Prostate cancer is an important disease; in fact, it’s the most common internal malignancy in American men. Prostate cancer is a variable disease; many cases are slow growing, even harmless, but some cases are aggressive and even lethal. And it’s a puzzling disease; some cases are passed down from father to son, but most occur without apparent rhyme or reason.
Important, variable, and puzzling — it is no wonder that men pay attention when research reports that common dietary elements appear to reduce risk. Two studies have attracted a lot of press; both contain surprising information, but to understand what they mean for you, you’ll have to dig behind the headlines and sound bites.
Most experts believe that diet has a major impact on the risk of developing prostate cancer. The prevailing wisdom is that trans fats and saturated fats increase risk, while omega-3 polyunsaturated fats, especially the fats in fish oil, are protective. But good scientists are not satisfied with received wisdom, so a team of researchers from around the U.S. set out to validate the hypothesis.
The subjects were selected from a group of over 18,000 men who volunteered for the Prostate Cancer Prevention Trial, which was chiefly designed to evaluate the effect of finasteride (Proscar, generic) on the occurrence of prostate cancer. The men were between the ages of 55 and 84 when they entered the study, and all had normal prostate-specific antigen (PSA) levels and digital rectal exams.
This blood fat study focused on 1,658 men who developed prostate cancer during the seven-year trial. The scientists compared these patients with 1,803 men who remained free of cancer throughout the trial. These healthy volunteers, collectively known as the control group, were selected to closely resemble the cancer patients in terms of age, use of finasteride, and family history of prostate cancer; however, the control group had a higher percentage of nonwhites than the cancer group.
At the start of the study, the scientists obtained information about the men’s alcohol consumption, smoking, education, exercise habits, diabetes status, height, and weight. They also obtained blood specimens at the start of the study and at regular intervals thereafter. The blood samples were frozen and stored for later analysis.
The researchers performed a detailed analysis of the fatty acid composition in each blood sample. They expected to find that men with prostate cancer would turn out to have the highest levels of “bad” fats in their blood, while the cancer-free men would have high levels of “good” fats. Instead, they found the reverse: men with the most docosahexaenoic acid (DHA, one of the two major omega-3 fats in fish oil) in their blood had the highest risk of aggressive prostate cancer, while men with the most trans fatty acids had the lowest risk. Levels of trans fatty acid and DHA were not linked to low-grade, slow-growing prostate cancers, and none of the other blood fats that were studied had an impact on prostate cancer. The apparent effect of blood fats was the same in the men who took finasteride and in the men who did not.
The study had several strengths, including careful blood fat analysis and the detailed pathologic study of prostate biopsies in the men who remained cancer-free as well as the men who developed the disease. But the study had limitations, too. It did not account for the possible impact of medications, such as the statin drugs, that may affect the risk of prostate cancer, and it did not analyze blood cholesterol levels, which have also been related to risk. In addition, the research did not evaluate the men’s diets, and it did not check for supplement use, even though the NIH-AARP Diet and Health Study reported that men who take multiple supplements may face an elevated risk of prostate cancer. Finally, though the study appeared to implicate DHA as a prostate cancer risk factor, it found no effect from the other major fish oil, eicosapentaenoic acid (EPA).
Quite apart from these rather technical pros and cons, men should not make lifestyle changes without considering how this blood fat study fits into the large body of pre-existing research and without understanding the limitations of the study’s methodology. We’ll get to both issues after we review the second surprising study.
The coffee connection
Coffee has been blamed for many things, praised for many others. But prostate cancer has not been on either list — until now, that is.
Starting in 1986, Harvard’s Health Professionals Follow-up Study tracked 47,911 men to see if there was a link between coffee consumption and prostate cancer. All the volunteers provided detailed information about their dietary patterns, alcohol use, smoking, multivitamin use, diabetes status, family history of prostate cancer, PSA testing, racial heritage, height, and weight. None of the men had been diagnosed with prostate cancer when they enrolled in the study, but over the next 20 years 5,035 men were diagnosed with the disease. The Harvard scientists divided the cases into three categories according to severity: nonadvanced (low-grade), advanced (high-grade), and lethal or potentially lethal.
When the results were tallied, coffee drinking did not affect the risk of nonadvanced prostate cancer, and it was linked to just a slight reduction in the risk of advanced but nonlethal cases. But coffee appeared to confer substantial protection against the most serious forms of prostate cancer. As compared to the men who drank the least coffee, the men who averaged more than six cups a day enjoyed a 60% lower risk of lethal prostate cancer. Men who drank smaller amounts of coffee enjoyed proportionally smaller benefits; an average of one to three cups a day was linked to a 29% reduction in the risk of lethal prostate cancer. The results were similar for caffeinated and decaffeinated coffee, and they remained valid after the scientists accounted for other dietary influences and prostate cancer risk factors.
