Carbohydrates and Cancer: Does Fiber Fight Cancer?

Almost 95% of Americans do not eat enough dietary fiber.  And it seems that we aren’t all that interested in learning about fiber, either.  I have written about dietary fiber before.  Unfortunately, that article didn’t get much attention.  I get it, fiber is boring.  We don’t like to read about it and we certainly don’t like to eat it.  But, as nutritional scientists keep uncovering more about this valuable macronutrient, fiber may just be the healthiest and most important of all carbohydrates. This article continues to explore the connection between carbohydrates and cancer and answers the question “Does Fiber Fight Cancer?”

With the help of doctors, dietitians, and recent scientific studies, I took on the huge task of exploring questions about carbohydrates and cancer.  As a breast cancer survivor, I initially sought to improve my own health.  But as I learned more, I wanted to help others understand what carbohydrates were and how they influence your cancer risk.

I have written a 3-part series as your guide to Carbohydrates and Cancer.  I encourage you to read all three articles to learn the current thoughts from the medical community on this important and fascinating topic.

Part 1: Carbohydrates and Cancer: Does Sugar Feed Cancer?

Part 2: Carbohydrates and Cancer: Does Fiber Fight Cancer?

Part 3: Carbohydrates and Cancer: Are Starchy Foods Linked to Cancer?

What is a Carbohydrate?

As we saw in Part 1, carbohydrate molecules are made up of various configurations of carbon, hydrogen, and oxygen molecules. It is important to remember that not all carbs are sugars.  We learned that there are both simple and complex carbohydrates.  Part 1 talked about sugars, the simple carbs.  In Part 2 and Part 3, I discuss complex carbohydrates. 


Sugars are the simplest form of carbohydrate.  This is usually what people refer to when describing “Low Carb” diets.  There are monosaccharides, such as glucose, that are made up of one sugar molecule. And there are disaccharides, such as sucrose (white table sugar), that have two sugar molecules bound together. 

These carbohydrates trigger insulin release.  Insulin helps you process, use, and store these nutrients.  But, as we learned in Part 1, eating too much sugar can lead to illnesses, such as diabetes, obesity and cancer.


Starches are strings of many sugar molecules linked together.  These complex carbohydrates are the energy storage molecules for both plants and animals.  Most starches can be broken down into glucose units.  Thus, these molecules raise blood sugar.  This provides energy to the body, especially muscle cells during workouts.  However, some resistant starches cannot be broken down.  These undigested molecules act as prebiotics to keep your gut microbiome healthy.  I talk more about starches in Part 3 of this series. 


Fiber is another complex carbohydrate.  These are even longer chains of sugar molecules with very tight bonds that are not digested by the body.  Thus, fiber cannot be broken down into base glucose molecules like some of the other carbohydrates. 

Carbohydrates and Cancer Part 2, which you are currently reading, talks about dietary fiber and its influence on cancer. 

What is Dietary Fiber?

Also known as “roughage” or “bulk”, dietary fiber are all the indigestible parts of plant foods. They are important carbohydrates that are often overlooked in our diets.  The average American eats only slightly more than half of the recommended dietary fiber.  It is obviously not our favorite food nutrient.  But it is arguably one of the most important dietary macronutrients when we talk about cancer. 

There are two basic types of fiber. 

Soluble fiber can be dissolved in water.  This type ferments in the intestines to help feed our good gut bacteria.  Thus, soluble fiber acts as a prebiotic.  The good bacteria in your gut make some important short-chain fats using soluble fiber.  This type of fiber will also absorb water, forming a gel, which helps make stools softer and easier to defecate. 

Some important soluble fiber includes psyllium, beta-glucan and inulin.

Insoluble fiber does not dissolve and provides the bulk of our stools.  This type helps food move through our digestive system to prevent constipation.  It also influences how other nutrients are digested and absorbed. 

Important insoluble fiber includes cellulose and lignin.

Carbohydrates and Cancer Part 2 of 3 Fiber

What are the Health Benefits of Fiber?

Fiber has an important role in many digestive and metabolic processes.  Here are 15 of the health benefits of fiber.

