Cancer cells eat glucose as their primary fuel, consuming it at rates 10 to 100 times faster than normal cells. But glucose is only the beginning. Cancer cells are remarkably flexible eaters, capable of scavenging proteins, fats, and even cellular debris from their surroundings to sustain their rapid growth.
Glucose: The Preferred Fuel
In the 1920s, scientist Otto Warburg noticed something strange: tumors were devouring enormous amounts of glucose compared to the tissue around them. Even stranger, they were processing it inefficiently. Normal cells fully break down glucose in their mitochondria, extracting maximum energy. Cancer cells instead convert most of their glucose into lactate, a partial breakdown product, even when they have plenty of oxygen and perfectly functional mitochondria.
This quirk, called the Warburg effect, seems wasteful at first glance. Each molecule of glucose yields far less energy through this shortcut. But the speed makes up for the inefficiency. Cancer cells burn through glucose 10 to 100 times faster than cells using the slower, more complete breakdown process, producing roughly the same total energy over any given time period. The speed also generates a flood of molecular building blocks that cancer cells repurpose to construct new DNA, proteins, and membranes for rapid division.
This glucose hunger is so reliable that doctors exploit it to find tumors. PET scans work by injecting a radioactive glucose look-alike into the bloodstream. Cancer cells, unable to resist, gobble it up in far greater quantities than surrounding tissue. The radioactive signal lights up on the scan, revealing tumors as small as a few millimeters and helping oncologists monitor whether treatment is working.
Glutamine: The Nitrogen Supplier
Glucose handles the energy and carbon needs, but cancer cells also need nitrogen to build DNA and new amino acids. That’s where glutamine comes in. Glutamine is the most abundant amino acid in the bloodstream, and many cancer cells are so dependent on it that researchers describe them as “glutamine addicted.”
Glutamine plays a dual role. Its carbon backbone feeds into the same energy-producing cycle that glucose supports, while its nitrogen atoms are stripped off and funneled into the production of nucleotides, the building blocks of DNA and RNA. This matters enormously for a cell that needs to copy its entire genome every time it divides. In some cancers driven by the oncogene c-Myc, the enzymes that channel glutamine’s nitrogen into nucleotide production are cranked up dramatically. As cancer becomes more aggressive, cells increasingly shift their glutamine use away from energy production and toward building the raw genetic material needed for rapid replication. When researchers deprive certain cancer cells of glutamine in lab experiments, the cells stall partway through copying their DNA and stop dividing.
Fats: Building New Membranes
Every time a cancer cell divides, it needs to build an entirely new cell membrane. That requires enormous quantities of lipids. Most healthy adult tissues simply absorb the fats they need from the bloodstream, but cancer cells reactivate the fat-manufacturing machinery normally reserved for specialized tissues like the liver.
This homemade fat production gives cancer cells a survival edge. The fats they synthesize tend to be more saturated than dietary fats, which makes their membranes more resistant to oxidative damage. Cancer cells also stockpile fat in lipid droplets, which serve as energy reserves. Ovarian cancers take this a step further: they activate fat-releasing signals in nearby fat cells, then absorb those released fats and burn them for energy. Cholesterol synthesis also ramps up, since cholesterol is a critical component of cell membranes and helps regulate their flexibility.
Lactate: Recycling Waste Into Fuel
For years, lactate was considered a dead-end waste product of the Warburg effect. That understanding has changed dramatically. Metabolic tracing studies in both mice and human lung cancer patients have shown that lactate is actually a major fuel source, not just for cancer cells but for many normal tissues as well.
Inside a tumor, this creates an efficient recycling system. Cells in oxygen-poor regions deep within the tumor rely heavily on rapid glucose breakdown, pumping out large quantities of lactate. Cells in better-oxygenated areas of the same tumor then absorb that lactate and feed it into their mitochondria for energy. This metabolic cooperation means tumors waste very little. What one cell discards, another cell eats.
Scavenging Proteins and Debris
When glucose and glutamine run low, cancer cells have a backup strategy that healthy cells rarely use: they swallow whole proteins and even chunks of dead cells from their surroundings. This process, called macropinocytosis, works like a cellular gulp. The cancer cell extends its membrane outward, engulfs a droplet of surrounding fluid (along with whatever proteins are dissolved in it), and pulls it inside. The captured material is then broken down in the cell’s recycling compartments to yield amino acids and lipids.
Pancreatic cancer cells are especially adept at this. When starved of amino acids, they internalize albumin, the most abundant protein in blood, and digest it to fuel their growth. They can also consume collagen, the structural protein that surrounds them, breaking it down into the amino acid proline for energy. Some cancer cells even practice a form of cannibalism called necrocytosis, engulfing debris from dead neighboring cells to harvest their molecular contents. This scavenging ability helps explain why tumors can keep growing in the nutrient-starved, chaotic interior of a solid mass where blood supply is poor.
How Fructose Feeds Tumors Indirectly
A common question is whether sugar in the diet, particularly fructose from sweetened foods, directly feeds cancer. The answer is more nuanced than a simple yes or no. A 2025 study from the National Cancer Institute found that most cancer cells lack the enzyme needed to break down fructose and can’t use it at all. But that doesn’t mean fructose is harmless.
When researchers grew cancer cells alongside liver cells and added fructose, the liver cells converted the fructose into various lipids. The cancer cells then absorbed one particular group of those lipids to build new membranes, and their growth increased dramatically. This kind of metabolic cross-talk, where healthy tissues unwittingly convert dietary nutrients into forms cancer can exploit, adds a layer of complexity. The cancer cells aren’t eating the fructose directly, but they’re benefiting from it all the same. Some tumor types, however, can use fructose directly as fuel, so the picture varies by cancer type.
Starving Tumors Through Diet
Given cancer’s dependence on glucose, the idea of a low-carbohydrate ketogenic diet as a way to “starve” tumors has generated significant interest. Animal studies have consistently shown that limiting carbohydrate intake can slow tumor growth, and the logic is straightforward: reduce blood glucose, reduce the fuel supply. Clinical investigations have confirmed that ketogenic diets lower levels of insulin-like growth factor 1, a hormone that promotes cell growth, while increasing ketone bodies that healthy cells can use but some cancer cells cannot.
The translation to human patients, however, has produced contradictory results. Some small trials suggest ketogenic diets may enhance the effectiveness of chemotherapy and radiation while protecting healthy cells from treatment side effects. Others show little measurable benefit. The challenge is that cancer cells are metabolically flexible. Blocking glucose doesn’t help if the tumor switches to glutamine, lactate, scavenged proteins, or fat. Newer approaches attempt to address this by combining dietary changes with drugs that simultaneously block multiple nutrient pathways. Whether this strategy will prove effective in large clinical trials remains an open question.
Outcompeting the Immune System
Cancer’s appetite doesn’t just fuel tumor growth. It actively weakens the body’s ability to fight back. T cells, the immune cells responsible for recognizing and killing cancer, also depend on glucose to power their attack. When cancer cells strip the surrounding tissue of glucose, T cells in the tumor’s neighborhood are left energy-starved and unable to mount an effective response. This nutrient competition creates a form of immune suppression that has nothing to do with the immune-evasion tricks cancer is better known for. It’s a simpler, more brutal tactic: cancer cells simply eat the fuel the immune system needs to destroy them.