Continuing my alphabetic tour, I arrive at coffee, one of the enduring pleasures in life. At last - an easy subject!
There are no coffee intervention clinical trials - we must draw conclusions from epidemiological studies, but such studies don't prove causality.
I am used to seeing bias in dietary studies. Researchers are sometimes looking for confirmation that saturated fat, red meat & coffee can cause cancer. Perhaps that influences the results?
Curiously, green tea gets a pass, while coffee has sometimes been dismissed as containing countless unknown substances that might be carcinogenic. After all, coffee is a stimulant with no dietary value - it must be bad for one.
Coincidentally, in the NY Times on Wednesday [1a]:
"An influential panel of experts convened by the World Health Organization concluded on Wednesday that regularly drinking coffee could protect against at least two types of cancer, a decision that followed decades of research pointing to the beverage’s many health benefits. The panel also said there was a lack of evidence that it might cause other types of cancer."
"The announcement [1b] marked a rare reversal for the panel, which had previously described coffee as “possibly carcinogenic” in 1991 and linked it to bladder cancer. But since then a large body of research has portrayed coffee as a surprising elixir, finding lower rates of heart disease, Type 2 diabetes, neurological disorders and several cancers in those who drink it regularly."
"A summary of the final evaluations is published today in The Lancet Oncology [1c], and the detailed assessments will be published as Volume 116 of the IARC Monographs."
[1d] I see bias in a 1964 Japanese study, which looked at national consumption & PCa death rates. At the top of Table 1 we have Sweden (8 kg coffee per person per year) & the highest PCa mortality rate. At the bottom, Japan, with almost no coffee intake - & almost no PCa.
"Japan is exceptionally low both in coffee consumption and in prostate cancer mortality." ... So?
Alas, the data doesn't behave & Eiji Takahashi goes into reasons why dark roasts might be more dangerous, etc.
He could have added that the Swedish method of boiling coffee (don't forget the eggshells) preserves volatile oils that are otherwise trapped by paper filters.
[1e] More bias in 1970. Here is the scope of the study:
"age-adjusted death rates in 1964-65 from cancers of different sites and the annual consumption of cigarettes, solid fuel, tea and coffee" ... No, it's not a joke.
"Coffee is positively related with the pancreas, prostate and leukaemia in males and with ovary and leukaemia in females."
 COFFEE as a PCa Risk Factor.
After 1970 & up until 2000, there were a number of studies (not cited) that found no PCa risk. But then, two negative studies:
[2a] (2000 - Canada) Used "participants in the 1970-1972 Nutrition Canada Survey (NCS)." "145 developed prostate cancer."
"Compared to those who reported no coffee drinking, men who averaged more than 250-ml per day experienced a 40% increase in risk ..."
[2b] (2007 - Italy) "Coffee consumption was directly associated with risk of prostate cancer (OR = 1.9 for the third versus the first tertile). "
 COFFEE - Filtered or Boiled?
[3a] (2010 - Sweden) "Västerbotten Intervention Project (64,603 participants, including 3,034 cases), with up to 15 years of follow-up"
"No associations were found for all cancer sites combined, or for prostate ... cancer."
[3b] (2015 - Norway) "An inverse relationship between number of cups per day and the risk of prostate cancer was present only for the boiled coffee type."
 COFFEE - The breakthrough.
From 2011 we see good news about coffee. Previous studies mostly failed to find risk, but did not report benefit. The two negative studies  are cherry-picked. The neutral studies have been ignored, since they did not support benefit.
[4a] (2011 - U.S.) "47,911 men in the Health Professionals Follow-up Study who reported intake of regular and decaffeinated coffee in 1986 and every 4 years thereafter. From 1986 to 2006, 5035 patients with prostate cancer were identified, including 642 patients with lethal prostate cancers, defined as fatal or metastatic."
"Men who consumed six or more cups per day had a lower adjusted relative risk for overall prostate cancer compared with nondrinkers (RR = 0.82 ...)"
"The association was stronger for lethal prostate cancer (consumers of more than six cups of coffeeper day: RR = 0.40 ..."
"The inverse association with lethal cancer was similar for regular and decaffeinated coffee (each one cup per day increment: RR = 0.94 ... for regular coffee and RR = 0.91 ... for decaffeinated coffee ..."