As an interesting aside, the Harvard study found no link between coffee consumption and the lower urinary tract symptoms of benign prostatic hyperplasia (BPH). But before you head to Starbucks for a cup of prostate protection, pause to look behind the headlines generated by both of these surprising studies.
The blood fat and coffee studies attracted public attention because their results did not follow the script. Remember, though, that medical knowledge is like a jigsaw puzzle. If a new piece does not fit into the rest of the puzzle, you should consider the big picture before you act on the odd result.
The blood fat study contradicted research that suggested omega-3 fats may reduce the risk of prostate cancer. But even if these results are accurate, they are no reason to do an about-face on your diet. Men are much, much more likely to die from heart disease than prostate cancer, and there is a huge body of evidence showing that trans fatty acids increase the risk of heart disease by raising LDL (“bad”) cholesterol levels and lowering HDL (“good”) cholesterol levels. That’s why the American Heart Association (AHA) recommends virtually eliminating trans fats from your diet — and it’s why food manufacturers are finally learning to make do without the partially hydrogenated vegetable oils that were once so prominent in baked goods, fast foods, and snack foods.
You should also consider the big picture for fish oils, including both DHA and EPA. Previous research reported that men who eat fish enjoy protection from both heart disease and prostate cancer. That’s why the AHA suggests eating oily fish at least twice a week, and it recommends daily fish oil supplements for high-risk individuals who do not eat enough fish.
The coffee study got attention because it counteracts the vague but prevailing belief that something as good as coffee must somehow be bad for you. In fact, high-quality research has debunked the notion that coffee is bad for the heart and for blood pressure, and studies suggest it may be linked to a reduced risk of ailments that range from gallstones to colon and breast cancers. And while little attention has been paid to this aspect of the Harvard study, it also argues against another common belief, that coffee can increase urinary tract distress in men with BPH.
On both counts, the study is good news for men who love their coffee and their prostates. Still, it’s premature to count on coffee to reduce the risk of aggressive prostate cancer, since earlier research reported that coffee does not affect the risk of prostate cancer for good or ill, and because we need to look more closely at the kind of study that produced these findings.
Mull the method
For just a moment, let’s pretend that the blood fat and coffee reports are the only studies on these topics and that their findings are accurate. Does that mean we agree with press reports that said that DHA causes prostate cancer while trans fats and coffee protect against the disease? The answer is no — not because of flaws in the research, but because both reports are observational studies; while this research method is useful and important, it can never establish cause-and-effect relationships.
Observational studies come in two varieties. The coffee study is an example of a cohort analysis, while the blood fat research is a case-control study.
Cohort analysis begins when researchers recruit a group of apparently healthy individuals. Next, the scientists establish health profiles for each member of the group. Their third step is to observe the cohort over time, relying on various combinations of questionnaires, medical tests, and health records to track the group. Finally, the investigators compare members of the cohort who have remained healthy with those who have fallen ill, trying to identify the factors associated with illness.
Cohort analysis is a powerful technique, but it’s slow, difficult, and expensive, typically involving thousands of subjects over many years. In the era of fiscal restraint and diagnostic urgency, researchers are turning to other ways to get results.
Case-control studies have the same goals as cohort analyses, but they proceed from the opposite direction. Instead of observing a group of initially healthy people, researchers begin case-control studies by identifying a group of patients who are already ill. Next, they compare the patients with an equal number of demographically similar healthy people to identify factors that may account for the difference between illness and health. Because case-control studies use many fewer subjects, they are much less difficult and expensive than cohort studies. And because they look back instead of forward, they are also much faster.
Both methods are useful, but neither can establish causality. At best, the coffee study tells us that the beverage is associated with a reduced risk, not that it actually reduces risk. Similarly, the blood fat study links DHA to an increased risk, but it does not show that the omega-3 fat actually increases risk — or that trans fatty acids are really protective.
What to do?
Despite the headlines in newspapers and the sound bites on the airwaves, it’s clear you should stick with your low–trans fat, high–omega-3 diet. As for coffee, don’t count on it for protection, but if you drink it for pleasure, the study may add a little sweetness to each cup.
The most important take-home message, though, is not restricted to fats, beverages, and cancer. Instead, it’s a reminder that you should always evaluate medical news with a good understanding of its context and research methods. It’s slow work and it can be hard, but an important part of our mission is to help you get past the simple “new hope” and “no hope” of headlines and sound bites to understand what medical research really means for you.