In our continuing series examining what long-term cancer survivors say helped them the most, one theme keeps coming up again and again: gaining control of blood sugar.
It’s a well known fact that cancer cells have an unusually high demand for glucose. In fact, this is how many tumors are detected in the first place. PET scans, for example, give doctors a window into how the body uses sugar. The cells that take in the most show up clearly—and cancer cells stand out because they depend so much on it.
That simple observation has led researchers to ask a powerful question: what happens when you stop constantly feeding that system?
For many survivors, this became a turning point. Instead of focusing only on treatments, they began looking at the internal environment—the “terrain”—and how it could be shifted.
Strategies like reducing sugar intake, spacing meals farther apart, and keeping blood sugar stable became part of that approach.
The Metabolic Theory of Cancer
One of the leading voices in this field is Dr. Thomas Seyfried, professor of biology, genetics, and biochemistry at Boston College and author of Cancer as a Metabolic Disease.
I had the opportunity to sit down with Dr. Seyfried to discuss this work in more detail:
His research brings attention to what is known as the metabolic theory of cancer. Instead of viewing cancer strictly as a genetic disease, this perspective looks at how cancer cells produce and use energy.
According to this model, many cancer cells are heavily dependent on glucose and fermentation for fuel. Healthy cells, on the other hand, are metabolically flexible. When glucose is limited, they can shift to alternative energy sources—such as ketones—to continue functioning.
That difference may be more important than we once realized.
It suggests that changing the body’s fuel supply could influence how cancer cells behave—potentially making the environment less supportive of their growth.
Why Fasting Has Drawn So Much Attention
Fasting is one of the fastest ways to influence this metabolic shift.
When the body goes without food for a period of time, insulin levels drop, blood sugar stabilizes, and the body begins transitioning toward fat-burning and ketone production. This is sometimes referred to as a “metabolic switch.”
Dr. Seyfried and other researchers have explored how this shift may place stress on cancer cells, which are less adaptable, while healthy cells continue to function.
There is also emerging research suggesting that fasting may support the body in several other important ways. It can help the body clean out damaged cells (a natural process often called “cellular cleanup”), improve how the body handles blood sugar, and calm inflammation—all of which are important when the body is under stress.
Some researchers are also looking at how fasting may help certain conventional treatments—like chemotherapy and radiation—work more effectively, while potentially helping healthy cells better tolerate those treatments.
Ketosis and Its Benefits
When the body goes for a period of time without food—or when carbohydrate intake is very low—it eventually shifts into a metabolic state known as ketosis. In ketosis, the body begins burning stored fat for energy and produces molecules called ketones, which many healthy cells can use efficiently as fuel.
Researchers studying cancer metabolism, including Dr. Seyfried, have explored whether this shift in fuel sources may create a less favorable environment for certain cancer cells that rely heavily on glucose for energy.
Fasting vs. Intermittent Fasting
Fasting doesn’t have to mean extreme protocols. It can be as simple as allowing longer overnight breaks between meals or avoiding constant snacking throughout the day. It’s a process often referred to as “intermittent fasting”.
Seyfried asserts that the fastest way to achieve optimal ketosis is to begin with a 3-5 day water-only fast. And research backs him up.
One such study published in Cancer Science states, “Emerging evidence suggests that fasting could play a key role in cancer treatment by fostering conditions that limit cancer cells’ adaptability, survival, and growth. Fasting could increase the effectiveness of cancer treatments and limit adverse events.”
If such a water-only fast is prohibitive for you, consider intermittent fasting. It’s a method in which you restrict your meals to a certain number of hours per day, with as much as 18 hours between dinner and breakfast the next morning. It’s not as difficult as it may sound, especially if you’re not a breakfast person. Just put off your first meal of the day until, say, noon. Then eat your last meal of the day before 6 pm.
Stabilizing Blood Sugar: The Missing Piece
Beyond fasting, many survivors say one of the most helpful changes was simply keeping their blood sugar steady throughout the day.
In practical terms, that means cutting out sugar and processed carbs, eating enough protein and healthy fats, and not constantly snacking.
