People have sought answers about the relationship between nutrition and health throughout history. In the past few decades, scientific research has uncovered important insights about our diets and cancer, including how food can affect our risk of developing the disease, the treatment of cancer, and living well after cancer.
Experts at Memorial Sloan Kettering Cancer Center (MSK) are at the forefront of this research. Hematologic oncologist Urvi Shah cares for people with plasma cell disorders, including multiple myeloma. Medical oncologist Neil Iyengar treats people with breast cancer. Both conduct research aimed at understanding the interplay between nutrition and cancer.
Drs. Shah and Iyengar recently conducted a review of studies that examine the relationship between diet and cancer. They focused on two of the most popular diets — a whole foods, plant-based diet and the keto, or ketogenic, diet.
Their review was published in JAMA Oncology in July 2022 and offers important new insights into which of these two very different approaches are best for people concerned with reducing their cancer risk and improving their health after treatment.
Why did you focus on plant-based and keto diets?
Dr. Shah: One reason is many of our patients ask about these two diets. Patients sometimes question: “Are sugars bad? Do sugars cause the cancer to get worse?” That’s why a keto diet may appeal to some patients — because it’s considered low-carb, so they think they are eliminating sugars, which may reduce cancer risk. Although this is not entirely true, and while we can all agree that refined carbohydrates are bad, consumption of whole grains have been associated with a reduced cancer risk.
We also get many questions about a plant-based diet, as there is more awareness of the health benefits of plant foods, especially the importance of fiber, which is only derived from plants. There is now a large and growing body of scientific research about these two diets. The concepts behind these diets are diametrically opposite. So comparing the research to understand differences about both of them is helpful.
Dr. Iyengar: There is growing evidence that weight loss is helpful for reducing cancer risk, particularly for obese and older people, including the postmenopausal age group. Both keto and plant-based diets have proven effective for weight loss. However, there is less available data to answer whether the benefits of these diets extend to cancer. Most of the available data support a whole foods, plant-based diet over a ketogenic diet for reducing the risk of cancer. After a cancer diagnosis, a plant-based diet still appears to be superior.
However, some studies in mice show that the keto diet is useful in very specific tumor types, such as a specific type of breast cancer with mutations in the PIK3CA gene. Ongoing research is investigating whether this is true for humans with this type of breast cancer. It is important to note that the keto diet made one type of blood cancer worse in the mouse study, so it is important to wait for data from the human studies before we can recommend the keto diet to people diagnosed with cancer.
What is a keto diet?
Dr. Shah: The aim is to force the body into using a different type of fuel, called ketone bodies, rather than relying on sugar that comes from carbohydrates like grains and fruits. This usually requires getting a very large percentage of calories from high-fat foods, including meat, dairy, fish, oils, and eggs.
Many diets that claim to be ketogenic are actually just low-carb, however. They don’t really force the body into the state of ketosis, where the body breaks down protein and fat for energy. That can make it challenging to compare keto-style diets for their beneficial effects, since they vary quite a bit.
Dr. Iyengar: For some cancers — including breast cancer — it’s also important to note that higher fat consumption has been associated with poorer long-term outcomes. And some diets claiming to be keto allow for fats from highly processed foods, which isn’t great. There are many so-called keto foods available at grocery stores, but some could actually be harmful if the fat content is higher than it should be.
For patients diagnosed with breast cancer, MSK has a keto clinical trial called TIFA that is very specifically controlled to avoid those pitfalls. For instance, we deliver preprepared meals to patients so the mix of nutrients is appropriate. We encourage patients to look for well-designed trials like the TIFA trial if they are interested in ketogenic diets.
What is a plant-based diet?
Dr. Shah: When we talk about a whole foods, plant-based diet, we mean the majority (at least 80% to 90%) of the food should be unprocessed plant-based foods — things like legumes, fruits, vegetables, seeds, whole grains, and nuts. Some people may end up eating minimal amounts of processed plant foods or animal-based foods like dairy or meat occasionally, but not on a regular basis.
What’s the difference between plant-based, vegetarian, and vegan diets?
Dr. Shah: I tell patients that vegan means zero animal products and is grounded in ethical or environmental or health reasons for a person who has decided very clearly what they want. A vegetarian diet may have similar reasons but allows dairy or eggs in varying proportions. A whole foods, plant-based diet can be similar to a Mediterranean diet or other diets that include eating meat rarely. Again, the majority (90%) of foods should be whole plant-based foods.
Dr. Iyengar: We don’t have strong enough evidence to conclude that vegan eating is superior to a semi-vegetarian diet for reducing the risk of cancer or cancer recurrence. MSK and other centers are running trials that will help address this question.
Which diet did your review find was better at reducing the risk of cancer — a plant-based diet or keto?
Dr. Shah: Our review of the evidence supports a plant-based diet for reducing cancer risk compared with a ketogenic diet. That conclusion is particularly apparent when you look at big populations, such as three large studies conducted in the U.S., U.K., and France. All three found that people eating more plants and less animal protein had less cancer overall. That’s consistent with other large studies and included in the guidelines by the American Institute of Cancer Research and American Cancer Society, which all recommend diets that incorporate high amounts of plant-based food.
Dr. Iyengar: There are many biologic processes in the body that are impacted by diet and energy balance in general. Many of these processes are involved in the growth of cancer, such as obesity and inflammation, hormones, insulin, the microbiome, and more. We examine several of these processes in our review. The bottom line for people looking to reduce their risk of cancer is that the current evidence supports a plant-based diet over a keto diet.
Research also shows that broad dietary patterns that people can stick to — like a plant-based diet — are generally better in the long-term than highly specific interventions like keto diets.
Which diet was better during treatment?
Dr. Iyengar: Currently, there is no evidence to suggest that a specific diet by itself can treat cancer. There is preliminary evidence to suggest that certain dietary patterns can help make some cancer treatments work better or reduce side effects. For instance, studies have shown that plant-based diets can help with some of the gastrointestinal toxicity from chemotherapy and joint pain from hormonal treatments for breast cancer.
There is also some evidence that the keto diet could reduce certain side effects, such as high blood sugar levels, from specific cancer treatments.
Of course, it’s important for people being treated for cancer to check with their oncologist about any specific changes to their diet.
Which diet was better for healthy living after treatment?
Dr. Iyengar: The majority of breast cancer is curable, but there can be long-term metabolic disorders from cancer treatment. In fact, many people treated for cancer are at increased risk for diabetes, obesity, and heart disease.
We know that a plant-based diet can help reduce the risk of these metabolic disorders, which is why I typically recommend this diet to my patients — we don’t want to just cure a person’s cancer; we want to also improve health overall.
Dr. Shah: It’s important that people — especially those facing cancer — think about the broad conclusions and common themes that emerge from dietary studies. For example, don’t focus too much on specifics like trying to avoid all sugar or carbohydrates; rather, cut out refined sugars and incorporate whole grains that are known to have health benefits. Trying to make changes with every study one comes across can drive a person crazy, especially if it says opposing things to the majority of data and cause a negative relationship with food.
Instead, concentrate on a healthy, plant-forward eating pattern that can be sustained, make gradual changes, and stick to it.