Hereditary fructose intolerance is a rare autosomal recessive disorder and, while most people are not born with it, its occurrence later in life is very common.
First of all, what it fructose malabsorption?
It is a digestive disorder in which the absorption of fructose (a type of sugar found in fruit and other foods) is hindered by faulty fructose carriers in the small intestine’s enterocytes. The typical cause is an increased concentration of fructose in the intestine.
Fructose malabsorption that develops over time is a secondary problem that arises from a functional problem in the gut. It is common to hear people report they had irritable bowel-type symptoms for a long time before eating apples, garlic and onions ever became a big problem.
It is becoming common practice for dieticians and gastroenterologists to implement a low FODMAP diet in patients with irritable bowel problems. (Here’s what FODMAPs stands for.)
For many people this can provide some symptom relief in the short term, but in the long term it is a diet that is not conducive to providing a repairing gut damage. Removing the fructose-containing foods will not solve it and, over time, it will become problematic itself. The very foods that are high in FODMAPs are the foods that act as the prebiotic fibres that fuel beneficial gut bacteria in the bowel where gut repair begins.
The primary cause of the fructose intolerance requires identification and treatment in the hope that, at some point in the future, fructose-containing foods can be tolerated once again.
The main foods that cause symptoms in patients are wheat, rye, onions, garlic, apples, pears and stone fruits. This helpful Monash university app updates regularly and uses a red light-green light system to identify foods to avoid and foods to include.
People with FODMAPs may also need to avoid:
- Sorbitol and other sugar alcohols; e.g. erythritol, xylitol
- Sugar-free products that commonly contain sugar alcohols; e.g. sugar-free chocolates, sugar-free lollies
- Raffinose (a combination of galactose and fructose) can be problematic for some, too; pulses and beans, for example, are raffinose containing foods
- Fructooligosaccharides and inulin – prebiotics that can be found in some probiotic formulas and nutritional supplements and should be avoided.
What happens to these sugars?
Carrier proteins from the GLUT family transport monosaccharide’s (broken down carbohydrates). GLUT 5 is the carrier required for the absorption of fructose. The presence of glucose stimulates the GLUT 5 transporter. While sorbitol blocks GLUT 5, it is only sparsely absorbed and is actually used therapeutically as a laxative. Abnormalities in the expression of GLUT 5 effects 30 per cent of the population.
Factors that promote uptake of fructose include diabetes, metformin (diabetic drug), high GI diet, co-ingestion of amino acids and topical and systemic use of corticosteroids. Inhibition of fructose uptake can be caused by low GI diet, stress, glucocorticoids and tumor necrosis factor alpha – meaning there is a connection between fructose uptake and inflammation occurring in the body.
Fructose is transported to the distal small intestine and the proximal large bowel where it causes an increase in osmotic load. That is, delivery of more water with it to the small intestine and large bowel. Fructose is fermented by bacteria which causes an increase in gas production and distention of the intestinal lumen; i.e. bloating.
There is also an increase in intestinal motility causing diarrhea due in part to increasing liquidity of the stool and byproducts of bacterial fermentation activating feedback pathways that regulate gut motility. Because fructose is the substrate for bacterial fermentation it also causes an alteration in the profile of bacteria in the GIT.
Signs and Symptoms of fructose malabsorption
The signs and symptoms that present usually are:
- Intestinal bloating
- Abdominal cramping
- Abdominal pain
- Diarrhea and/or constipation
- Some people will also experience headache
- Depression has been associated with fructose malabsorption and low tryptophan (serotonin precursor) levels were observed in one study
Primary causes of fructose malabsorption
• Small Intestinal Bacterial Overgrowth (SIBO) – more on this here
This refers to the bacterial colonisation of the distal small intestine, a part of the gastrointestinal tract that should remain sterile and bacteria free. When SIBO is present bacteria migrate to the small intestine to gain access to sugars to ferment them. SIBO is thought to cause damage to gastric mucosa of the small intestine, thus causing an inability to properly transport sugars. Causes of SIBO include GI surgery, malnutrition, immune deficiency, low stomach acid, H2 antagonist (anti histamine) and PPI use.
• Acute gastroenteritis
Some patients report that their symptoms began after an infection or antibiotic therapy. The pattern of colonic flora is thought to determine the severity of the symptoms in patients with fructose malabsorption; that is, the individual who experiences an infection will have high amounts of infectious, opportunistic organisms in the gut flora and an imbalance and/or inadequate amount of beneficial bacteria, thus indicating onset or increased severity of symptoms.
• Coeliac disease and Crohns disease
These diseases cause impairment of the epithelial surface of the small intestine which impedes absorption of carbohydrates. In irritable bowel disease there are often a number of things that will cause fructose malabsorption. These include SIBO, shortened bowel length and inflammatory mediators that may reduce absorption of sugars.
• Increased intestinal permeability (IP)
Intestinal permeability (also known as leaky gut syndrome) refers to perforations in the gut wall that cause inflammation in the gut wall and damage to microvilli, where digestive enzymes are found. Absorption of nutrients and the gut wall as a defence barrier are reduced when IP is present. Causes of IP include medication use, poor nutrition, stress, coeliac disease, infections and pathogens, food allergies, fructose and lactose intolerance.
• Inflammation of the gut/intestinal damage
This is seen in coeliac disease, irritable bowel disease, irritable bowel syndrome and in IP. Inflammatory mediators in the intestinal tract are thought to contribute to the reduced absorption of sugars.
• Unsupported digestive function
Low stomach acid and digestive enzyme insufficiency are the main issues that arise here. Low stomach acid leads to a less sterile environment in the small intestine and causes SIBO. Poor digestive enzyme efficiency means less of what is eaten is broken down and effectively absorbed and assimilated, leading to poor nutritional status which contributes to fructose malabsorption, too.
What we can do to treat this condition
Further necessary testing required includes hydrogen breath test for SIBO, Comprehensive Digestive Stool Analysis and Intestinal Permeability testing. With the use of nutritional and herbal supplementation, SIBO, intestinal permeability, inadequate digestion, inflammation and damage to the gut wall can be addressed and treated.
Once gut repair is complete high FODMAP foods can be trialled and re-introduced successfully in many people.