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Alicia Morgans: Hi. I'm so excited to be here with Dr. Stacy Loeb where we're talking about a recently published paper that she had in Cancer, looking at the effects of a plant-based diet in prostate cancer survivors. Thank you so much for being here with me today, Dr. Loeb.

Stacy Loeb: Well, thank you so much for having me and for collaborating on the study.

Alicia Morgans: It was truly my pleasure and honor to participate in this work, which I know has been a passion of yours over many years. And I'd love to hear from your perspective, what was it that kind of prompted this work? I know there's been a lot of back and forth in nutrition research in prostate cancer and in all cancers for many years, but this has been an area that you've really dug into, and I'd love to hear where you were starting and what prompted this study.

Stacy Loeb: Absolutely. So, first of all, part of the motivation was really from clinical practice. I've actually had patients who have adopted a whole foods plant-based diet, and they no longer are using their Viagra. And so, that really got me interested in studying this scientifically because obviously we can't just go on anecdotal evidence. So, first, we actually looked at sexual health in people without prostate cancer, and that was actually also in the Health Professionals Follow-Up Study, the same large cohort that was used for the current study in prostate cancer. And what we found was that basically eating more plant-based and less animal-based foods was associated with a lower risk of incident erectile dysfunction for men in their 60s. So then I was very interested in looking at this also in patients with prostate cancer. This is a really difficult population because patients with prostate cancer have a high risk of erectile dysfunction with treatment.

So, in addition to all the normal things that often we see with aging, cardiovascular disease, atherosclerosis that are challenges for enough blood flow for erections, in addition, prostate cancer treatment, surgery or radiation can affect the nerves, and so they have even a greater challenge to overcome. So, I really wasn't sure if there would be any difference in sexual function or other quality of life measures with differences in diet in the prostate cancer population. And in fact, actually, there was a previous study that looked at the Mediterranean diet, and they actually didn't see any difference in sexual or urinary function scores with the Mediterranean diet in that study. So I wasn't sure what we would find here, but I thought it was really important to look at plant-based diets since they're just becoming a lot more popular and have many other health benefits.

Alicia Morgans: Absolutely. And it's so important, I think, to study diet and to understand these differences because I feel like a cancer population, like the prostate cancer population, is a vulnerable one that is at risk of following various forms of dietary change. And if we don't have the data to back up that change, it could cost them a lot of money, time, and they could lose weight depending on how drastic and dramatic those changes are. And so, it's so important to me that there is some data, some evidence to at least suggest an association of a benefit before I encourage patients down a path. And so, I'm so grateful that you're interested in this and that you were studying it. Now, can you tell me a little bit about the cohort that you used to really investigate this? It's a really interesting cohort, it's such an important one that looked at men over time, many thousands of men, actually.

Stacy Loeb: That's right. So this was data from the Health Professionals Follow-Up Study, so this is a really large cohort study out of Harvard University. And as you mentioned, what's great about it is longitudinal data, so they actually have serial questionnaires for diet since people's diets can change over time. And they also have serial questionnaires for quality of life, including things like sexual and urinary measures. And so, we were able to take all of that data and look at longitudinal models that take into account how these things may change even in an individual over time. So it's a really excellent data source for prospective cohort studies that's very well-annotated.

The other nice thing about this population is that they collect a lot of other data for potential confounding factors. So, for example, we were able to adjust for things like physical activity, smoking, body mass index because they collect so many other variables. So, it is an observational study, it's not a randomized control trial, but to the extent possible, the models were adjusted for a really extensive list of potential confounders to try to really parse out what is the association of the diet specifically after you take into consideration that maybe people who eat more plant-based exercise more or smoke less.

Alicia Morgans: Absolutely. So, what did you find as you investigated this within the health professional study?

Stacy Loeb: Yeah. So we looked at more than 3,500 patients with prostate cancer, most of them had been treated either with surgery or radiation. And what we found was significantly higher scores on questionnaires for sexual function, urinary control, and also vitality in people who consume more plant-based and less animal-based foods.

