Do you like sweets? Chocolate? Maybe a sweeter fruit like melon? Well, I have news for you — cancer cells like it too.

If we could watch a cancer cell in real time, you’d probably expect to see “uncontrolled division,” “constant multiplication.” The truth is a little different — and, paradoxically to everything that spreads on social media, quite a bit more interesting. What we’d actually observe is a ravenous hunger for glucose.

Healthy cells use the building blocks of carbohydrates as well as proteins and fats, including vitamins and minerals. Cancer cells, on the other hand, are metabolically “hungry” almost exclusively for sugar. But before we get to the question of whether restricting sugar actually makes a tumor “starve,” it’s worth understanding where this hunger comes from.

The science — where it all begins

About 100 years ago, Otto Warburg made a major discovery: healthy cells use oxygen efficiently, while tumor cells use it inefficiently, even when plenty of oxygen is available. Sounds strange, right? Let’s look at why.

Our cells have two main ways of generating energy. Glycolysis takes place in the cytosol, while the Krebs cycle and oxidative phosphorylation take place in the mitochondria — the organelles known as the cell’s “power plants.” They have their own DNA, distinct from the DNA in the nucleus, and a large share of diseases actually trace back to mutations in mitochondrial DNA specifically.

Glycolysis happens with or without oxygen, but yields only about 2 molecules of ATP — relatively little energy. Like lighting a piece of paper — it flares up fast, but doesn’t warm you for long. The Krebs cycle, by contrast, only works when oxygen is present, but yields many times more ATP — much more energy, though more slowly, through more steps. Like lighting a thick oak log — it takes time, but burns long and hot.

Cancer cells aren’t particularly patient. They prefer the fast, if less efficient, path — glycolysis — even when there’s enough oxygen available to use the more efficient Krebs cycle. This is exactly the Warburg effect.

Picture a cancer cell like a teenager who’s just had their phone taken away and is waiting to play again. They could go for a walk — a lot more useful and sustainable. But they prefer the quick hit of pleasure, even if it’s less efficient in the long run.

By the way — the Warburg effect is exactly what PET scans are based on. Specially tagged glucose concentrates more in the tumor than in healthy tissue, because the tumor is literally “starving” for sugar.

From there, the science gets considerably more complex. In many tumors, mitochondrial respiration actually remains functional, working — the shift to glycolysis doesn’t necessarily mean “broken” mitochondria, but rather active metabolic reprogramming, driven by oncogenes like AKT, MYC, and RAS.

The ketogenic diet — “the hidden cure” or “a lot of noise about nothing”?

Here we’re entering territory that’s both exciting and delicate.

If you drink cyanide, you’ll die — but your tumor won’t. Why? Because cyanide damages the respiratory chains in the mitochondria, and the tumor doesn’t rely on oxygen for energy anyway. It relies on fermentation — a process where energy comes from carbohydrates without oxygen. The fuel for this fermentation is two molecules: glucose and glutamine.

This is exactly what Warburg studied in rats a century ago.

Ketosis is a metabolic state in which the body burns fat instead of glucose for energy. The hypothesis behind the ketogenic diet for cancer is appealing: if the tumor depends on glucose, and we drastically reduce glucose, could we make it “starve”?

Here’s the catch. For the body to use ketone bodies for energy, it needs healthy mitochondria, capable of oxidative phosphorylation. And the defining characteristic of tumor cells is exactly the lack of that capacity. If a mitochondrion has lost its structure — the cristae (the folds) where the respiratory chains sit — it can’t process ketone bodies. It will reach back for fermentation, but unsuccessfully, if we’ve also reduced glucose.

Seyfried’s theory — what it claims and why it’s controversial

Professor Thomas Seyfried of Boston College is the most prominent proponent of the view that cancer is primarily a metabolic, not a genetic, disease. In his book Cancer as a Metabolic Disease (2012), he proposes the so-called Press-Pulse strategy — a combination of induced ketosis (to restrict glucose) with temporary glutamine suppression, aimed at weakening tumor cells enough for the immune system to “clear” them.

It’s important to say this clearly: this is a hypothesis under investigation, not an established treatment. Mainstream oncology remains skeptical for several reasons — the genetic mutations found in tumors are well documented and hard to explain by metabolic dysfunction alone, and the glutamine inhibitor Seyfried proposes (DON) is toxic and still not approved for clinical use outside strictly controlled trials.

What we actually have in terms of clinical evidence

Despite the enormous interest, clinical evidence supporting the ketogenic diet in cancer patients remains limited. Preclinical animal and in vitro studies show that the ketogenic diet can suppress tumor proliferation in certain cancer types, but clinical evidence for an effect on tumor progression in humans is still limited.

