The Bitter Truth About Sugar & Cancer

You know that sugar causes tooth decay. You know it is high in calories, contributes to weight gain, and raises blood sugar. You may even be aware that it affects your mood.

But did you know that sugar also promotes cancer growth?

Glucose Fuels Cancer Cells

Sugar and other dietary carbohydrates are broken down in your digestive tract into glucose, a simple carbohydrate that is your body’s primary source of energy. Glucose is converted in the mitochondria, the “powerhouses” of your cells, into ATP, and ATP drives everything from muscle movement and nerve impulse transmission to basic metabolic reactions. 

Glucose is also the preferred fuel of cancer cells, but they use it in a different way to generate energy. Normal cells require oxygen to produce ATP in the mitochondria. Cancer cells, however, metabolize glucose in a fermentation process known as the Warburg effect, which enables rapid cell proliferation.  

Cancer cells are glucose hogs, consuming about 200 times more glucose than normal cells. You can actually see this increased consumption on PET scans, a common imaging tool for detecting and staging cancer that visualizes glucose utilization in the body. Tumors light up like a Christmas tree! 

Is Cancer a Metabolic Disease? 

The Warburg effect and its role in cancer was discovered nearly 100 years ago, but was largely ignored by cancer researchers until the last couple of decades. It is now clear that glucose metabolism plays a key role in driving cancer growth. 

Boston College Professor Thomas Seyfried, PhD, and other experts believe that cancer is actually a mitochondrial metabolic disease. Dr. Seyfried explains that cancer cells revert to fermentation to produce energy because of defects in their mitochondria—and that it is this abnormal metabolism, fueled by glucose, that spurs unbridled cell growth.

I am not suggesting that glucose is the only factor in the proliferation of cancer, but it is most certainly an important factor, and it is one you can control. How? By reducing your intake of sugars and other carbohydrates. 

Calorie Restriction and Fasting Starve Cancer Cells

Scientists have long known that fasting and calorie restriction (reducing calorie intake by about 40%) are highly therapeutic. Animal studies reveal that both approaches not only enhance all aspects of health but also increase longevity. 

When you stop eating or dramatically reduce your food intake, blood glucose decreases, insulin goes down, and your body starts mobilizing fat. In the absence of glucose, fat is broken down in the liver to form ketones, which your cells seamlessly switch over to using for energy. This adaptation, which evolved to ensure survival in times of food scarcity, optimizes energy efficiency and actually improves mitochondrial function. 

It also puts the brakes on cancer growth. Whereas healthy cells thrive on ketones, cancer cells, with their dysfunctional mitochondria, do not. Deprived of fuel (glucose), cancer growth is dramatically curtailed. 

Ketogenic Diet as a Cancer Therapy 

The problem with fasting and calorie restriction is they’re hard to stick with. The good news is that you can achieve similar benefits with a ketogenic diet. (Check out the ketogenic calculator here). 

Ketogenic diets have become quite popular in recent years for weight loss, diabetes, and more. There are many versions, but all are very low in carbohydrate and high in fat with moderate amounts of protein. The ketogenic diet is easier to stick with for several reasons. You don’t have the blood sugar swings and food cravings that can occur with a high-carb diet. Fat is quite satiating, so it tides you over and you tend to eat less. Plus, you still have a lot of food choices, so it eliminates the mental challenges of fasting and strict calorie restriction.

I recommend a ketogenic diet for most of my patients who have cancer. They admit that it takes some getting used to, but they usually report better quality of life—and their cancer markers routinely improve.

Can Cutting Out Sugar Prevent Cancer?

Although there is no definitive research proving the benefits of a low-sugar, low-carb diet for cancer prevention, it just makes good sense. 

A high-carb diet is linked with insulin resistance, which occurs when the cells lose their sensitivity to insulin’s signals to clear glucose out of the blood and into the cells. As a result, both blood glucose and insulin remain elevated, and a high level of circulating insulin is associated with an increase in cancer risk and progression. 

Insulin resistance is also the underlying problem in metabolic syndrome, a cluster of conditions that include abdominal obesity, lipid abnormalities, and increased risk of diabetes. Obesity and diabetes are well-known risk factors for cancer. They are marked by inflammation, impaired insulin signaling, and increased glucose uptake and metabolism—abnormalities that create a more hospitable environment for cancer. 

Say No to Sugar

Bottom line, what you eat matters. Our bodies are not adapted for the enormous amounts of carbohydrates that make up the bulk of the average American diet, and it is taking a toll on our health. You don’t have to eat a ketogenic diet to reduce your risk of cancer, but you do need to eliminate or at least cut back on sugar and fast-burning carbohydrates that are quickly converted into glucose. 

I know this is easier said than done. Sugar is comfort food. It’s birthday cakes, cookies and milk after school, and special treats for good behavior. It is also addictive. The craving for sweets and the satisfaction you get from eating them works on the same reward system in your brain as drugs and alcohol. No wonder it’s hard to eat just one chocolate!

If you’re not ready to go cold turkey, start by reducing your sugar intake by 50%. Simply eliminating sodas and other sweetened drinks is a giant step in the right direction. Occasional indulgences are fine, but remind yourself that when you eat sugar, you are feeding potential cancer cells. 

A Final Word

At the Cancer Center for Healing, we track the hemoglobin A1c (HbA1c) levels of all our patients. This simple blood test measures the average blood sugar level over the previous three months. If your level is elevated, you need to get serious about diet changes and other measures for lowering your blood sugar. Even if you don’t have cancer, keeping your blood sugar in the normal range is essential for optimal health.   


Seyfried TN, et al. Can the Mitochondrial Metabolic Theory Explain Better the Origin and Management of Cancer than Can the Somatic Mutation Theory? Metabolites. 2021;11(9):572. doi:10.3390/metabo11090572 

Weber, D. D., et al. Ketogenic diet in the treatment of cancer – Where do we stand? Molecular metabolism. 2021;33, 102–121. 

Gallagher, E. J., et al. Hyperinsulinaemia in cancer. Nature reviews. Cancer. 2020;20(11), 629–644.

Scully T, et al. Obesity, Type 2 Diabetes, and Cancer Risk. Front Oncol. 2021;10:615375. doi:10.3389/fonc.2020.615375

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Dr. Leigh Erin Connealy, MD

Leigh Erin Connealy, M.D., is the Medical Director of the Cancer Center For Healing and the Center For New Medicine in Irvine, CA. Dr. Connealy’s multidisciplinary treatment protocols, a team of healthcare professionals, and taking the best of all sciences approach to health and healing have made the Centers the largest most technologically advanced integrative/functional medicine clinics in North America, visited by more than 67,000 patients from all over the world. Author of The Cancer Revolution and Be Perfectly Healthy and a sought-after speaker on numerous TV and radio shows, webinars, and podcasts, Dr. Connealy has been named one of the Top Functional & Integrative Doctors in the United States.