Alicia Morgans: That's really interesting and exciting. Can you tell me though, how did you quantify the proportion of foods that were really plant-based? Because I could imagine someone saying, "Well, you're not there judging these people. Did they self-report? How valid is the self-reporting measure?" What did you use?

Stacy Loeb: Right. That's a very good question. So this is all from Food Frequency Questionnaires, which is a validated dietary measure, but importantly, we used the plant-based diet index. So it is literally a score that gives you plus points for more plant-based foods, things like vegetables, fruits, grains, legumes, and you get minus points for animal-based foods, so things like meat, dairy, and eggs. And so, it literally is just calculating the proportion of plant-based to animal-based food. So the people in this population actually were not vegetarian or vegan; they were really largely an omnivorous population whom we categorize mathematically based on calculating the proportion of plant to animal-based foods and broke them down into five groups from the least plant-based, most animal-based to the most plant-based, least animal-based.

Alicia Morgans: That's great. And that's, I think, what makes this kind of information more generalizable in many ways because it is a spectrum, and many of our patients are going to be somewhere on that spectrum, not necessarily completely vegan or plant-based, but could be somewhere along the spectrum. And it is helpful for them to kind of put it into context and also to think about change in their own lives, in their own diet that they don't have to go from fully meat to fully plant. Although maybe that would be helpful to them, they can sort of find themselves on the spectrum in a way that they might be able to find a diet sustainable, which is the other thing that can be challenging as we're trying to make major changes in our dietary habits. So, as you think about this, and there were some benefits seen, what were some of the limitations of this project?

Stacy Loeb: I think with any observational study, we just have to keep in mind that there could be other factors that we didn't measure that are potential confounders. So we can't conclude definitively that eating more plant-based caused less erectile dysfunction, for example, in these people. This wasn't a randomized control trial where people were assigned to a specific diet, and all these other factors were similar between the groups. We really did try, to the extent possible, to adjust the models for everything that we could think of that might potentially confound this relationship, but there could still be other factors. So, I think that's really the main limitation, but the strengths are that it was such a very large population of people who we were able to break down based on their actual dietary patterns and scores over time, over many, many years after prostate cancer. And I was surprised and excited that we actually saw statistically significant associations here.

So I would say, despite the fact that this isn't a randomized control trial or we can't prove causality, plant-based diets are already something that is recommended in the American Dietary Guidelines as a healthy dietary plan at all life stages. And also that, according to the American Dietary Guidelines, it is associated with a reduced risk for many other chronic diseases, including cardiovascular disease, which is actually the leading cause of death in patients with prostate cancer and is also linked with erectile dysfunction. So, I think it's really just a win-win to tell our patients to consider eating more plant-based.

Alicia Morgans: I agree. And as we wrap up, how do you use this information in your daily clinical practice, and what would your final words be for practitioners who are trying to think through this, maybe especially those who have not really considered a plant-based diet recommendation for their patients before?

Stacy Loeb: I think it's time that we start incorporating nutritional counseling into our patient counseling. Patients are constantly asking if there is anything that they can do, and this is something that is easy that they can do that may help with their quality of life but may also help with a lot of other things for which there are large randomized controlled trials, such as reducing diabetes, weight loss, and blood pressure. So, this is a very teachable moment, and I think it's truly a missed opportunity if we don't use it to try to help our patients become healthier overall.

Alicia Morgans: Wonderful. Well, thank you for sharing your perspectives. Congratulations on a fantastic study, really a large study that, even though it is not absolutely showing cause and effect, is showing a very strong association in a way that we have not seen before when it comes to a plant-based diet, and something that is very thought-provoking. I think it encourages us to make these changes in our recommendations to patients and practice each day. So, thank you for your time and for your expertise.

Stacy Loeb: Thank you.