Fructose intolerance – the NET Effect

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 When I cast my mind back to my very first surgery, I remember all sorts of ‘plumbing’ issues but was told it would take a while for my adjusted plumbing to repair and do its work once more. It’s not gone fully back to normal, but I accept minor side effects of getting rid of cancer (metastatic with small intestine NET primary). 

My diet is slightly different nowadays as I endeavour to return to that normality, and I have a good handle on what makes me move faster AND slower. Constipation is not good either, so I try to find a middle ground.  

I started taking pancreatic enzyme replacement therapy in 2018 and often use that to offset a meal containing more fat than other meals, similar reducing dosages for lower fat meals.  

I do like the odd sweet (candy), but not a big fruit eater if I was to be honest but I do eat a fair amount of well-cooked broccoli (well-cooked to avoid any high residue issues).  After a comment in my private group about fructose intolerance, I set about researching that subject to be able to document it in this blog. I found some potential linkages to NET treatment.  I think this is just another piece of the Diarrhea jigsaw coming the consequences of surgery and/or side effects of treatment. 

What is Fructose?

Fructose is a natural sugar that occurs naturally in fruits, fruit juices, certain vegetables, and honey. In these forms, fructose sugars can be part of a healthy diet. However, fructose is also a component of high-fructose corn syrup (HFCS), which manufacturers make from corn starch and add to not so healthy foods such as sodas and candies/sweets.  Like everything else though, is not necessarily a health concern when a person consumes it in moderation.

Fructose is digested in the liver to produce mainly glucose (~50%), and minor amounts of glycogen (>17%), lactate (~25%) and a small amount fatty acids.

What is Fructose Intolerance and what causes it?

People with fructose intolerance cannot digest these sugars as normal and may experience bloating, abdominal pain, and diarrhea.   Sound familiar?  I see those symptoms on a daily basis in my private group.  But they are symptoms of many conditions.  Often, it’s difficult to sort out the symptoms and find the root cause.  For some people, this may be one of the causes and trial and error is necessary to rule out. 

Causes and types of fructose intolerance

The term “fructose intolerance” can be misleading as there are two types:

  1. Dietary fructose intolerance.
  2. Hereditary fructose intolerance.

Dietary fructose intolerance

If you have dietary fructose intolerance, cells in your intestine don’t absorb fructose as they should. Looking at various studies on the internet, it appears fairly common.  In people with dietary fructose intolerance, eating foods high in fructose can lead to uncomfortable symptoms, including:

  • Gas.
  • Bloating.
  • Diarrhea.
  • Stomach pain.

Fructose intolerance is sometimes called fructose malabsorption. 

Hereditary fructose intolerance

This is a genetic condition in which people are born without an enzyme that breaks down fructose. Without this enzyme, they can’t digest fructose at all. Symptoms show up soon after babies start eating their first solid foods. These babies typically show signs including:

  • Nausea.
  • Bloating.
  • Diarrhea.
  • Vomiting.
  • Abdominal pain.
  • Low blood sugar.
  • Strong dislike of sweets.
  • Problems with growth and development.

Genetic testing can confirm a diagnosis of hereditary fructose intolerance. I very much suspect that most people with a heredity version will have known for some years and already be taking action to reduce or totally eliminate fructose consumption.  

Hereditary fructose intolerance will not be covered further as there is no known linkages to NETs. 

Causes and risk factors

Fructose malabsorption may result from inflammation or damage to the small intestine that prevents it from fully absorbing fructose, meaning fructose remains in the gastrointestinal tract.

However, there are a number of risk factors that might be contributing to the overall condition or severity of the condition.  Most are related to the intestines but also include stress factors. 

