Dietary Strategies in Cancer: The Role of Fasting and Ketogenic Therapy

Part 2 of 2 on Optimizing Diet and Metabolic Health to Reduce Cancer Risk

In my practice, more and more patients are asking about fasting, ketogenic diets, and other metabolic strategies. Many have done their homework and come in with thoughtful questions, but they are also feeling overwhelmed. They want to know what is real, what is safe, and what might actually help.

This article reflects how I approach those conversations in the clinic. There is no one-size-fits-all answer. I try to understand the whole person: their goals, overall health, nutritional status, and their interest in using diet as part of their care. I use these strategies with some patients, not all, and always with careful consideration and a focus on safety.

To be honest, I find this topic challenging too. Even after years in oncology and a genuine interest in metabolism and prevention, I still run into research that is hard to interpret. Nutrition is complicated. Cancer is even more so. A lot of what we are working with involves early data, mixed results, and unanswered questions.

That is why I wrote this piece. It reflects my current understanding, shaped by clinical experience, conversations with colleagues, and ongoing reading. It is not a comprehensive review, and I have simplified some of the science to keep it accessible. But it is honest, and I hope it is helpful.

Photo credit: Amy Paturel, Cedars-Sinai

Why I Care About Metabolism?

Most of my career has focused on delivering conventional cancer treatments. I have seen how powerful tools like immunotherapy and precision oncology can be. We have made remarkable progress. But I have also seen moments when those tools were not enough.

That led me to ask different questions. What internal conditions allow cancer to thrive? And is there anything we can do to shift that environment?

My interest in strategies like fasting and ketogenic diets has grown because new research suggests metabolism might influence cancer. The studies are still small and early, but they are worth paying attention to.

So let’s start with a basic question:

What does cancer need to grow, and what might that tell us about how to slow it down?

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The Metabolic Terrain: What Cancer Needs

Cancer does not just arise from faulty genes or bad luck. It grows in a biological environment shaped by how our bodies manage energy, inflammation, stress, and immune function.

Here are a few key features of that environment:

• Glucose metabolism shifts: Many cancers rely heavily on glucose, even when oxygen is available. This is known as the Warburg effect.

• Mitochondrial function declines: Inefficient mitochondria push cells toward growth rather than repair.

• Insulin and growth signals rise: High insulin and IGF-1 levels encourage abnormal cell division and survival.

• Chronic inflammation develops: This supports tumor growth and weakens immune defense.

• Immune evasion occurs: Cancer cells often find ways to hide from the immune system.

• The microenvironment changes: Blood vessels, connective tissue, and signaling molecules can help cancer grow.

• The gut microbiome matters: The bacteria in our gut influence immune and metabolic health and can affect treatment response.

Understanding this terrain may help explain why some cancers grow quickly, why some treatments fail, and why two patients with the same diagnosis can have different outcomes.

Fasting and Cancer: Where It Fits

Fasting is not a new concept, but interest has surged. There are animal studies, small trials, and helpful reviews, including:

• The Clinical Impact of Time-restricted Eating on Cancer: A Systematic Review

• Prolonged Nightly Fasting and Breast Cancer Prognosis

After reviewing the research and discussing with patients, here is where I land:

• Fasting may help with prevention. It improves markers like insulin and inflammation. These changes could reduce cancer risk over time.

• The role during treatment is less clear. Some small studies suggest fasting might reduce chemo side effects or protect healthy cells. Results are mixed.

• Adherence is the challenge. During treatment, fasting can be tough. Patients may already be dealing with fatigue, appetite loss, or weight changes.

Because of this, I do not routinely recommend fasting during treatment. In some survivorship or prevention cases, it may be helpful. But it needs to be individualized and safe.

For many patients, improving metabolic health through sustainable habits like a Mediterranean-style diet, regular exercise, and good sleep is more practical and may be just as effective.

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The Ketogenic Diet and Cancer: A Closer Look

Of all the metabolic strategies patients ask about, the ketogenic diet sparks the most curiosity. The theory of "starving cancer of sugar" feels intuitive.

A ketogenic diet is high in fat, moderate in protein, and very low in carbohydrates, usually 25 to 50 grams per day. This shifts the body toward using fat for energy and producing ketones. Normal cells can use ketones. Some cancer cells cannot.

A 2019 systematic review examined 39 studies involving 770 cancer patients. The findings were mixed. Some studies suggested benefit, but overall evidence was limited. Adherence was a major issue.

These findings highlight a key point. We do not yet know if the ketogenic diet changes outcomes. What we do know is that sticking with it is hard for many people, especially during cancer treatment.

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Signs of Promise in Glioblastoma

Glioblastoma (GBM) is an aggressive brain cancer that relies heavily on glucose. That makes it a logical target for dietary strategies.

A 2024 prospective study of 18 GBM patients found that among the six who stayed on the ketogenic diet for at least six months, four survived more than three years. Of the twelve who did not follow the diet, only one reached that mark.

These results are early and not definitive, but this has helped launch studies, like the DIET2TREAT trial, which is now underway.

Again, the challenge remains sustainability. Even if the science is promising, the real-world application is difficult without the right support.

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Where Research Is Headed

One lab that has drawn attention in this field is Dr. Thomas Seyfried’s group at Boston College. While some of their ideas have been seen as controversial, their work has helped spotlight cancer metabolism. Even in their own research, they acknowledge that ketogenic diets alone are not enough.

Their recent work focuses on combining keto with other metabolic therapies. One such compound, JHU-083, blocks how cancer cells use glutamine, another major energy source. The idea is to deprive tumors of both glucose and glutamine while allowing normal cells to adapt.

There is also new interest in the PIK3CA pathway. This pathway is involved in insulin signaling, and mutations in it are common in breast and endometrial cancers. These cancers may be especially sensitive to high insulin levels. Some studies are now testing whether lowering insulin through diet or medication can improve outcomes.

We still have much to learn. But these are good questions. They deserve more research, more clinical trials, and thoughtful integration into care.

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What I Tell My Patients

I’ve had many patients ask if they should try a ketogenic diet. For some, especially those with insulin resistance or metabolic syndrome, it might be worth exploring. When done with support and in the right context, I’ve seen it improve metabolic health.

But it is not right for everyone. For people already dealing with fatigue, weight loss, or nausea, keto can be difficult to sustain. Like any intervention, it requires planning, timing, and support.

My approach is to use it thoughtfully, when it makes sense, and as part of a bigger plan that includes proven treatments and a deep understanding of the person in front of me.

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Final Thoughts

Metabolism and nutrition are becoming a more important part of the cancer conversation, and for good reason. These approaches are not cures, but they may help shift the environment in the body to support healing.

I believe we should stay curious and careful. We need to keep asking the right questions, support the right trials, and focus on care that is not just effective, but also personal and compassionate.

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If you're just beginning to explore how nutrition connects with cancer risk and recovery, I’ve written a few deep dives that might be helpful. Both were inspired by the thoughtful, important questions I hear every day in clinic: questions about what to eat, what to avoid, and how diet might influence outcomes: