This study is the first, to our knowledge, to evaluate the relative effects of biochemically-based nutrient pathways and breast cancer risk. Using hierarchical modeling and data from a large, population-based case-control study, we observed a decrease in breast cancer risk with increasing overall intake of nutrients involved in glycemic control, although the effect estimates were imprecise. In general, estimates from the hierarchical model were less inflated, and more precise compared to those from a multivariable model that included all nutrients, and this modeling approach allowed for estimation of an overall effect of groups of nutrients that operate along a specific biological pathway. We observed essentially null associations with breast cancer risk with increasing intake of nutrients involved in one-carbon metabolism, in phytoestrogens and antioxidants. A stratified analysis yielded similar results regardless of supplement use.
The finding of an effect for exposures related to glycemic control is in agreement with previous findings from our research group that indicated that high consumption of sweets was associated with increased breast cancer risk (5
). The importance of glycemic control in cancer etiology is gaining attention. Recent evaluations of dietary glycemic index/glycemic load have yielded generally null findings (32
), yet the utility of this measure is controversial (35
); instead, our analysis considers a set of nutrients in this pathway rather than this specific measure. Regular intake of foods that increase serum glucose, such as dietary carbohydrate, increases insulin activity, result in chronic hyperinsulinemia, which is in turn associated with increased levels IGF-1 (36
). Insulin, released from the pancreas in response to elevated serum glucose, is used for glucose transport and utilization (37
), protein synthesis, and cellular proliferation (39
). In addition to its direct mitogenic effects, insulin enhances growth hormone (GH) stimulated insulin-like growth factor-1 (IGF-1) synthesis (36
) which independently promotes tumor development by increasing cell proliferation and inhibiting apoptosis (40
). Previous epidemiologic studies have shown elevated serum levels of IGF-1 to be associated with breast and colorectal cancer (41
), implicating this hormonal milieu in carcinogenesis. While excess dietary carbohydrate intake may be one factor in the hyperinsulinemic state (36
), several other nutrients, which were considered in our hierarchical analysis, are thought to play important roles. Fiber slows the absorption of glucose into the small intestine thereby blunting the ensuing insulin response (43
) while calcium, magnesium, and zinc are important cofactors involved in glucose homeostasis and are primarily involved with insulin secretion (10
In the hierarchical models, although we observed a biologically plausible increased risk for developing breast cancer in association with the glycemic control pathway, effects were not observed in the other pathways considered in this study. These include the one-carbon metabolism, anti-oxidant, and phytoestrogen pathways, all of which have strong biologic plausibility and for which we had previously reported inverse associations with breast cancer incidence when each was considered separately (1
). Stratification by supplement use did not alter our findings. Thus, reasons for these unexpected findings are unclear, but could be due to the issues that motivated us to conduct hierarchical regression modeling, namely to reduce the impact of artifactual influences of multicollinearity and multiple comparisons, which are always of concern in epidemiologic studies of nutrition and cancer. Our findings suggest that future research efforts should also utilize methods to address these concerns.
Our analysis employed hierarchical regression, a semi-Bayesian modeling approach that allows prior information about parameter estimates to be incorporated into the model that relates multiple related exposures to a disease outcome. This technique has been useful in analyses with multiple correlated exposures (45
) and has been successfully applied to several analyses of dietary factors and cancer risk (28
) by considering the effect of individual foods by incorporating nutrients in the second stage. The univariate model does not consider examination of multiple nutrients that may be highly correlated, and the multivariable model is often overparameterized. The hierarchical model improves upon the univariate and multivariable models by allowing for incorporation of multiple correlated nutrients in a single model, while simultaneously considering biologic pathways. Recently, Carmichael and colleagues (15
) have successfully applied it to a study of the effects of dietary exposures and neural tube defects, utilizing relevant biochemical pathways as the second-stage grouping, which is the strategy we adapted, given that no previous breast cancer study has explored this nutrient-pathway based approach. The authors of this study found that hierarchical modeling resulted in more precise and less inflated estimates of the dietary intakes in their analysis when compared to the multivariable approach. Hierarchical modeling allows the researcher to consider relationships between multiple exposures that may or not be related to each other. Serial analysis of a set of related exposures is likely to yield spurious associations, since an association may be found for a benign exposure that is simply correlated with another one which is responsible for a true effect. Including all of the correlated exposures simultaneously in a traditional regression framework is similarly ill-advised as the resultant multicollinearity will induce instability in the model. The hierarchical approach employed here “shrinks” parameter estimates of exposures that operate along a similar pathway towards a common value, while allowing them to have their own individual effect on risk of breast cancer. This approach improves model stability and yields more plausible effect estimates compared to a simple multivariable technique.
In addition to our use of a method to incorporate prior biologic knowledge into our analysis, this study benefitted from a large, population-based sample that includes pre- and post-menopausal women and the comprehensive assessment of anthropometric, lifestyle, and dietary factors. While these notable strengths lend credibility to our findings, they should be evaluated in light of a few limitations. With an even larger sample, future studies could consider stratification by breast cancer subtype and the potential for effect modification by other important exposures such as alcohol. The use of a FFQ to ascertain usual diet has a number of well-known issues, however they have been shown to adequately rank dietary intakes across individuals (6
), which is the approach utilized for the current analysis. Differential recall in case-control studies is always a possibility, in that the controls may over-report healthy behaviors with greater frequency than cases in an effort to appear more socially acceptable. However, the fact that a null effect was observed for three of the four pathways considered here would appear to be in contrast with that scenario—it would be unlikely that the cases would differentially report behaviors related to a single biochemical pathway. Additional considerations for future studies would include utilizing different methodologies, including: factor analysis, principal components, and consideration of food intake. However, these alternative approaches do not incorporate prior information regarding biologic pathway, and therefore would not provide an estimation of biologic-pathway based effects.
In conclusion, this analysis is the first to consider, in a single model, nutrients and food constituents that operate along specific biological pathways involved in the etiology of breast cancer. Our findings suggest that dietary factors related to glycemic control may be relatively more important than those related to one-carbon metabolism and antioxidant activity or those with estrogenic properties. Additional research should focus on the importance of the glycemic control pathway in the etiology of breast cancer, and future analyses of diet could benefit from a similar pathway-based approach to highlight relevant biologic mechanisms, while reducing the impact of issues related to multicollinearity and multiple comparisons.