A number of studies published in the past few years have found that higher dietary fat intake is associated with an increased risk of prostate cancer.
However, the findings of these studies conflict with a number of others that have revealed no such link. To put the association between diet and prostate cancer into perspective, Renal & Urology News spoke with Stephen J. Freedland, MD, who has conducted extensive research on this topic. Dr. Freedland is Associate Professor of Surgery (Urology) and Pathology at the Duke Prostate Center at Duke University Medical Center in Durham, N.C.
Do you believe there is a relationship between dietary fat and prostate cancer?
Dr. Freedland: Fat is one of three macronutrients. It exists with protein and with carbohydrates. When a person eats more of one, they eat less of another, but there’s also a very important factor of just how much they eat: total calories. It becomes very worrisome when I see papers that have focused on one little area [such as dietary fat] without talking about the whole big picture. Ultimately it’s diet. You don’t say, “I’m going to eat fat at this meal; I’m going to eat protein at that meal; I’m going to eat carbohydrates at this meal.” You eat a piece of chicken; you eat a piece of bread. You eat a diet; a whole meal.
People talk about tomatoes being healthy. You hear a lot about that in terms of prostate cancer: “Tomatoes have lycopene, which is good for you; eat tomatoes!” Well, lycopene, and many of the other nutrients in tomatoes, are fat-soluble. So if you just sit down and eat a tomato like you would an apple or a pear, you don’t really absorb many of the nutrients. But if you have it in a sauce or you have it on a pizza with greasy cheese, or with meatballs, you actually absorb the nutrients of the tomato better.
There are some data linking fat with prostate cancer, but I think there’s actually more data coming out now that actually says there’s no relationship between dietary fat and prostate cancer. In our mouse work we’re finding that a really-low-fat diet can slow tumor growth. But interestingly, an Atkins diet—very, very low-carb, but very high-fat — works even better. So we’re starting to look at this and come up with some conceptual models about macronutrients’ availability, and fats and carbs and what tumors like and what they don’t like.
But at the end of the day, tumors like calories. The number-one thing they want is calories. And if you cut down calories you reduce inflammation, you reduce reactive oxygen species, and you lower your insulin levels. All those are growth factors or drivers for cancer development and progression.
Why do some studies find a correlation between dietary fat and prostate cancer while others do not?
Dr. Freedland: Many of the studies are case-control [investigations] of a few hundred people here and a few hundred there. But when you look at data from very large 80,000-person cohort studies, there’s not nearly as strong of a link. And [in studies relying on self-reports], people may have “recall bias,” where they think since they have cancer, they must have eaten a high-fat diet all those years, when that might not necessarily have been the case. Yet in studies with tens of thousands of people who are asked about their diets and then are followed for years and years, we’re not really seeing men who had a particularly high-fat diet have worse outcomes. But we do see obesity resulting in a worse outcome.
Some of the studies are not even necessarily saying it’s the fats themselves that are the problem, but perhaps what we do with the fats: We overcook them, we process them, we add preservatives. Look at Europe: The highest fat intake is in the Mediterranean countries—Greece, Italy. And which European countries have the lowest rates of prostate cancer? Greece, Italy.
And certainly people in Asian countries have a lower-fat diet than we do. But they also eat [fewer] calories, they don’t eat the refined carbohydrates that we do, they eat a lot of soy products, they drink a lot of green tea.
So I’m very leery of pointing the finger at any single [dietary] entity. I think dietary patterns more so than just fat intake will be important. Diet is the sum of the whole.
Is it pointless, then, to try to identify dietary influences on prostate cancer risk or progression?
Dr. Freedland: No. I think it absolutely needs to be done. The difficulty is, for better or for worse, everybody is studying this, my own team included. So you get 100 articles a year, and half say dietary fat increases risk, and half say it has no effect. That’s why I try not to get excited about every new study.
But I worry that we’ve become so fat-phobic. Interestingly, in the 1970s and 1980s the nutrition guidelines came out saying that fat is bad for you, and over the last 20 or 30 years, fat intake in the American diet as a percentage of calories has gone down by about 5% – but obesity rates are way up. That’s because we’re eating fat-free ice cream, and the sugar content of fat-free ice cream is through the roof! But, it’s a low-fat diet, so it’s okay, right?
We say, “You’ve got to cut the fat,” but we’re missing the bigger problem, which is, number one, you have to cut calories. Because I can tell you unequivocally that obesity – the end result of caloric excess – is linked with fatal prostate cancer. Cutting calories is the number-one thing we should be talking about.
I’m very proud to have had a hand in influencing the nutrition guidelines from the Prostate Cancer Foundation [www.ProstateCancerFoundation.org]. They are written by four nutrition and exercise experts from around the country, and they really don’t discuss dietary fat much at all. They focus on appropriate exercise, minimizing refined carbohydrates, eating fruits and vegetables, and importantly cutting calories in general.
What about cholesterol that is in dietary fat? Is it relevant in the study of prostate cancer?
Dr. Freedland: The cholesterol research is interesting. There is increasing data to suggest higher serum cholesterol levels may promote aggressive prostate cancer, and there are potential mechanisms through which that can occur.
But there’s not always a good correlation between what you eat and what’s in your blood. People have actually gone on [high-fat] Atkins diets and don’t raise their cholesterol levels at all. If anything, they tend to lower their bad cholesterol and raise their good cholesterol, even though they’re eating meat and bacon all day long.
There is also increasing data—and lots of interest, from my group as well—in the role of statins in prostate cancer. Statins lower cholesterol, but they also inhibit inflammation, perhaps via cholesterol mechanisms but they also inhibit some cellular processes that may be key for cancer-cell growth, independent of cholesterol. The data here are much stronger for prostate cancer progression than prevention.