What Is Fructose Intolerance?

Key takeaways from the article

Overview

Fructose is a natural sugar found in honey, table sugar, most fruits and vegetables, and sorbitol (sweetener). It contributes to the body’s overall energy production, which helps fuel various physiological processes.

Just like glucose, fructose is a simple sugar; however, unlike glucose, fructose is not directly used as energy. It is converted first into glucose so it can be stored for later use.

Fructose digestion mainly occurs in the small intestines, where it is then absorbed through a specific protein channel called GLUT - 5  and transported to the liver through blood. Once in the liver, fructose undergoes conversion by 2 enzymes, fructokinase and Aldolase B, to form compounds that ultimately produce energy.¹

Fructose intolerance

Fructose intolerance is the body’s inability to properly handle and metabolise fructose. This differs from fructose malabsorption, which is a type of dietary fructose intolerance identified by the impaired absorption of fructose in the small intestines, leading to gastrointestinal symptoms.²

Types of fructose intolerance

Hereditary fructose intolerance (HFI)

This is a rare genetic disorder caused by a mutation in the aldolase B gene, which leads to a deficiency in the enzyme Aldolase B. Aldolase B is the enzyme responsible for the breakdown of fructose in the liver. This leads to a build-up of toxic substances as fructose is not broken down to produce energy.³

HFI is often diagnosed in children as they are being weaned off breast milk or formula milk. It is rarely diagnosed among adults. When an adult is diagnosed with HFI, they usually have a history of avoiding sweets, honey, or sugary drinks and fruits.

Dietary fructose intolerance (DFI)

Unlike HFI, DFI is characterised by the inability of the small intestine to absorb fructose. The cells in the small intestine are unable to fully break down and absorb fructose. When unabsorbed fructose reaches the large intestine, gut bacteria can ferment it, leading to the production of gases such as hydrogen and methane. This fermentation process can result in symptoms such as bloating, gas, abdominal pain, and diarrhoea.

DFI is more likely to develop in adults.⁴

Symptoms

The severity of symptoms can vary among individuals depending on the amount of fructose a person can tolerate.

Common symptoms of Hereditary Fructose Intolerance include:

• Jaundice - yellowing of the eyes and skin
• Stunted growth in children
• Vomiting
• Bloating
• A decrease in phosphate and glucose in the blood
• Elevated levels of fructose in the blood and urine
• Nausea
• Abdominal pain
• Dislike fruit or sweet things.

The build-up of fructose in the body can be dangerous as it can lead to serious complications such as seizures, coma, and even damage to organs such as the liver and the kidneys.⁵

Some other complications can include:

• Gout
• Liver failure
• Bleeding
• Low blood sugar (hypoglycemia)
• Death due to severe hypoglycemia or organ failure

Dietary fructose intolerance symptoms can include:⁶

• Bloating
• Gas
• Diarrhoea
• Stomach aches

Fructose intolerance can, however, have a potential overlap of symptoms with other GI disorders, which may hinder the absorption of fructose. These disorders can include:⁷

• Celiac disease
• Crohn’s disease
• Gastroenteritis
• Irritable bowel syndrome
• Inflammation
• Stress
• Use of antibiotics

Diagnosis

A thorough dietary history and assessment of a patient presenting with symptoms suggestive of fructose intolerance should be done. The symptoms of fructose intolerance are similar to other health conditions; hence, it's important to get a definitive diagnosis.

A hydrogen breath test is a simple test that does not involve drawing blood. It involves consuming a sugar substrate, followed by periodic breath samples collected and analysed to measure hydrogen gas levels produced by gut bacteria breaking down the sugar substrate. Increased hydrogen levels in breath indicate malabsorption.⁸

Genetic testing for abnormalities in the Aldolase B gene is done for those with suspected HFI.⁹ Other useful tests can include:

• Liver sample
• Liver function tests
• Blood sugar test
• Urinalysis

Treatment

Definitive treatment of fructose intolerance involves eliminating fructose, sorbitol and sucrose-containing foods from the diet, hence why one has to undergo dietary modifications to avoid high fructose foods.¹⁰ FODMAPs are a type of fermentable carbohydrate that can cause gastrointestinal symptoms such as bloating, gas, abdominal pain, and diarrhoea in people who are sensitive to them.¹¹ A low FODMAP diet has been designed to help ease the symptoms associated with fructose intolerance. The diet involves restricting foods high in FODMAPs for a specified period, typically a few weeks.¹²

Foods to avoid

Other foods:

• Foods and beverages sweetened with high fructose corn syrup
• Meats marinated and seasoned with fructose foods
• Sugars like table sugar, honey, agave, molasses, syrup

Low-fructose fruits and vegetables

Vegetables and fruits that are low in fructose and can be tolerated by those with fructose intolerance include;

Living with fructose intolerance Practical tips for managing a low fructose diet

Adopting a low-fructose diet can be beneficial for individuals with fructose intolerance. Here are some specific recommendations to consider when following a low-fructose diet:

