In this population-based study of Icelandic men, consumption of total salted or smoked fish and intake of fish oil during different periods of life was not associated with risk of prostate cancer overall. In contrast, the risk of advanced prostate cancer was increased following high intake of smoked or salted fish during adolescence and late life, and substantially reduced among men who consumed fish oil at older age. Although our results for total fish intake in midlife are in line with a recent meta-analysis on 12 cohort studies 
, we emphasize that our study was conducted in a population with a uniquely high intake of particularly lean fish 
. Hence, our reference group of up to two fish meals per week could in fact also be classified as a “high intake group” in other study populations. Thus, we cannot rule out the possibility that a potential beneficial threshold level might already have been reached by our reference group. Furthermore, it has been proposed that different methods of cooking fish might affect the prostate cancer risk where it is suggested that avoiding high-temperature cooking methods for white fish might lower the risk 
. Although we do not have information on cooking methods in present study, information from a national nutrition survey conducted in 1990 showed that 64% of total fish consumed as a main meal was boiled or baked, while 36% was fried 
To our knowledge only one other study (population-based case-control) assessed early life fish intake and prostate cancer risk, and found a marginally increased risk following frequent fish consumption 
. The discrepancy with our findings could be due to different study design, dissimilar fish species consumed, and different methods of collecting data on diet. We have previously reported that residency in seaside villages, with exceptionally high fish consumption for the first twenty years of life, was not associated with prostate cancer risk 
Unexpectedly, we discovered a positive association between frequent salted or smoked fish consumption both in early- and late life and advanced prostate cancer. At least three case-control studies have reported findings on this subject. A study from China assessed intake five years prior to diagnosis and reported positive association between salted fish consumption and total prostate cancer 
. However a study from Poland 
showed an inverse association between frequent consumption of smoked or dried fish or liver and a study from Canada also found an inverse association between frequent consumption of smoked/dried/salted fish and prostate cancer 
. These mixed results could be due to different species of fish being preserved along with different methods of preserving the fish. In addition none of these studies presented data on advanced and localized prostate cancer separately. The mechanism behind our finding on salted or smoked fish is unclear and could be due to the salt content and/or presence of mutagens as a result of the preservation process 
. Salt induces muscle lipid oxidation in fish 
and lipid oxidation in n-3 or n-6 fatty acids generates α, β -unsaturated aldehydes supporting different functional groups containing oxygen, which are currently being considered as possible causal agents of different types of cancer 
. Our findings could also be due to ineffective preservation processes ensuing in infectious microorganisms being present in the fish; genitourinary infection has been suggested to play a role in the etiology of prostate cancer, although specific infectious agent has yet to be identified 
We are not aware of studies that have examined smoked or salted fish consumption in early life in relation to prostate cancer, yet early life rural residency in Iceland (compared with early life city residency) examined in a larger cohort during the beginning and mid of 20th
century was associated with increased risk of advanced prostate cancer 
. At that time high intake of milk, salted or smoked fish, meat and rye bread was typical. Thus, although we previously suggested that high milk consumption could explain our findings for rural residency, we cannot rule out that salted or smoked fish intake might explain, in part, the positive association between early life rural residencies and advanced prostate cancer risk.
We found no association between fish oil consumption in early-and midlife and prostate cancer, but late life consumption was inversely associated with advanced prostate cancer risk. This finding suggests a role in disease progression rather than etiology and fits with results on high prediagnostic plasma 25-hydroxyvitamin D predicting improved prostate cancer prognosis 
The ability to study fish and fish oil consumption across the life course is a particular strength of our study design. Other important strengths are the extensive background data allowing control for potential confounding factors and the complete follow-up. For analyses that include prevalent cases, our results are vulnerable to recall bias because men with prevalent prostate cancer may evaluate their past dietary consumption differently from men without prostate cancer. However, for salted or smoked fish consumption, we only found associations with advanced prostate cancer not total, and only for early life intake, a pattern of findings unlikely to arise due to different recall between cases and controls. Furthermore, findings on current diet in late life were based on incident cases only.
The validation study on current food consumption in the AGES-Reykjavik did not show acceptable results for fish meals, possibly due to the inability of the 3-day food record used as a reference method, to adequately reflect individual intake of food items that are consumed 1–2 times per week or 3–4 times per week 
. The validation study on midlife food consumption in the AGES-Reykjavik showed that participants were acceptably ranked by their intake of several important food groups 
. Still, there is uncertainty in assessing dietary habits stretching over a 40 to 50 year period of time but this would typically lead to underestimation of the observed associations and failure to observe true associations. The validity of the early life dietary assessment has not and cannot be investigated. Yet, the data importantly show similar residency-dependent variation in dietary habits as documented in a household study conducted in Iceland in 1939 
. It has indeed been reported that food related memory from childhood over four decades later can be as accurate as from current diet, especially for food items eaten rarely or daily 
. Another limitation to our study is the lack of information about total energy intake and fat intake; however we adjusted for body mass index measured in midlife, which may give indirect indication of total energy- and fat intake. Furthermore, we adjust for adult height, which can reflect nutritional status in early life 
. Lastly, the frequency of fish oil consumption was not assessed in greater detail beyond “daily intake” which limited our opportunities for assessing dose-response. Daily dosage is however recommended on the product, which is 10 ml per day.
In summary, salted or smoked fish may increase risk of advanced prostate cancer, whereas in a setting with very high fish consumption no association was found between overall lean fish consumption in early life or midlife and prostate cancer risk. Potential exposure to carcinogens in salted or smoked fish needs further study. We observed reduced risk associated with fish oil consumption in late life, but not in early life or midlife, which may be an indication of a mechanism involving n-3 PUFAs and/or vitamin D and opens for studies on the potential protection of fish oil on the progression of prostate cancer. Improved understanding of potential dietary factors affecting prostate cancer risk, especially for advanced disease, could have a major public health impact.