"We observed a strong inverse association between coffee consumption and risk of lethal prostate cancer."
This study suggests that coffee consumption perhaps becomes more important after PCa initiation.
[4b] (2012 - Scotland) "Men consuming 3 or more cups of coffee per day experienced 55% lower risk of high Gleason grade disease compared with non-coffee drinkers"
[4c] (2013 - Sweden) Weaker findings, but still good:
"For localized PCa, each one cup increase in daily coffee consumption was associated with a 3% reduced risk ... For advanced and fatal PCa, we found a non-significant inverse association; each one cup increase was associated with a 2% reduced risk of advanced ... and fatal PCa ..."
[4d] (2013 - Japan) "There was a significant inverse association between coffee consumption and the incidence risk of prostate cancer. Compared with those who did not drink coffee, the multivariate adjusted HRs were 0.81 .., 0.73 .., and 0.63 ... for those who drank coffee occasionally, 1–2 cups per day, and ⩾3 cups per day, respectively ..."
[4e] (2013 - Sweden) "Coffee intake was not associated with overall prostate cancer risk. Risk of fatal prostate cancer was inversely, but not statistically significantly, associated with coffee intake, with an odds ratio of 0.64 ... for men consuming greater than 5 cups per day compared to men drinking less than 1 cup per day. The highest intake of coffee was associated non-significantly with lower risk of advanced disease (OR = 0.73 ...) and associated significantly with lower risk of high-grade cancer (Gleason 8-10; OR = 0.50 ...). Risk of localized, grade 7, and low-grade cancers was not associated with coffee intake."
[4f] (2013 - U.S. data / Netherlands) "... participants were men diagnosed with prostate cancer in 2002–2005 in King County, Washington ..."
"... 630 patients with a median follow-up of 6.4 years, during which 140 prostate cancer recurrence/progression events were recorded. Approximately 61% of patients consumed at least one cup of coffee per day. Coffee consumption was associated with a reduced risk of prostate cancer recurrence/progression; the adjusted HR for ≥4 cups/day versus ≤1 cup/week was 0.41"
 COFFEE - & Meta-analyses.
With four important studies published in 2013, there was a spate of meta-analysis studies the next year. Such studies can report mixed results, depending on the inclusion criteria, but all have suported coffee as having benefit, so are not included here.
 COFFEE - & the blood, etc.
From 2015, we see interest in the blood chemistry of coffee drinkers.
[6a] (2015 - U.S. / Canada) "We used Luminex bead-based assays to measure serum levels of 77 immune and inflammatory markers in 1,728 older non-Hispanic Whites."
The study reported "Lower circulating levels of inflammatory markers among coffee drinkers ..."
[6b] (2016 - Norway) A cell & mouse study that looked for an effect on NF-kB activation. NF-kB is chronically activated in PCa & is responsible for cell survival proteins, including those responsible for inflammation.
"In vivo imaging revealed a 31% lower NF-κB-luciferase activation in the xenografts of the mice receiving 5% coffee compared to control mice."
"Furthermore, coffee modulates transcription of genes related to prostate cancer and inflammation. Our results are the first to suggest mechanistic links between coffee consumption and prostate cancer in an experimental mouse model."
(i) Why did boiled coffee beat the other kind in [3b]? Possibly the oils. I use an AeroPress coffee maker. It makes great coffee because it eliminates two causes of bitterness: high brewing temperature & excessive time of grounds in the water. It has a small filter & a pack lasts for ever - you can even reuse them. But 3 years ago I invested in a metal filter. The taste is different. I can't say better or worse - just different. So an adjustment is required. But I decided that I wanted some of those oils.
(ii) Studies often use "cups" as a measure of consumption. It's not a good substitute for ounces. Better still is the volume of beans used. My mug of coffee is made with a third of a cup of beans. Some like weaker coffee. "4 cups" / day is not as precise as it sounds.
(iii) Milk in coffee. The casein proteins in milk bind to some of the polyphenols in coffee. This is well-documented in tea studies, where a high percentage of the beneficial tannins can be lost. (The conjugates are indigestible.) Coffee studies never take the milk into account.
Incidentally, with the elimination of bitterness by the AeroPress method, the tendency is to make the coffee stronger (because of the improved signal-to-noise ratio). & less milk & sweetener is therefore needed to balance the taste. (But some just love sweet milky coffee.)