Fructose Malabsorption (formerly Dietary Fructose Intolerance)
Which FODMAPs are involved?
Fructose is a monosaccharide (mono - single, saccharide - sugar) that has the molecular formula C6H12O6. While this is the same molecular formula as glucose, its structure is different. If you're into chemistry, this means it is an isomer of glucose. Fructose commonly occurs in fruits, and has earnt itself the name "fruit sugar." However, it not only occurs as a monosaccharide but also as di- or oligo-saccharides (sucrose or fructans respectively) in plants and their fruits/vegetables.
Fructans, or fructooligosaccharides (FOS), are chains of two to sixty fructose molecules that end with a glucose molecule. Just like all oligosaccharides, fructans are not absorbed or digestible by mammalian intestines due to the type of glycosidic bond that holds the polysaccharide together – this is why cabbage and beans have been linked with bad gas, even before the association was made with fructans.
The mechanism behind fructose malabsorption
How is fructose normally absorbed?
In a normally functioning gut, fructose is absorbed independently of simple glucose via the GLUT5 transport proteins of the enterocytes (cells) lining the lumen of the small intestine; however, glucose can also aid in the transportation of fructose across the luminal membrane of the small intestine by “co-transport” – basically it drags a molecule of fructose along with it through its own protein transport system (called SLC5A1 if you are interested). This is why fructose in the form of sucrose is more readily absorbed than simple fructose alone.
What happens in Fructose Malabsorbers?
Essentially, the GLUT5 transport proteins malfunction, impeding fructose absorption. Hence, the fructose continues along the alimentary canal and into the lumen of the large intestine. Here, it comes into contact with a host of bacteria (good and bad); this is where the problems begin, as the bacteria ferment the fructose and fructans that reach them, producing hydrogen, carbon dioxide and methane gas as well as short chain fatty acids. The higher concentration of fructose/fructans in the colonic lumen also causes an increase in the levels of water in the large intestine through a process known as osmosis. This all leads to the symptoms we’ve grown to hate.
What are the Symptoms of Fructose Malabsorption?
The “short” list:
- Altered bowel function/movements
- Stomach pain
- Abdominal pain
- Vomiting (if +++ amounts are consumed)
- Bags under your eyes
- Even symptoms like mental depression!
How is fructose malabsorption diagnosed?
There are two main methods to test for fructose malabsorption, the breath test and an elimination diet.
Hydrogen/methane breath test
Breath tests are non-invasive and simple ways to investigate whether a certain carbohydrate is being malabsorbed. The subject must follow a restricted diet before the exam (check with your testing centre).
A baseline reading of both gases is taken before the subject drinks a concentrated solution containing 50 g of fructose. After waiting for thirty minutes, the subject will then proceed to exhale into the collection bag at specific intervals, until hydrogen gas production either increases by 20 ppm, or 3 hours has passed (whichever comes first). An increase in hydrogen by 20 ppm is considered a positive diagnosis.
Either individually or as part of the entire low FODMAP elimination diet, as per your dietitian’s discretion, fructose will be removed from you diet for a set period of time before being reintroduced in increasing amounts to test for tolerance.
Elimination diets are good for those who either don’t have access to the breath test or for those who feel that they still might have an issue with fructose/fruits and vegetables, regardless of the breath test result. If you are part of the latter group, speak to your health practitioner, as other components of foods can cause similar symptoms as fructose malabsorption (see differential diagnoses bellow).
How do you manage fructose malabsorption?
All the symptoms associated with fructose malabsorption (FM) stem from the body’s inability to absorb anything more than a small fructose load. People with functioning GLUT5 transport proteins (the method by which fructose is absorbed from the small intestine into the blood stream) can absorb up to 50 g of fructose per sitting; those of us with FM absorb less than 25 g of fructose per sitting and the fructose that is absorbed is done so via the glucose co-transport method.
The fructose that reaches the colon is fermented by the resident anaerobic bacteria, which produce short chain fatty acids, carbon dioxide, hydrogen and methane gas and in turn reduces water retention – the colon is the region of your gut in which water not required for expulsion is reabsorbed into your body.
To manage your FM, you will need to monitor your dietary intake of fructose, fructans and possibly other FODMAPs – depending on your exact sensitivities. It will be difficult, there will most likely be tears and pain and bloating to begin with; mistakes will be made but you can do it and it will be worth it. I highly recommend seeing a dietitian or a nutritionist for some expert advice but in the meantime, have a look at the following to tide you over/refresh your memory.
What do you need to avoid?
Fructose and Fructans (FOS) belong to a subset of carbohydrates called FODMAPs.
By definition, fructose, fructans and polyols are fermentable sugars. If fructose and fructans come into contact with something that can ferment them, e.g. the flora in your colon, then they will be metabolised into carbon dioxide, methane, hydrogen gas and short chain fatty acids. Other examples of FODMAPS include lactose, sorbitol and xylitol etc.
Sorbitol and xylitol are examples of polyols, also known as sugar alcohols. They occur naturally in some fruits and vegetables, as well as being popular artificial sweeteners. Research has shown that polyol consumption can exacerbate fructose malabsorption by binding to the GLUT-5 transporters; they are also not well absorbed in the small intestine, especially those bigger than 6 carbon chains. Excessive polyol consumption can create a laxative effect in the colon, which you can imagine, if coupled with an excess of fructose, will make a bad situation worse.
That being said, people with FM do not need to avoid all FODMAPS, unless they have other sugar intolerances. Foods high in fructose/fructans and their derivatives, as well as some of the larger chain polyols should be avoided or tested as part of an elimination diet. One of the key things to remember is that it’s all about serving size; you might tolerate a small portion of a food, such as broccoli, which causes you to react when over-consumed.
If you have FM and are still having issues while on a strict low fructose/fructans and polyols diet, it might be worth being tested for lactose or sorbitol malabsorption, or simply cut out all FODMAPS to see if it makes a difference. Remember, it is always best to work with a professional.
Strategies to reduce your fructose load
According to Dr Sue Shepherd, and many other resources, the following are useful methods.
- Limit your intake of "free fructose" to less than an excess of 0.5g/100g of glucose.
- Sprinkle glucose on foods that may have excess free fructose to even out the fructose/glucose ratio.
- Limit your total fructose load per meal (regardless of glucose intake).
- Limit your intake of fructans, do not eat foods with more than 0.5g per serving.
Could Fructose Malabsorption be the cause of your Irritable Bowel Syndrome? A detailed overview of fructose malabsorption and how it leads to digestive distress.
How to manage your Irritable Bowel Syndrome with the Low FODMAP Diet: Linking fructose malabsorption, lactose intolerance and IBS with the low FODMAP diet.