Background Little is known about fish intake throughout the life course and the risk of breast cancer. Methods We used data on the first residence of 9,340 women born 1908–1935 in the Reykjavik Study as well as food frequency data for different periods of life from a subgroup of the cohort entering the AGES-Reykjavik Study (n = 2,882). Results During a mean follow-up of 27.3 years, 744 women were diagnosed with breast cancer in the Reykjavik Study. An inverse association of breast cancer was observed among women who lived through the puberty period in coastal villages, compared with women residing in the capital area (HR = 0.78, 95% CI: 0.61, 0.99). In the subgroup analysis of this Icelandic population, generally characterized by high fish intake, we found an indication of lower risk of breast cancer among women with high fish consumption (more than 4 portions per week) in adolescence (HR 0.71, 95% CI, 0.44, 1.13) and midlife (HR 0.46, 95% CI, 0.22, 0.97), compared with low consumers (2 portions per week or less). No association was found for fish liver oil consumption in any time period which could be due to lack of a reference group with low omega-3 fatty acids intake in the study group. Conclusion Our findings suggest that very high fish consumption in early to midlife may be associated with a reduced risk of breast cancer. Impact Very high fish consumption in early adulthood to midlife may be associated with decreased risk of breast cancer.

Recent studies indicate that lifestyle factors in early life affect breast cancer risk. We therefore explored the association of high consumption of meat, milk, and whole grain products in adolescence and midlife, on breast cancer risk. We used data from the population based AGES-Reykjavik cohort (2002–2006), where 3,326 women with a mean age of 77 years (SD 6.0) participated. For food items and principal component derived dietary patterns we used Cox proportional models to calculate multivariate hazard ratios (HR) with 95% confidence intervals (95% CI). During a mean follow-up of 8.8 years, 97 women were diagnosed with breast cancer. For both adolescence and midlife, daily consumption of rye bread was positively associated with breast cancer (HR 1.7, 95% CI 1.1–2.6 and HR 1.8, 95% CI 1.1–2.9, respectively). In contrast, persistent high consumption of oatmeal was negatively associated with breast cancer (0.4, 95% CI 0.2–0.9). No association was found for other food items or dietary patterns that included rye bread. High rye bread consumption in adolescence and midlife may increase risk of late-life breast cancer whilst persistent consumption of oatmeal may reduce the risk.

Objective To study the association of fish and fish-liver oil consumption across the lifespan with CHD later in life among Icelandic women, with special emphasis on the effects of consumption in adolescence. Design Prevalence association study. Logistic regression was used to estimate odds ratios and 95 % confidence intervals of CHD according to fish or fish-liver oil exposure. Models were adjusted for age, education, concurrent diet and other known risk factors. Setting The study was nested within the AGES-Reykjavik Study, conducted in Reykjavik, Iceland. Subjects Participants were 3326 women aged 66–96 years, with available information on CHD status at entry to the study and information on fish and fish-liver oil consumption during midlife and adolescence. Dietary habits were assessed retrospectively using a validated FFQ. Results CHD was identified in 234 (7·9 %) women. Compared with women with no intake of fish-liver oil in adolescence or midlife, women who consumed fish-liver oil at least three times weekly in adolescence or in midlife had a decreased risk of CHD (OR = 0·62; 95 % CI 0·45, 0·85 and OR = 0·68; 95 % CI 0·50, 0·94, respectively). No associations were observed between fish intake (>2 portions/week v. ≤2 portions/week) in adolescence or midlife and CHD in this population with high fish intake. Conclusions Fish-liver oil consumption, from early life, may reduce the risk of CHD in older women. Lifelong nutrition may be of importance in the prevention of CHD in older women.

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Growth rate is regulated by hormonal pathways that might affect early cancer development. We explored the association between rate of growth in height from age 8 to 13 (childhood) years and from age 13 to height at study entry (adolescence), and risk of breast and prostate cancer. Participants were 2,037 Icelanders born 1915 – 1935, who took part in the Reykjavik Study established 1967. Height measures were obtained from school records and at study entry. We used multivariable Cox regression models to calculate hazard ratios with 95% confidence intervals of breast and prostate cancer by rates of growth in tertiles. During a mean follow-up of 66 years (women) and 64 years (men), 117 women were diagnosed with breast cancer and 118 men with prostate cancer (45 w/advanced). Women in the highest tertile of growth rate in adolescence had increased risk of breast cancer, hazard ratio 2.4, 95% confidence interval: 1.3, 4.3, compared with women in the lowest tertile. A suggestive inverse association was observed for highest adolescent rate of growth in men and advanced prostate cancer, hazard ratio 0.4, 95% confidence interval: 0.2, 1.0. Rapid growth, particularly in adolescence may affect cancer risk later in life.

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