FRUCTOSE INTOLERANCE

What is Fructose Intolerance?

Fructose is a naturally occurring sugar. It is also known as “fruit sugar” because it is primarily found in fruits. Despite fruits, fructose can also be found in food obtained from plants, for instance honey, sugar cane and vegetables. Fructose intolerance occurs when your body is not able to absorb or break down fructose ingested (What is Fructose Intolerance?, 2021). Fruits have always been a vital portion of a healthy diet, but for individuals with fructose intolerance, fruits could be fatal.

Type of Fructose Intolerance

Hereditary Fructose Intolerance
Dietary Fructose Intolerance

HEREDITARY
FRUCTOSE
INTOLERANCE

Hereditary fructose intolerance is a genetic disorder where people with hereditary fructose intolerance have mutated ALDOB gene that encodes for an enzyme called aldolase B in the liver. Mutations in the ALDOB gene cause deficient aldolase B produced.

Fructose that is ingested and absorbed into the bloodstream needs to be converted into intermediates of glycolysis (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate) in order to generate usable energy. Aldolase B plays an important role in breaking down fructose-1-phosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. However, if the aldolase B is absent, fructose-1-phosphate accumulates principally in the liver, causing toxicity in the liver. Besides, increased levels of fructose inhibit both glycogenolysis and gluconeogenesis, resulting in hypoglycemia (Hegde & Sharman, 2021).

Signs and symptoms develop in affected infants when they begin to ingest foods containing fructose. Since hereditary fructose intolerance is an inborn disorder, signs and symptoms appear even in infants. They may show:

If the babies repeatedly ingest fructose, their kidneys and liver may be damaged due to death of cells caused by toxic accumulation of fructose-1-phosphate. Consequently, the babies will develop symptoms like jaundice, hepatomegaly, and cirrhosis. If the condition is left untreated, the babies will result in seizure, coma and even death (Hereditary Fructose Intolerance, 2020).

nausea,
bloating,
abdominal pain,
diarrhea,
vomiting,
hypoglycemia, and
growth delay.

SIGNS AND SYMPTOMS

HOW TO KNOW IF A BABY HAS HEREDITARY FRUCTOSE INTOLERANCE

According to Fructose Intolerance, Hereditary (2007), if a baby shows above-mentioned symptoms, doctors will carry out several diagnostic tests to confirm hereditary fructose intolerance.

Enzyme assay to determine the activity of aldolase B in the liver. The enzyme, aldolase B, is obtained from the patient’s liver via liver biopsy. Baby with hereditary fructose intolerance should have lowered activity of aldolase B.
Fructose tolerance test. Fructose is given intravenously to the patient under controlled condition, the body’s response to the fructose is assessed. If the fructose level in the blood does not decrease over time, this means that the patient may have fructose intolerance.

However, the two tests mentioned above may be too risky for newborn, thus, a newer, safer diagnostic test has been developed:

DNA test. Although a negative result does not guarantee absence of hereditary fructose intolerance, presumptive diagnosis can be made based on positive results with clinical symptoms.

It is estimated that 1 in 20,000 to 30,000 individuals develop hereditary fructose intolerance (Hereditary Fructose Intolerance, 2020). It is relatively rare as it is inherited in autosomal recessive manner where an individual must receive two mutated ALDOB genes from both parents in order to develop hereditary fructose intolerance.

IS HEREDITARY FRUCTOSE INTOLERANCE COMMON?

DIETARY FRUCTOSE INTOLERANCE

According to Olsen (2018), dietary fructose intolerance is also called fructose malabsorption. In individuals with fructose malabsorption, absorption of fructose by intestine mucosal cells is impaired due to deficiency in fructose carriers on the mucosal surface. It shares some similar symptoms with hereditary fructose intolerance:

nausea,
diarrhea,
abdominal pain, and
bloating.

Fructose malabsorption can develop any time in lifetime, but what triggers it remains unknown. Impaired absorption causes accumulation of fructose in the intestine. The decreased osmotic pressure in the intestine brings water by osmosis into the lumen of intestine, causing symptoms like diarrhea (Villines, 2020).

CAUSES OF DIETARY FRUCTOSE INTOLERANCE

HOW TO DIAGNOSE FRUCTOSE INTOLERANCE?

If an individual shows symptoms after ingesting food containing fructose, fructose malabsorption is suspected. To diagnose, following tests are done:

Hydrogen breath test. High levels of hydrogen in the breath may indicate that the individual is having difficulty digesting and absorbing fructose. Hydrogen gas in the breath is produced by intestinal bacteria when they ferment accumulated fructose for nourishment.
Elimination test. Suspected individuals are required to avoid all foods containing fructose and the body’s response is monitored (Olsen, 2018).

Although the statistics in Malaysia are unclear, it is very common in Western countries, approximately 40% of the population develop dietary fructose intolerance (Villines, 2020).

IS DIETARY FRUCTOSE INTOLERANCE COMMON?

Treatment for Hereditary and Dietary Fructose Intolerance

Unfortunately, there is no cure for both hereditary and dietary fructose intolerance. Both hereditary and dietary fructose intolerance patients are to follow a restrictive diet in which fructose is either absent or reduced. Hereditary fructose intolerance patients should avoid all foods containing fructose, but they need to find alternatives to compensate for other nutrition that they may have missed in foods. On the other hand, individuals with hereditary fructose intolerance should reduce their fructose intake until the symptoms improve and they can slowly increase fructose intake to assess their tolerance towards fructose. Since the symptoms are not fatal, it is possible for patients with dietary fructose intolerance to do so (Villines, 2020).

According to What is Fructose Intolerance? (2021), foods that patients with fructose intolerance should avoid:

Fruits (e.g., watermelon, apples, pears, mango, grapes, etc.)
Honey
Coconut sugar
Molasses
Agave syrup
Sodas

There is no clear information regarding the gold standard diagnostic test for hereditary fructose intolerance or dietary fructose intolerance. Enzyme assay to determine the activity of aldolase B and fructose tolerance test remains the most common diagnostic tests for hereditary fructose intolerance while hydrogen breath test is most used for diagnosis of dietary fructose intolerance. However, it is very important to have a gold standard diagnostic test for a medical condition. Gold standard diagnostic test is the best available test to determine the presence of a medical condition, it is vital as sometimes the symptoms of a medical condition could be confusing with other related or unrelated medical conditions.

References:

Fructose Intolerance, Hereditary. (2007). National Organization for Rare Disorders. Retrieved from https://rarediseases.org/rare-diseases/fructose-intolerance-hereditary/#:~:text=A%20diagnosis%20of%20HFI%20can,fructose%20feeding%20is%20carefully%20monitored.

Hegde, V. S., Sharman, T. (2021, June 29). Hereditary Fructose Intolerance. Treasure Island, Fluorida: StatPearls Publishing. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK559102/

Hereditary Fructose Intolerance. (2020, August 18). MedlinePlus. Retrieved from https://medlineplus.gov/genetics/condition/hereditary-fructose-intolerance/#causes

Olsen, N. (2018, October 5). What is fructose malabsorption? Healthline. Retrieved from https://www.healthline.com/health/fructose-malabsorption

Villines, W. (2020, February 24). What to know about fructose intolerance. Medical News Today. Retrieved from https://www.medicalnewstoday.com/articles/fructose-intolerance#diagnosis

What is Fructose Intolerance? (2021, June 1). WebMD. Retrieved from https://www.webmd.com/digestive-disorders/what-is-fructose-intolerance#:~:text=Certain%20people%20can't%20absorb,intolerances%20are%20dietary%20and%20hereditary.