Cancer’s favorite food is glucose, the simple sugar your body makes from carbohydrates. Nearly a century ago, scientist Otto Warburg discovered that tumors consume dramatically more glucose than normal tissues, and this remains one of the most consistent findings in cancer biology. But the full picture is more interesting: cancer cells are remarkably flexible eaters, and glucose is just the beginning of what they consume.
Why Cancer Cells Crave Sugar
Healthy cells process glucose efficiently, extracting maximum energy through a slow, oxygen-dependent process. Cancer cells do something different. Even when oxygen is plentiful, they ferment most of their glucose into lactate, a waste product normally associated with hard-working muscles. This seems wasteful, and it is. Cancer cells extract far less energy per molecule of glucose than normal cells do.
So why do it? The answer has to do with speed rather than efficiency. Rapidly dividing cancer cells need more than just energy. They need raw building materials to construct new DNA, new proteins, and new cell membranes for every daughter cell. By running glucose through this faster, less efficient pathway, cancer cells generate the chemical precursors they need to keep dividing. They also regenerate a molecule called NAD+, which powers dozens of the chemical reactions involved in building new cell components. In other words, cancer cells don’t just eat glucose for fuel. They eat it for construction materials.
This glucose hunger is so reliable that doctors use it for diagnosis. PET scans work by injecting a radioactive form of glucose into the body and watching where it accumulates. Tumors light up because they absorb so much more glucose than surrounding tissue.
The Second Favorite: Glutamine
If glucose is cancer’s main course, glutamine is the essential side dish. Glutamine is the most abundant amino acid in your blood, and many cancer cells consume it at rates that researchers describe as “addiction.” Some tumors can’t survive without it.
Glutamine serves purposes that glucose can’t. It provides nitrogen, which cells need to build DNA and new proteins. It feeds into the cell’s central energy cycle, generating both energy and the precursors for fats and other amino acids. It also helps cancer cells produce glutathione, a powerful antioxidant that protects them from the internal stress of rapid growth. In pancreatic cancers driven by the KRAS mutation, glutamine is essential for maintaining the cell’s chemical balance and supporting continued growth.
When glutamine runs low, cancer cells adapt. They can scavenge nitrogen from other amino acids like leucine, isoleucine, and valine. Some cancer cells even learn to manufacture their own glutamine internally by stabilizing an enzyme called glutamine synthetase, a trick that lets them keep proliferating when external supplies dry up.
Some Cancers Prefer Fat
Not all cancers follow the glucose playbook. Prostate cancer cells, for example, depend heavily on burning fatty acids for energy. Research from the American Association for Cancer Research found that prostate cancer cells burn fat at roughly three times the rate of normal prostate cells. This distinct metabolic preference is one reason prostate tumors don’t always light up well on standard PET scans.
A 2024 study published in Nature revealed another fat connection. Researchers found that fructose (the sugar in high-fructose corn syrup and fruit) fueled tumor growth, but not in the way you might expect. The tumors couldn’t use fructose directly because they lacked the necessary enzyme. Instead, the liver converted fructose into a specific type of fat that cancer cells then consumed to build new cell membranes. Interestingly, the mice in this study didn’t gain weight or develop blood sugar problems on a high-fructose diet, suggesting the cancer-promoting effect worked through fat metabolism rather than obesity.
Cancer Cells Recycle Their Own Waste
One of the more surprising discoveries in cancer metabolism is that tumors recycle lactate, the “waste” product of their glucose fermentation. Researchers at the National Cancer Institute found that rather than simply expelling lactate, cancer cells transport it into their mitochondria and use it to generate lipids for new cell membranes and to fuel additional energy production. Studies in human lung tumors confirmed that lactate serves as a genuine energy source, not just a byproduct.
This creates an efficient internal economy within tumors. Cells on the outer edge with good blood supply may burn glucose in the traditional wasteful way, producing lactate. Cells deeper inside the tumor, where glucose is scarce, then absorb that lactate and use it as fuel. The tumor essentially feeds itself.
Metabolic Flexibility Is the Real Superpower
What makes cancer metabolism truly dangerous isn’t its preference for any single nutrient. It’s the ability to switch between fuel sources when one becomes scarce. Glucose and glutamine are the two preferred nutrients, but cancer cells compensate for a shortage of either by increasing their use of the other. When both run low, some cancer cells activate survival programs that let them scavenge amino acids and fats from the body’s own muscle and fat stores.
This scavenging is part of what drives cancer cachexia, the severe muscle wasting that affects many people with advanced cancer. Tumors shift the body’s hormonal balance toward breakdown rather than building, accelerating the loss of muscle and fat tissue. The nutrients released by this breakdown feed the tumor’s continued growth.
Does Eating Sugar Feed Your Cancer?
This is the question behind the question, and the answer is more nuanced than social media suggests. You cannot starve a tumor by cutting sugar from your diet. Your body tightly regulates blood glucose levels, and even on a zero-carb diet, your liver will manufacture glucose from protein and fat to keep blood sugar stable. Cancer cells will still get their glucose.
That said, chronically high blood sugar and the elevated insulin levels that come with it do create a more favorable environment for tumor growth. Insulin is a growth-promoting hormone, and consistently high levels are associated with increased cancer risk. This is one reason obesity and type 2 diabetes are linked to higher rates of several cancers. The diabetes drug metformin, which lowers blood sugar and insulin, has shown some ability to inhibit tumor growth, and researchers are studying it as an add-on to standard cancer treatments.
The ketogenic diet, which drastically cuts carbohydrates to force the body into fat-burning mode, has generated significant interest as a potential way to exploit cancer’s glucose dependence. Animal studies show promising results: reduced tumor growth, protection of healthy cells during chemotherapy, and enhanced drug effectiveness. But human clinical trials have produced contradictory findings, and there is no consensus yet on whether ketogenic diets meaningfully improve cancer outcomes in people. Some studies suggest improved quality of life and better tolerance of chemotherapy, but the evidence is not strong enough to make firm recommendations.
The practical takeaway is that a diet low in added sugars and processed foods reduces your risk factors for cancer through multiple pathways: lower insulin levels, reduced inflammation, and healthier body weight. But the relationship between diet and tumor metabolism is indirect and complex, not as simple as “sugar feeds cancer.”