1. Choose low-fructose fruits: Opt for fruits that are lower in fructose content, such as berries (strawberries, blueberries, raspberries), kiwi, cantaloupe, and citrus fruits (oranges, lemons, limes).
2. Limit high-fructose fruits: Avoid or consume high-fructose fruits like apples, pears, cherries, and grapes in moderation.
3. Be cautious with sweeteners: Use glucose, rice syrup, or dextrose as alternative sweeteners instead of high-fructose sweeteners like honey, agave, and high-fructose corn syrup.
4. Read food labels: Check ingredient labels for hidden sources of fructose in packaged and processed foods. Look for terms like "fructose," "high-fructose corn syrup," and "agave nectar."
5. Mindful portions: Pay attention to portion sizes and monitor your tolerance levels. Some individuals can handle small amounts of fructose without triggering symptoms.
6. Focus on whole foods: Emphasise whole, unprocessed foods like vegetables, lean proteins (chicken, fish, turkey), eggs, nuts, seeds, and whole grains (quinoa, rice, oats).
7. Limit certain vegetables: While many vegetables are generally well-tolerated, be cautious with high-fructose vegetables like onions, artichokes, and asparagus. Consider cooking them well to reduce fructose content.
8. Cooking techniques: Cooking or steaming vegetables can help break down some of the fructose, making them easier to digest.
9. Monitor dairy intake: Dairy products can contain lactose, which may be an issue for some individuals. Choose lactose-free options or food with lower lactose content.
10. Stay hydrated: Drink plenty of water throughout the day to stay hydrated and support digestion.
11. Work with a dietitian: Consider consulting a registered dietitian experienced in managing fructose intolerance to create a personalised meal plan and ensure balanced nutrition.
12. Keep a food diary: Keep track of your meals, symptoms, and reactions to different foods to identify patterns and better manage your diet.
13. Cook at home: Preparing your meals home gives you more control over ingredients and cooking methods.

Remember, individual tolerance levels may vary, so it is important to listen to your body and make adjustments accordingly. A low-fructose diet should aim to minimise symptoms and improve your overall well-being. If you have concerns or questions, consult a healthcare professional or registered dietitian for personalised guidance.

FAQs

What is the prognosis of fructose intolerance?

Patients who receive prompt diagnosis and treatment have a great prognosis.

What are the symptoms of fructose intolerance?

Common symptoms include bloating, gas, abdominal pain, diarrhoea, and discomfort after consuming foods high in fructose or certain sweeteners.

If diagnosed with fructose intolerance, can I consume any fructose at all?

Many individuals with fructose intolerance can tolerate small amounts of fructose without showing symptoms. Finding your tolerance level through trial and error is important.

How long does it take for fructose to be digested?

It normally takes about 6 hours for fructose to be broken down. However, if one is fructose intolerant, it can take several days to weeks for fructose to be eliminated.

What foods should I avoid with fructose intolerance?

Foods high in fructose, such as apples, pears, honey, and sweeteners like high-fructose corn syrup, should be avoided. It's important to read labels and watch for hidden sources of fructose.

Can fructose intolerance be cured?

Fructose intolerance is typically a lifelong condition, but managing it through dietary changes can significantly reduce symptoms and improve quality of life.

Is fructose intolerance the same as lactose intolerance?

No, they are different conditions. Fructose intolerance involves difficulty digesting fructose, while lactose intolerance is the inability to digest lactose, a sugar found in dairy products.

Can fructose intolerance lead to other health issues?

If left untreated, fructose intolerance can lead to nutrient deficiencies, disrupted gut health, and potential complications. Proper management is essential.

How do you diagnose a person with a fructose intolerance?

Diagnosis involves clinical evaluation, symptom assessment, and hydrogen breath testing after consuming fructose. Genetic testing and exclusion diets may also help diagnose fructose intolerance.

Summary

The body's inability to effectively digest fructose is known as fructose intolerance. It can be divided into two categories: dietary fructose intolerance (DFI), which is defined by reduced fructose absorption in the small intestine, and hereditary fructose intolerance (HFI), a rare genetic condition brought on by a deficiency of the enzyme aldolase B. The severity of the symptoms varies. These symptoms include jaundice, problems with growth, bloating, abdominal pain, and others. Genetic testing and a hydrogen breath test are also helpful diagnostic tools. Eliminating foods high in fructose from the diet is the mainstay treatment for fructose intolerance. In order to follow a low-fructose diet, one must select low-fructose fruits, avoid high-fructose fruits, use substitute sweeteners, study labels, prioritise whole foods, and consult a dietitian. Furthermore, customising portion sizes, cooking techniques, and monitoring dairy intake are also essential. Lastly, staying hydrated, consulting healthcare professionals, and maintaining a food diary can also contribute to the successful management of fructose intolerance.