Does Sugar Feed Cancer?

Written by Annie Cavalier, MS, RDN, LD

Many cancer patients are, understandably, very concerned about the food that they put into their bodies and how it affects their prognosis. Misinformation and a lack of clear guidelines on these topics only add to these anxieties. A question that commonly comes up when talking about cancer and nutrition is whether sugar feeds cancer cells.

In this article, we will talk about the basics of how carbohydrates are metabolized and how this relates to cancer, as well as what that means for how you should be eating during your cancer treatment.

Spoiler alert: As with most things in nutrition, it all comes down to moderation.

What are carbohydrates and how does your body respond to them?

First things first, it’s important to understand that sugar feeds all cells in your body, regardless of if it is a healthy cell or a cancerous cell.¹ How does this work?

There are three types of macronutrients that your body can use for energy – carbohydrates (which are all different types of sugar), protein, and fats. Of these three nutrients, carbohydrates are the preferred source of energy for all cells in the body because they are the easiest to break down.

Many cells, such as those in the brain, rely almost exclusively on carbohydrates for energy.² Red blood cells, which are responsible for transporting oxygen throughout your body, are actually incapable of using either protein or fat for energy and can only metabolize (convert into energy) carbohydrates.³ In fact, carbohydrates are so important for various biological functions that your body will actually break down proteins and fat and turn them into sugar if there is not enough.⁴

Carbohydrates are found naturally in grains, starchy vegetables (potatoes, corn, beans, peas, lentils), fruit, and dairy products. Carbohydrates can also be processed and refined and added to foods such as sweets, desserts, and sugar-sweetened beverages. When you eat any of these foods, your body breaks these carbohydrates down into smaller sugars (primarily in the form of glucose) so that they can be absorbed into your blood stream, causing your blood sugar to rise. This triggers your pancreas to release a hormone called insulin, which tells the cells in your body to take up some of these sugar molecules, which they can then use for energy or store for later use in the form of either glycogen or fat.⁵

It is important to note that your body does not respond to all carbohydrates in the same way. Natural sugars such as those found in whole grains, fruits, vegetables, beans, and dairy products typically do not cause your blood sugar to rise as much as the simple sugars found in sugar-sweetened beverages, honey, desserts, or candy.⁶ Foods that are high in fiber also reduce spikes in blood sugar by slowing down the rate of absorption as they are digested.⁵ This is why foods such as whole wheat bread or brown rice are preferred to white bread or white rice, which have had most of their fiber removed during processing. We will talk more about tips for controlling your blood sugar later.

How are sugar and cancer related?

The phrase “sugar feeds cancer” that is seen on some headlines and on social media has understandably caused people to become fearful of consuming carbohydrates. However, the science shows that the relationship between sugar and cancer is not as direct as you may think.

As mentioned above, excess glucose that enters a cell becomes stored as fat to burn later if you are not consuming enough calories. Because of this, chronic overconsumption of carbohydrates (and therefore calories) may lead to overweight or obesity, which has been shown to increase the risk for several types of cancer, including breast cancer, as well as increase the risk for poor cancer outcomes.^7,8

Being overweight or obese also puts individuals at increased risk for insulin resistance, a condition in which the pancreas continues to release insulin in response to elevated blood sugar levels, but the cells in the body do not respond appropriately by absorbing the glucose. This leads to chronically elevated blood sugar and insulin levels.⁹ Even if you do not have insulin resistance, your body can still make excess insulin if your blood sugar is higher than normal, such as after having a meal that contains a lot of simple sugars.

Elevated levels of insulin can also cause higher-than-normal production of a compound called insulin-like growth factor 1 (IGF-1).¹⁰ While both insulin and IGF-1 are essential to normal body functions, high levels of these hormones is associated with increased tumor growth, proliferation, and cancer cell survival.^11–14

Another reason that people may be concerned about sugar and cancer is because of the Warburg Effect. Like all cells in the body, cancer cells use glucose for energy, but they use it at a faster rate than non-cancerous cells. Cancer cells take a short-cut in their metabolism, and instead of using a more efficient method of metabolism that creates even more energy (oxidative phosphorylation), they use a process called aerobic glycolysis which creates energy at a faster rate, enabling them to divide and grow rapidly.¹⁵

What does this mean for how you should be eating?

Does this mean that you should cut out all carbohydrates from your diet and only eat protein and fat-based foods? No.