  1. Binds to and lowers blood cholesterol
  2. Reduces inflammation
  3. Lowers blood sugar levels
  4. Balances acid in your digestive system
  5. Prevents constipation
  6. Let’s you feel full and satisfied at mealtime
  7. Supports a healthy gut microbiome
  8. Aids in weight loss
  9. Lowers risk for metabolic disease
  10. Helps balance body hormones
  11. Detoxifies the digestive system
  12. Aids in absorption of other nutrients from food
  13. Helps with water absorption and balance
  14. Helps prevent obesity
  15. Reduces risk for some cancers
Carbohydrates and Cancer Part 2 of 3 Fiber

How Much Fiber Should I Be Eating?

According to the World Cancer Research Fund, the average adult should be eating at least 30 grams of fiber per day. This means eating at least 15 ounces (400 grams) of vegetables. 

So, all you need to do to get all these amazing health benefits is eat 2 to 2 1/2 cups of vegetables every day.  Seems easy enough, right?  So why aren’t we eating enough? 

Most Americans are not getting enough fiber because we eat too much processed food.  We are taking the fiber right out of our diets.  Juicing removes fiber.  Flours have reduced fiber since we remove the bran and grind it all down into digestible starches.  We eat a lot of simple sugars, refined grains, saturated fats, processed proteins, and very little fiber. 

A typical Western Diet is devoid of whole grains, legumes, vegetables and fruits, which is where dietary fiber comes from.

So, is a lack of dietary fiber leading to increased cancer rates? Does fiber fight cancer? Well, this complex carbohydrate may help in the battle against cancer.

How Does Dietary Fiber Decrease Risk For Cancer?

It is well known that dietary fiber decreases risk for colorectal cancers. But fiber may also decrease risk for other types of cancer too, such as breast, ovarian, and  prostate cancers. So, how does fiber do this? 

Fiber lowers cancer risk in several ways.

1. Fiber Helps Control Body Fatness

Higher body fat is linked to higher risk of cancer.  Fiber helps you feel fuller during a meal, so you eat less calories.  Eating less calories can help you lose weight.

2. Fiber Controls Sugar and Insulin Levels  

As you read in Carbohydrates and Cancer Part One, high insulin levels encourage growth of cancer cells and tumors.  Eating high fiber foods can help control spikes of insulin. 

This is one of the reason that sugars in whole fruits are not bad for you. The fiber in the fruit balances out the sugar. Whole fruit is okay to eat. Juicing fruit removes the fiber. A recent study found that drinking fruit juice may increase the risk for cancer.

“In this large prospective study, the consumption of sugary drinks was positively associated with the risk of overall cancer and breast cancer. 100% fruit juices were also positively associated with the risk of overall cancer. They suggest that sugary drinks, which are widely consumed in Western countries, might represent a modifiable risk factor for cancer prevention.” – Published in the British Medical Journal 2019

3. Fiber Detoxifies the Digestive System.

Fiber improves bowel movements.  You will defecate more and more often. And, in each bowel movement, you will be eliminating more harmful toxins from your body.  Thus, dietary fiber is a natural way for your body to get rid of some carcinogenic compounds. 

4. Fiber Increases Short-Chain Fatty Acids

Fiber is used by the good bacteria, called probiotics, in your intestines to make short chain fatty acids, such as butyrate. These fatty acids have anti-cancer actions and make tumors less likely to develop. 

What is the Best High Fiber Food to Decrease Cancer Risk?

Not all food fiber is equal in its ability to decrease cancer risk. The best type of fiber comes from pulses, such as beans and lentils.  Fiber from cereal grains, such as corn and wheat, seem to have less of an ability to ward off some cancers.    

How Do I Increase My Fiber Intake?

Here are 10 ways to get more fiber into your diet.

  1. Eat whole fruits and vegetables, not juices. 
  2. Add chia seeds or whole flaxseeds to baking. 
  3. Avoid refined grains, such as white bread, white pasta, and white rice.
  4. Include at least one vegetable at every meal.
  5. Snack on air-popped popcorn.
  6. Make your own cereal or granola from unprocessed whole grains, oats, nuts and seeds.
  7. Bake with flours that have higher fiber levels, such as whole wheat. Avoid products made with white flour.
  8. Substitute meats with legumes in at least one meal per week.
  9. Eat the skin of some fruits, such as apples, pears, grapes, plums, and cucumbers.
  10. Look for delicious recipes that include vegetables, fruits, legumes, nuts and seeds.

