Ph.d. thesis: "Whole grain products — Determinants of intake and cancer risk" by Rikke Egeberg

The ph.d. thesis "Whole grain products - Determinants of intake and cancer risk" by Rikke Egeberg was successfully defended on October 2nd, 2009 at the Danish Cancer Society in Copenhagen. The thesis was based on a study of different dietary, lifestyle, anthropometric and socioeconomic determinants for the intake of whole grain products and three studies on whole grain products in relation to colorectal, breast and prostate cancer.

Abstract of thesis:

Whole grain products have for long been recommended as part of a healthy diet and are now believed to be a dietary practice of considerable importance for public health. Existing epidemiological research almost consistently have linked a high intake of this food group to a lower risk of cardiovascular diseases and type 2 diabetes, but in particular in relation to cancer, an increased knowledge of their role in relation to prevention is still needed.

The aims of this PhD thesis were 1) to evaluate dietary, lifestyle, anthropometrical and socioeconomic determinants of intake of whole grain products and 2) to evaluate whether intake of whole grain products is associated with risk of breast, colon and rectal, and prostate cancer. The thesis is based on data from the Danish prospective Diet, Cancer and Health study. This thesis reviews the current literature on the relation between whole grain product intake and risk of cancer, with particular focus on cancer of the breast, colon and rectum, and prostate. The results obtained in this PhD have been put into perspective and discussed in relevant chapters. Areas covered in this thesis include:

1. Introduction to whole grain products
2. Estimation of whole grain product intake and its limitations
3. Determinants of whole grain product intake
4. Potential mechanisms linking whole grain products and cancer
5. Whole grain products and cancer risk.

Knowledge of the association between intake of whole grain products and other disease risk factors is important in correctly interpreting the increasing number of studies linking high whole grain product consumption to lower disease risk. Earlier studies have indicated that high consumption of whole grain foods is associated with healthier dietary and lifestyle habits. It is possible that these associations may vary between populations because of differences in food and lifestyle habits. In paper I, we investigated the association between intake of whole grain products in association with other dietary, lifestyle, anthropometrical and socioeconomic factors among Danish men and women. In this middle-aged population, higher whole grain product intake was positively associated with a number of other dietary factors. The strongest positive associations were seen for intake of vegetables and processed meat, whereas the strongest negative associations were seen for intake of red meat and refined-grain products. In addition, a higher intake of whole grain products was positively associated with cycling, taking dietary supplements and high school education, but negatively associated with intake of alcohol, BMI and smoking. Results from Paper I were used in the subsequent cancer-endpoint papers (Paper II-IV) as part of their analysis strategy.

In this thesis, breast, colon and rectal, and prostate cancer were the endpoints of interest. For a better understanding of the role of whole grain products in risk of these cancer types, evaluations of associations in relation to different subtypes or specific characteristics of cancer tumours were also central in this thesis. Results from Paper III confirm and add to the findings from earlier epidemiological studies that high whole grain product consumption lowers risk of colorectal cancer. We found that higher total whole grain product intake was associated with a lower risk of both colon and rectal cancer, in men, but not in women. For colon cancer, this association was confined to intake of rye bread and whole grain bread. Tendencies towards a lower risk of both proximal and distal colon cancer in men were found for a higher intake of total whole grain products as well as rye bread and whole grain bread. The epidemiological evidence on the role of whole grain products in the prevention of breast and prostate cancer is limited and inconsistent. In paper II, we investigated whole grain product intake in relation to risk of breast cancer overall and by tumour receptor status and tumour histology. However, whole grain products were not found to be associated with risk of overall or specific breast cancer subtypes. In addition, use of hormone replacement therapy did not modify the associations. The association between intake of whole grain products and risk of prostate cancer has not previously been investigated in a prospective design setting. In paper IV, we found that a higher intake of rye bread was statistically significantly associated to a lower risk of prostate cancer among Danish men. Associations between whole grain products (total or specific type) and risk of prostate cancer did not differ according to grade of disease defined by the Gleason score. That rye in particular may be important in prostate cancer prevention instead of total whole grain products has not previously been shown in any other epidemiological study and therefore needs confirmation by future studies; however this finding confirms those results based on experimental animal studies.

In conclusion, the studies included in this thesis strengthen the insights into the impact of what determines whole grain product intake and their role in risk of cancer. Although, the strengths of associations potentially varies according to specific cancer types or that only specific types of whole grain products may be related to risk for specific cancer types, recommending a higher intake of whole grain products as part of a healthy diet will probably have a valuable public health impact.