Cutting out all carbohydrates means that you are significantly reducing the amount of nutrient dense foods in your diet, such as whole grains and fruit, both of which have shown anti-cancer properties and are associated with improved cancer survival rates.^16–20

A low carbohydrate diet is also typically low in fiber, a nutrient that plays many important roles in your body, such as helping manage cholesterol levels, promoting a healthy gut, and even helping to lower the risk of certain types of cancer.²¹ In fact, several studies have shown that a high fiber diet is associated with a reduced risk of breast cancer compared to a low fiber diet.²² The American Institute of Cancer Research specifically recommends incorporating high-quality carbohydrates, such as whole grains, fruit, and beans, into your diet while limiting added sugars and refined grains.^23,24

The takeaway from most of the studies looking at whether carbohydrates feed cancer is that it is important to have well-controlled blood sugar levels to prevent chronically elevated insulin and IGF-1 levels as well as to prevent unintentional weight gain. Thankfully, there are several dietary and lifestyle modifications you can make to help with this which don’t involve cutting out carbohydrates all together:

• Pay attention to the type of carbohydrates you eat. Instead of cutting back on all carbohydrates, try to focus on reducing the amount of added sugars in your diet. Foods that are natural sources of carbohydrates, like whole grains, fruit, and beans, are typically more nutrient dense, and some of these nutrients help slow the absorption of sugars into your blood. Added sugars, on the other hand, like those found in desserts, candy, and sweetened beverages, are digested very rapidly, which leads to a sharp spike in blood sugar levels and therefore a rapid release of insulin into your blood.²⁵ Cutting back on added sugars also helps reduce the total number of excess calories you are taking in, therefore helping prevent unintentional weight gain, which is another risk factor of cancer. The general recommendation is to keep added sugars to less than 10% of your total calorie intake,²⁶ though the American Heart Association recommends keeping them to less than 6% of your total calorie intake, or 24 grams or less per day for most women.²⁷
• Incorporate physical activity as you’re able. Being physically active (if approved by your doctor) is a great way to help prevent spikes in your blood sugar following a meal. Even a short 2-5 minute walk after you eat can help lower your blood sugar.²⁸ Always talk to your doctor before incorporating physical activity into your lifestyle to see what activities are appropriate for you at each stage in your treatment.
• Think about how you combine foods. The type of foods that you eat together also have a tremendous impact on your blood sugar levels. Fat and protein slow the entire digestive process and therefore the rate of carbohydrate absorption. Instead of eating carbohydrates by themselves, try to pair your carbs with a healthy fat or protein to avoid a sharp spike in your blood sugar.^29,30 A slower rise in your blood sugar will reduce the amount of insulin needed and therefore lower the activation of IGF-1.
• Focus on fiber. Like protein and fat, fiber also helps slow the digestive process and limit spikes in your blood sugar and insulin levels.²¹ To increase the amount of fiber in your diet, try whole wheat bread or brown rice instead of eating white bread or white rice. Instead of fruit juice, try eating the intact fruit itself. Including a wide variety of whole grains, fruits, vegetables, beans, and legumes in your diet is the best way to ensure that you are getting the recommended amount of fiber each day, which is 25 grams for women.²¹

What about carbs during cancer treatment?

Sometimes, the side effects of some cancer treatments may make it difficult to tolerate many foods other than refined grains or other carbohydrate-based foods. While it is not ideal that these are the only foods you are eating, it is better that you are eating something rather than nothing at all.

If you are having a hard time taking in adequate calories or eating balanced meals, the American Cancer Society recommends using an oral nutrition supplement.²⁴ Examples of these include Ensure, Boost, Premier Protein, Glucerna, Fairlife, Orgain, and Kate Farms, just to name a few. There are even instances when doctors and registered dietitians recommend high-protein milkshakes to cancer patients who are at high risk for malnutrition, even though they contain added sugars, to help prevent further weight loss or promote weight gain for those who are underweight. Remember, your non-cancerous cells need fuel to support you, too!

What about a keto diet?

There is some discussion as to whether a ketogenic (keto) diet, which is a very low carb, high fat diet, is appropriate for cancer patients. The research is still ongoing on this topic, but there are some definite areas of concern. For example, a ketogenic diet promotes having approximately 70% of your calories come from fat.³¹ However, multiple studies show that high fat diets are associated with increased breast cancer risk and poor cancer outcomes and that decreasing total fat intake is associated with reduced risk of cancer recurrence.^32–34 Research on this topic is still ongoing, and it is likely that the types of fat included in the diet also play a significant role.  Always talk to your doctor or registered dietitian before implementing any kind of restrictive diet.

Disclaimer: The information provided in this article should serve as a general guide and should not be used for the diagnosis or treatment of a medical condition. Always talk to your doctor or registered dietitian to see if there are specific nutrient guidelines and restrictions that you need based on your specific medical conditions and treatment plans. NBCF does not endorse any products named in this article.

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