast cancer risk in premenopausal women, although this increase was not seen in premenopausal women who reported high levels of physical activity. Although alcohol intake has been associated with an increased risk of breast cancer (5) , we found a small increase in risk in nondrinkers, but not in drinkers, who had a high glycemic diet. Evidence is mixed, but some studies suggest that moderate intake of alcohol may weakly improve insulin sensitivity (33) . Our finding that breast cancer risk is somewhat increased with increasing dietary GL in women who had been diagnosed with hypertension, hypercholesterolemia, or diabetes at baseline, suggests that lifestyle or metabolic factors associated with these conditions may amplify the effects of a high glycemic diet. An alternative explanation is that women with a history of these conditions are more likely to have dramatically changed their diets as a consequence of their diagnoses, and that changes in dietary intakes were not fully captured by the dietary questionnaire. Because of power limitations, we were unable to examine these conditions separately. We cannot rule out the possibility that these results are chance associations found because of the multiple comparisons we made and are not the reflection of a causal relationship between dietary GL, dietary GI, and breast cancer.
In this cohort, dietary GL is positively associated with reported physical activity. If the effect of a high dietary GL diet is modified by physical inactivity, then any residual confounding by physical activity within strata would be expected to bias the risk estimates toward the null, implying that true risk may be greater than our estimates. A previous study of exercise and breast cancer in this group of women found an inverse association between energy expended in physical activity and breast cancer risk, although the association was stronger in postmenopausal women than in the cohort as a whole (premenopausal women were not examined separately; Ref. 28 ).
In conclusion, in this prospective cohort study we did not find evidence that dietary GL or overall GI increases overall breast cancer risk. However, we did find an increase in risk in women who were premenopausal or of uncertain menopausal status at baseline and were in the lowest tertile of reported physical activity, suggesting that the effects of a high glycemic diet may be modified by interrelated lifestyle and hormonal factors. Future prospective studies of larger sample size and longer duration are warranted to confirm our findings.
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Simin Liu, Harvard Medical School and Brigham and Womens Hospital, 900 Commonwealth Avenue, Boston MA 02215.
Received 5/ 8/03; revised 7/21/03; accepted 9/ 4/03.
| References |
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