  • Celiac disease: Although this autoimmune disease mainly involves the malabsorption of gluten, a small study found that 7 out of 30 people with celiac disease also showed fructose malabsorption.
  • Inflammatory bowel disease (IBD): Crohn’s disease and ulcerative colitis may cause inflammation of the small intestine, leading to the malabsorption of fructose and other important nutrients.
  • Irritable bowel syndrome (IBS): In a study of 31 children and adolescents with this chronic gastrointestinal disorder or abdominal pain, researchers found fructose malabsorption in 9 of the 10 participants with IBS. Those with fructose malabsorption also had a higher body mass index.
  • Acute gastroenteritis: Commonly known as stomach flu, this intestinal infection may cause inflammation as well as malabsorption of fructose and other nutrients.
  • High fructose diet: Consuming large amounts of products made with high fructose corn syrup overwhelms the small intestine’s ability to absorb fructose and significantly reduces the production of beneficial intestinal bacteria, according to one small study(Trusted Source).
  • Stress: Mental stress can affect the gastrointestinal tract. Nearly 40% of people with IBS have significantly increased anxiety and depression.
  • Antibiotics: A study (Trusted Source) found that some antibiotics alter microorganisms in the gut, increasing the risk of IBD.

The NET Effect

Unlike my similar article covering Lactose Intolerance, the evidence of linkages to dietary Fructose Intolerance is not as easy to find.  I also discovered that the exact causes are not known and there are many overlaps with Irritable Bowel Syndrome (IBS) and Small Intestine Bacterial Overload (SIBO). It’s a conundrum. 

Surgery to the small intestine.  When you consider the risk factors above, e.g. “Fructose malabsorption may result from inflammation or damage to the small intestine that prevents it from fully absorbing fructose, meaning fructose remains in the gastrointestinal tract“, it follows that surgery on the small intestine may be involved in reducing tolerance causing the symptoms listed above.  This is the same effect as per Lactose Intolerance. 

Sorting out the Symptoms

Although fructose elimination diets can give clues, another clue can be made via a Hydrogen breath test.  It is normal to test for both lactose and fructose individually as part of the same test scheduled event(s). 

Hydrogen breath test. After you drink a liquid that contains high levels of lactose or fructose, your doctor measures the amount of hydrogen in your breath at regular intervals. Breathing out too much hydrogen indicates that you aren’t fully digesting and absorbing lactose or fructose.

Credit: Australian Family Physician Volume 41, Issue 5, May 2012 Fructose and Lactose Testing

Fructose or lactose breath hydrogen tests do not diagnose an illness or abnormality; malabsorption of lactose and/or fructose is a normal phenomenon. Whether symptoms are severe can depend on many factors (including whether there is an underlying gastrointestinal disorder or disease), so the breath test on its own may not find the real causal problem of the symptoms.

What can you do about it?

This is something to be discussed with your doctors and if you have one, your registered dietitian

For people with dietary fructose intolerance, there is no one-size-fits-all solution.  Fructose intolerance symptoms can sometimes overlap with other conditions, such as irritable bowel syndrome (IBS) and small intestine bacterial overgrowth (SIBO), there’s no single test we can do to be 100% sure. 

Most sites I researched seemed to indicate a reduction is all that may be necessary to make adequate improvements.  Limiting your intake of fructose can be difficult since it eliminates so many fruits and vegetables. As above, to make sure that you’re getting adequate nutrition, you can work with a dietician or specialist. 

Supplementary reading - click the picture to read more
Click to read more about a similar problem – Lactose intolerance


I am not a doctor or any form of medical professional, practitioner or counsellor. None of the information on my website, or linked to my website(s), or conveyed by me on any social media or presentation, should be interpreted as medical advice given or advised by me.  Neither should any post or comment made by a follower or member of my private group be assumed to be medical advice, even if that person is a healthcare professional as they are not members of the private group or followers of my sites in any official capacity.  Please also note that mention of a clinical service, trial/study or therapy does not constitute an endorsement of that service, trial/study or therapy by Ronny Allan, the information is provided for education and awareness purposes and/or related to Ronny Allan’s own patient experience. This element of the disclaimer includes any complementary medicine, non-prescription over the counter drugs and supplements such as vitamins and minerals.

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