What Are High Fiber Foods?

Most plant based whole foods are high in fiber.  When many of these foods are processed, the fiber is removed.  This is especially true of the insoluble fiber, since this can make foods less appealing and harder to eat.

The best sources of fiber are vegetables, legumes, whole grains, nuts and seeds.  Try some of these 30 healthy high fiber foods.  Eat them whole to get maximum benefits. Avoid processed foods.

Carbohydrates and Cancer Part 2 of 3 Fiber

Fiber supplements are not necessary for most people and may cause side effects from too much fiber. 

Be sure to ask your doctor about your specific individual needs. 

What Happens If I Eat Too Much Fiber?

As with most things, too much of a good thing is not good.  So, even when it comes to wholesome foods rich in fiber content, don’t go overboard.  It is possible to eat too much fiber. 

If you are just starting to add in more fiber, then add it in slowly, gradually increasing over time

Remember that soluble fiber is processed by your good gut bacteria.  It will take time for your microbiota to flourish so that it can meet the demands of a new higher fiber diet. 

Signs you may need to back off and add fiber in more slowly are…

  • Abdominal cramps
  • Bloating
  • Diarrhea
  • Constipation
  • Flatulence
  • Deficiency of other nutrients, including calcium and iron
  • Dehydration
  • Nausea

If you are increasing the amount of fiber in your diet, then also drink more water to help lessen some of these side effects.

Can Too Much Fiber Cause Colon Cancer?

Just like most things, too much of a good thing isn’t necessarily good.  And this is likely even true of dietary fiber.  But can very high levels of fiber cause cancer?  We don’t know enough about fiber to answer this question yet.  And since almost 95% of people don’t eat enough fiber, we may never find out. 

Theoretically, if eating too much fiber prevents you from having frequent bowel movements, you won’t be eliminating harmful toxins as often.  And eating too much fiber could cause dysbiosis, which may lead to poor health.  But more research in this area is needed before any conclusions are made.

Should I Take a Fiber Supplement?

Eating too much fiber is more likely to happen if you are using a fiber supplement.  Fiber can also bind minerals, such as calcium, magnesium and zinc.  This is not a problem if you are eating whole foods, as these foods contain a lot of these minerals.  Nature has a way of providing balance.  But if you are getting too much fiber from a purified supplement, mineral deficiencies could arise over time.  

Only use supplements with the guidance of a medical health professional

Should Cancer Patients Eat Fiber?

So, yes, people with cancer should eat adequate amounts of fiber, as they are able.  This should be in the form of whole foods, as much as possible.  Only use a fiber supplement on the advice of your doctor or registered dietitian.

Cancer, chemotherapy, and other treatments can cause digestive issues.  So, it is important to talk with your oncologist or cancer care dietitian to discuss your specific nutritional needs.   

Related Articles:

Prebiotics, Probiotics, Postbiotics, A Healthy Gut and Cancer

Dietary Fiber: Are you Eating Enough?

Do You Drink Enough Water? 

12 Foods to Avoid During Chemotherapy


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  1. great info and I love fiber, except I really love my japanese white rice tho Im trying to alternate it as well with mix rice and using whole wheat flour into baking. Its good to be mindful and learn about these things.

    1. Yes, you are correct. Did you get a chance to read Part 1 of Carbs and Cancer? I talk about the dangers of sugars in that article.

  2. Fiber should be at the center of most healthy diets. Foods that are good sources of fiber are also good sources of vitamins, minerals, and antioxidants. Some people may have conditions that make tolerating fiber, or other nutrients that accompany it, difficult. You mentioned many benefits of fiber including roles in weight management and immunity and ways to get more fiber in the diet, gradually and with more water. I also like how you pointed out that one can get too much fiber, too. This is a great, informative post – thank you for your research and for sharing!

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