The most promising results are in brain tumors — glioblastoma — where clinical and experimental studies show a satisfactory safety profile. Even there, though, we’re talking about safety and tolerability, not proven effectiveness against the tumor.

Glucose and glutamine — why it’s not that simple

If glucose can be restricted through diet, glutamine can’t — at least not safely. Glutamine makes up about 60% of the free amino acids in muscle and about 20% of those in blood. It’s critically important for the immune system, gut health, and the urea cycle.

This is also where the practical problem with the idea of “starving” the tumor lies: the amino acid that fuels cancer cells is the same one our healthy cells — especially immune cells — depend on for survival.

Mild ketosis, obesity, and inflammation — the bigger picture

There’s an interesting biological logic to the idea that mild, sustained ketosis — from reduced carbohydrates and an active lifestyle — keeps mitochondria healthy.

The flip side is better documented: obesity has already overtaken even smoking as the leading cause of preventable disease, and fat tissue releases inflammatory molecules like IL-1 and TNF-alpha. Chronic inflammation damages mitochondria, and damaged mitochondria produce more free radicals (ROS) — unstable molecules that damage DNA and lead to mutations.

In other words — metabolic health probably (but not certainly) matters for cancer risk. But that’s a very different statement from “the keto diet cures cancer.”

Fructose — researchers’ new focus

For a long time, research into the “fuel” of cancer cells was directed almost entirely at glucose. It turns out, though, that some cancer cells have a backup plan — they also use fructose when they can. Like a car that, besides gasoline, also runs on gas if it’s more convenient.

Fructose is processed differently from glucose — mainly in the liver. Some tumors obtain it directly from the blood through a special transport channel called GLUT5. Others are even more inventive — they produce fructose themselves from glucose through a series of reactions called the polyol pathway.

In pancreatic cancer, scientists have found that tumor cells produce more GLUT5 — and when more fructose is available, these cells multiply faster and send out signals to build new blood vessels. In some lung tumors, an enzyme (AKR1B1) involved in producing fructose inside the cell itself has been found — blocking it slows tumor growth under laboratory conditions.

Does this mean fructose causes cancer? No. Current data doesn’t show that fructose by itself causes cancer — only that an already existing tumor can use it as an additional energy source.

Does this mean you should stop eating fruit? No. Fruit carries fiber, vitamins, minerals, and antioxidants whose benefits far outweigh the fructose they contain. The problematic source of fructose is sweetened drinks, energy drinks, and heavily processed foods with added sugar — not the apple.

The practical takeaway

If we had to sum all of this up in one sentence for a patient or family member: the biology behind “sugar feeds cancer” isn’t a myth — but it’s also not grounds for self-directed starvation or a strict ketogenic diet without medical supervision.

ESPEN — the European Society for Clinical Nutrition — recommends exactly the opposite of restriction for most cancer patients: adequate calorie and protein intake, to prevent malnutrition and cachexia (the body’s outright wasting), which worsen prognosis far more reliably than any potential benefit of a strict diet.

The sensible message is: limit added sugar, not natural sources like fruit. Beyond that:

This article is for educational purposes and doesn’t constitute medical advice. The metabolic theory of cancer and the therapeutic use of the ketogenic diet remain an active area of research — not an established standard of care. Any decision related to diet during cancer treatment should be made together with your treating physician.

Sources

  1. Warburg, O. — original observations on aerobic glycolysis in tumor cells (1920s), summarized in: Potter, M., Newport, E. & Morten, K. J. The Warburg effect: 80 years on. Biochemical Society Transactions, 44 (2016).
  2. Seyfried, T. N. Cancer as a Metabolic Disease: On the Origin, Management, and Prevention of Cancer. Wiley (2012).
  3. Salido-Bueno et al. Effects of ketogenic diets on cancer-related variables: A systematic review and meta-analysis of randomised controlled trials. Nutrition Bulletin, 49 (2024).
  4. Phase 1 safety and feasibility trial of a ketogenic diet plus standard of care for glioblastoma. Scientific Reports (2025).
  5. Systematic Review: The Role of the Ketogenic Diet in Managing Glioblastoma. Cancer Neuroscience / MDPI (2024).
  6. Frezza, C. Fructose: the sweet(er) side of the Warburg effect. Cell Death & Differentiation (2024).
  7. WCRF — Ask the expert: does sugar feed cancer? World Cancer Research Fund.
  8. ESPEN Practical Guideline: Clinical Nutrition in Cancer (2021).

Further listening: The Peter Attia Drive, ep. #187 — “The Warburg Effect: Otto Warburg’s Cancer Metabolism Theory” with Sam Apple.