Does sugar cause cancer? The truth is far more complicated (February 2025)

By Carrie Dennett, MPH, RDN

If you want to reduce your risk of cancer — or increase your odds of surviving it—should you avoid sugar? If you go looking for information on the internet, or even in certain books, you might think the answer is a definite “Yes.” However, while there is a connection between sugar (aka glucose) and cancer, that connection is far more nuanced than many people realize.

The idea that sugar causes cancer or contributes to its progression by feeding it started almost 100 years ago when cancer researcher Otto Warburg observed that cancer cells use more glucose than normal cells, converting it into the lactic acid that supports tumor growth. However, this observation has been taken out of context for several decades, falsely leading people to think that if they cut all sugar and refined carbohydrates out of their diet, that they will be able to “starve” their cancer.

It’s true that cancer cells use glucose (sugar) from our bloodstream for fuel, but so do our other cells. It’s also true that cancer cells are rapidly growing and reproducing, requiring extra fuel, but some normal cells have high fuel demands, too. These include other very active and/or quickly dividing cells such as brain cells, heart cells and the cells that line our intestines.

If you did try to starve cancer cells by cutting out sugar from your diet so you can cut off cancer’s supply of glucose, it would be a futile effort. The body’s fail-safe system is smarter and stronger than our actions, and it works hard to maintain a blood sugar level necessary for life. It can free and release storage forms of glucose in our muscles, and it can even convert protein from food (or by breaking down muscle) into glucose. Also, when our blood sugar is low, we can’t simply direct what sugar there is to healthy cells, which means you can’t starve cancer cells without also starving healthy cells.

Another factor is that cancer cells adapt in order to survive and thrive. Although scientists once thought cancer cells depended on glucose for fuel, more recent research shows that they are flexible, and can take advantage of whatever fuel is available. If necessary, cancer cells can break down fatty acids (the building blocks of fats) and amino acids (the proteins) for energy, too.

This “metabolic flexibility” is considered one of the hallmarks of cancer, but it’s also worth noting that high consumption of glucose isn’t even found in all cancer cells or tumors. One reason may be that there are a variety of genetic mutations that contribute to cancer. And if there’s a silver lining to cancer’s appetite for glucose, it’s that many cancer therapies are exploiting it, slowing tumor growth by targeting enzymes that play a role in converting glucose into lactic acid.

Also keep in mind that many of the foods that may help reduce cancer risk are plant foods—vegetables, fruits, whole grains, pulses (such as dried beans and lentils), nuts and seeds—that are rich in carbohydrates. These foods supply antioxidants and phytochemicals (natural plant compounds) that may help control cell signals and gene expression. So avoiding these nutritious forms of carbohydrates, which, yes, contain natural sugars, would mean avoiding key protectors in our diets.
But what about added sugar? Evidence doesn’t support a direct effect of reducing added sugars—and doesn’t support avoiding them completely, for lower cancer risk. However, there are some indirect links. Looking at the bigger picture, a diet that’s very high in added sugars and other refined carbohydrates can promote inflammation, suppress immune function and, over time, contribute to insulin resistance (when insulin can’t effectively move blood sugar into cells, so the pancreas keeps pumping out more insulin), all of which can create conditions in the body that may encourage the initiation and progression of cancer.

So, for people who do consume high amounts of added sugar, it’s possible that cutting back to more moderate amounts—as part of an overall balanced diet that includes nutritious, fiber-rich plant foods—could lower cancer risk indirectly by calming inflammation and reducing insulin resistance.

However, it’s important to note that many lifestyle and environmental factors can also contribute to inflammation and insulin resistance. For example, smoking, stress, lack of physical activity, poor sleep, high intake of saturated fat, excessive alcohol intake, and exposure to chemicals and toxins in our environments. Some of those factors, along with genetics, also play a direct role in cancer risk.

The bottom line is that striving to avoid all sugar and restrict other forms of carbohydrate feeds into diet and wellness culture and can create or strengthen food fears. If today sugar is the enemy, what food might become the enemy tomorrow? Going down this path can not only lead to a limited diet and nutrient deficiencies, but it can retrigger eating disorder behaviors or be the catalyst for developing them in the first place. If you know you are at higher-than-usual risk of developing cancer, whether because of family history or other reasons, get preventive health screenings, pay attention to the modifiable factors mentioned above, and take a gentle, not extreme, approach to eating to help reduce your risk.

References:

National Cancer Institute. “Cell Metabolism and Cancer.” Center for Cancer Institute Horizons.

DeBerardinis RJ, Chandel NS. Fundamentals of cancer metabolism. Sci Adv. 2016;2:e1600200.