There is common awareness of the term “lactose intolerance,” however most are unfamiliar with other types of food intolerances. Sucrose, fructose and various food chemicals like nitrates/nitrites, salicylates and benzoates are less commonly known food intolerances. This article specifically focuses on fructose intolerance. 

What is a Food Intolerance?

To start, let’s review the difference between a food intolerance versus a food allergy or sensitivity. The primary difference is that a food intolerance does not involve the immune system, whereas a food allergy or sensitivity does involve the immune system. Intolerances relate to impairment in the digestive process, provoking primarily digestive symptoms. Conversely, allergies or sensitivities present a wider range of symptoms potentially impacting all body systems. 

Typically, food is consumed and broken down into tiny particles that the body absorbs and uses for various functions. After eating a carbohydrate-containing food, the sugars from that food break down into their smallest components, referred to as monosaccharides [one-sugar].

There are 3 types of monosaccharides including glucose, fructose, and galactose. One may assume that the smallest level a food can possibly break down would not provoke any digestive side effects, however one would be incorrect!

What is Fructose?

Fructose is a 6-carbon monosaccharide also referred to as fruit sugar. You’ll find fructose in apples, pears, watermelon, cherries, juices, agave and honey. Popular recognition of fructose comes from high-fructose corn syrup in processed foods. 

Fructose exposure comes from pure fructose, sucrose (table sugar) and other sweeteners, and fibrous polysaccharide chains such as inulin, fructans and fructo-oligosaccharides (FOS) found in various fruits, vegetables and grains. All humans lack the enzyme needed to break down these fibrous chains; therefore, bacterial fermentation occurs in the digestive tract. This explains why TOO much fiber is not your friend as it inevitably leads to excess gas and bloating. 

Throughout history, most humans consumed <20 grams of fructose per day primarily from fresh fruit. Fast forward to today where most Americans consuming 85 to 100 grams of fructose per day primarily from processed and sweetened foods and beverages. 

Symptoms of Fructose Intolerance:

Fructose intolerance symptoms include diarrhea or loose stools, abdominal pain, and gas after consuming a specific dose of fructose. These symptoms develop due to the fructose entering the large intestine where they may wreak havoc. Fructose changes the osmotic pressure in the large intestine, causing more water to be pulled in to help regain osmotic balance. This often results in loose stools and diarrhea. Additionally, the large intestine houses most of the body’s bacteria, who “eat” the fructose and produce gas as the byproduct. Excess gas can also cause discomfort, pain, and bloating. 

Reasons for Fructose Intolerance:

The term fructose intolerance indicates that a specific load of fructose is not properly digested and absorbed leading to a variety of gastrointestinal symptoms. This may be due to a number of factors. Some possess genetic variants that impact the production of enzymes that digest fructose. A deficiency in hepatic enzyme aldolase B, fructokinase, or fructose-1, 6-bisphosphatase are hereditary causes of fructose intolerance. 

For others, fructose is poorly tolerated due to the limited number of fructose transporters in the small intestine that are required for fructose absorption. These transporters include GLUT-2 and GLUT-5, with GLUT-5 being primarily responsible for most fructose transport using a concentration gradient. GLUT-5 activity increases as fructose intake increases, but can still only handle a certain load at one time. Consuming fructose in equal balance to glucose seems to assist in the fructose absorption process. 

Lastly, inflammation may negatively impact fructose absorption. More intestinal inflammation impairs the transport of fructose across the gut wall and allows it continue on to the large intestine. This may be why individuals with compromised gut function are more likely to experience fructose intolerance than healthy individuals.

Can I Test for a Fructose Intolerance?

One may test for genetic variants to diagnose a potential fructose enzyme deficiency. One may also complete a hydrogen and/or methane breath test to assess for poorly digested fructose.  Review test results in conjunction with apparent gastrointestinal symptoms after consuming fructose. 

It is estimated that ~50% of the U.S. is unable to absorb 25 grams of pure fructose. Also, up to 80% of healthy controls in clinical trials are unable to absorb 50 grams of fructose. Each individual possess their own fructose tolerance level so identify the fructose load that your body can handle. 

What Should I Do If I Have a Fructose Intolerance?

First, if you suspect a fructose intolerance or have a positive diagnosis, it is vital to modify your diet.

Start by limiting free fructose to <25 grams per day. Additionally, limit the intake of foods containing large fructose doses and especially avoid them in large quantities. Keep in mind that as fruit ripens, it’s sugar content increases and may provoke a reaction. Lastly, condensed sugars found in dried fruits and fruit juices are most problematic as they flood the body with excess fructose load in a short amount of time. 

Next, as digestive symptoms improve begin reintroducing these foods to find the dose that you may tolerate. Here’s a chart indicating net fructose grams in commonly consumed foods. Foods most likely to cause problems include apples, black currants, cherries, pears, raisins, watermelon, honey, and high fructose corn syrup. Note that tolerance improves with simultaneous consumption of fructose and glucose. 

For more information on fructose intolerance or other adverse food reactions, please contact me today to schedule an appointment. I’m here to help you improve your digestive health and feel your best! 



The Health Professional’s Guide to Gastrointestinal Nutrition 

Clinical and Translational Gastroenterology (2015) 6, e120; doi:10.1038/ctg.2015.50 & 2015 the American College of Gastroenterology


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