When blood sugar stays steady, things start to feel different. Energy becomes more consistent, cravings settle down, and you’re not riding that constant up-and-down cycle.
And just as important, you’re no longer feeding the kind of environment that allows abnormal cells to thrive.
A Shift in Perspective
Many long-term survivors say this was one of the biggest mindset changes they made.
They stopped asking only, “How do I fight cancer?”
And started asking, “What kind of environment does cancer thrive in—and how do I change that?”
For them, fasting and blood sugar control weren’t about deprivation. They were about creating conditions where the body could function more efficiently—and where abnormal cells might have a harder time gaining ground.
It’s a different way of thinking.
But it’s one that continues to gain attention as more people begin to explore the connection between metabolism, nutrition, and long-term health.
Resources Include:
Tiwari S, Sapkota N, Han Z. Effect of fasting on cancer: A narrative review of scientific evidence. Cancer Sci. 2022 Oct;113(10):3291-3302. doi: 10.1111/cas.15492. Epub 2022 Aug 10. PMID: 35848874; PMCID: PMC9530862.
Nencioni A, Caffa I, Cortellino S, Longo VD. Fasting and cancer: molecular mechanisms and clinical application. Nat Rev Cancer. 2018 Nov;18(11):707-719. doi: 10.1038/s41568-018-0061-0. PMID: 30327499; PMCID: PMC6938162.
Caffa I, Spagnolo V, Vernieri C, Valdemarin F, Becherini P, Wei M, Brandhorst S, Zucal C, Driehuis E, Ferrando L, Piacente F, Tagliafico A, Cilli M, Mastracci L, Vellone VG, Piazza S, Cremonini AL, Gradaschi R, Mantero C, Passalacqua M, Ballestrero A, Zoppoli G, Cea M, Arrighi A, Odetti P, Monacelli F, Salvadori G, Cortellino S, Clevers H, De Braud F, Sukkar SG, Provenzani A, Longo VD, Nencioni A. Fasting-mimicking diet and hormone therapy induce breast cancer regression. Nature. 2020 Jul;583(7817):620-624. doi: 10.1038/s41586-020-2502-7. Epub 2020 Jul 15.
Erratum in: Nature. 2020 Dec;588(7839):E33. doi: 10.1038/s41586-020-2957-6. PMID: 32669709; PMCID: PMC7881940.
Blaževitš O, Di Tano M, Longo VD. Fasting and fasting mimicking diets in cancer prevention and therapy. Trends Cancer. 2023 Mar;9(3):212-222. doi: 10.1016/j.trecan.2022.12.006. Epub 2023 Jan 14. PMID: 36646607.
Vernieri C, Fucà G, Ligorio F, Huber V, Vingiani A, Iannelli F, Raimondi A, Rinchai D, Frigè G, Belfiore A, Lalli L, Chiodoni C, Cancila V, Zanardi F, Ajazi A, Cortellino S, Vallacchi V, Squarcina P, Cova A, Pesce S, Frati P, Mall R, Corsetto PA, Rizzo AM, Ferraris C, Folli S, Garassino MC, Capri G, Bianchi G, Colombo MP, Minucci S, Foiani M, Longo VD, Apolone G, Torri V, Pruneri G, Bedognetti D, Rivoltini L, de Braud F. Fasting-Mimicking Diet Is Safe and Reshapes Metabolism and Antitumor Immunity in Patients with Cancer. Cancer Discov. 2022 Jan;12(1):90-107. doi: 10.1158/2159-8290.CD-21-0030. Epub 2021 Nov 17. PMID: 34789537; PMCID: PMC9762338.
Antunes F, Erustes AG, Costa AJ, Nascimento AC, Bincoletto C, Ureshino RP, Pereira GJS, Smaili SS. Autophagy and intermittent fasting: the connection for cancer therapy? Clinics (Sao Paulo). 2018 Dec 10;73(suppl 1):e814s. doi: 10.6061/clinics/2018/e814s. PMID: 30540126; PMCID: PMC6257056.
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