Diet Matters in Prostate Cancer, but It’s Complicated

Diet is increasingly seen as a modifiable risk factor in prostate cancer.

Recent studies have shown that ultralow-carbohydrate diets, weight loss diets, supplementation with omega-3 fatty acids, pro- and anti-inflammatory diets, fasting, and even tea drinking may affect prostate cancer risk or risk for progression.

In October, a cohort study involving about 900 men under active surveillance for early stage prostate cancers found that those who reported eating a diet that adhered closely to the US government’s recommendations as indicated by the Healthy Eating Index (HEI) saw a lower risk for progression at a median 6.5 months follow-up.

These findings follow results from an observational study, published in May, that followed about 2000 men with locally advanced prostate tumors. Men consuming a primarily plant-based diet (one closely adhering to the plant-based diet index) had less likelihood of progression over a median 6.5 years than those consuming diets low in plant-based foods.

“There is an increasing body of literature that says your diet matters,” said urologist Stephen J. Freedland, MD, of Cedars-Sinai Medical Center in Los Angeles and director of its Center for Integrated Research in Cancer and Lifestyle. “At the same time, there are a lot of things that could explain these associations. People who can afford lots of plant-based foods tend to have higher socioeconomic status, for example.”

What’s needed, Freedland said, are more randomized trials to test the hypotheses emerging from the longitudinal cohort studies. “That’s where I’m going with my own research,” he said. “I’d like to look at a study like [one of these] and design a trial. Let’s say we get half of patients to eat according to the healthy eating index, while half eat whatever they want. Can dietary modification change which genes are turned on and off in a tumor, as a start?”

Oncologist and Nutritionist Collaborate on Multiple Studies

Nutritionist Pao-Hwa Lin, PhD, of Duke University in Durham, North Carolina, has been working for several years with Freedland on trials of nutrition interventions. A longtime researcher of chronic disease and diet, she first collaborated with Freedland on a study, published in 2019, that looked at whether insulin could be driven down with diet and exercise in men treated with androgen deprivation therapy.

Not only are high levels of insulin a known contributor to prostate cancer growth, Lin said, but “insulin resistance is a very common side effect of hormone therapy. And we saw that the low carb diet was very helpful for that.” The finding led Freedland and Lin to design further trials investigating carbohydrate restriction in people with prostate cancer.

Lin said randomized trials tend to be smaller and shorter in duration than the observational cohort studies because “interventions like these can be hard to maintain, and recruitment can be hard to sustain. A very well controlled and intensive nutrition intervention is not going to be super long.” Short trial durations also mean that prostate cancer progression can be difficult to capture. Risk for progression has to be measured using surrogate markers, such as the doubling time for prostate-specific antigen (PSA).

In 2020, Freedland and Lin published results from a pilot study of 57 men who had been treated with surgery or radiation for localized prostate cancer but had a PSA recurrence and were randomized to an ultralow-carbohydrate diet or no restrictions for 6 months. The investigators saw that PSA doubling times, an intermediate measure of tumor growth rate, were slower among those consuming the low-carb diet.

Currently they are wrapping up a trial that randomizes men who have been scheduled for radical prostatectomy to daily supplementation with walnuts, a natural source of polyphenols and omega-3 acids. This time, the aim is to determine whether gene expression in tumors changes in response to supplementation.

The researchers are also recruiting for a study in men being treated for metastatic prostate cancer. This study randomizes patients to a fasting-mimicking diet, which is a type of intermittent fasting, or no dietary restrictions for 6 months.

Developed by biologist Valter Longo, PhD, of the University of Southern California, the fasting-mimicking diet has been shown to boost treatment effects in women with hormone receptor–positive breast cancer. In 2023, Longo and his colleagues published results from a small pilot study of the same diet in men with prostate cancer, reporting some positive metabolic findings.

Longo, who is consulting on Lin and Freedland’s trial, “has proven that the diet is helpful in treatment outcomes for breast cancer. So we connected and decided to test it and see if it’s helpful in prostate cancer as well.”

More Than One Approach Likely to Work

Though Lin and Freedland have focused most of their investigations on carbohydrate restriction, neither dismisses the potential for other dietary approaches to show benefit.

“There are two main schools of thought in terms of the relationship between diet and prostate cancer,” Lin said. “One is the insulin angle, and that’s what we hypothesized when we first tested the low-carb diet. The other is the inflammation angle.”

Studies have shown greater adherence to the HEI — a diet quality indicator that favors grains, fruits, dairy, vegetables, beans, and seafood — or the plant-based diet index to be associated with lower biomarkers of inflammation, she noted.

Insulin resistance, Lin explained, “is also highly related to inflammation.” (Several of the diets being investigated in prostate cancer were originally studied in diabetes.)

Moreover, weight loss caused by low-carb diets — or other healthy diets — can have a positive effect on insulin resistance independent of diet composition. “So it is a very complicated picture — and that doesn’t exclude other pathways that could also be contributing,” she said.

On the surface, a low-carb diet that is heavy in eggs, cheeses, and meats would seem to have little in common with the HEI or a plant-based diet. But Freedland noted that there are commonalities among the approaches being studied. “No one’s promoting eating a lot of simple sugars. No one’s saying eat a lot of processed foods. All of these diets emphasize whole, natural foods,” he said.

Lin hopes that she and Freedland will one day be able to test a diet that is both lower-carb and anti-inflammatory in men with prostate cancer. “Why not combine the approaches, have all the good features together?” she said.

But Freeland pointed out and explained why most clinicians don’t make dietary recommendations to their newly diagnosed patients.

“A new prostate cancer patient already gets easily an hour discussion of treatment options, of pros and cons. Patients often become overwhelmed. And then to extend it further to talk about diet, they’ll end up even more overwhelmed.” Moreover, he said, current evidence offers doctors few take-home messages to deliver besides avoiding sugar and processed foods.

Multiple dietary approaches are likely to prove helpful in prostate cancer, and when the evidence for them is better established, patients and their doctors will want to consider lifestyle factors in choosing one. The best diet will depend on a patient’s philosophy, tastes, and willingness to follow it, he concluded.

“At the end of the day I’m not rooting for one diet or another. I just want to get the answers.”

Lin disclosed no financial conflicts of interest. Freedland disclosed serving as a speaker for AstraZeneca, Astellas, and Pfizer and as a consultant for Astellas, AstraZeneca, Bayer, Eli Lilly, Janssen, Merck, Novartis, Pfizer, Sanofi-Aventis, and Sumitomo.

Jennie Smith is a freelance science writer.