Diet Tips:The facts about glucose and fructose

By Dr Jaci Barrett (APD) and Caroline Tuck (APD)

Early research demonstrated that if you add glucose sugar to fructose sugar, fructose is better absorbed. This is because there are two ways that fructose is absorbed, firstly when it is absorbed slowly on its own, and secondly when it is absorbed in combination with glucose. 

Many people do not have a great ability to use the first pathway, where fructose is absorbed on its own, and this can change over time. This causes fructose to be poorly absorbed, and in IBS, cause symptoms of bloating, abdominal pain and possibly diarrhoea. The second pathway, where fructose is absorbed together with glucose, is much more efficient and works well in the vast majority of people.

We therefore test foods for their level of fructose and glucose, and only foods that contain levels of fructose higher than glucose, are considered high fructose, and avoided as part of the low FODMAP diet.

Studies, therefore, looked at whether you could add glucose to fructose so that the sugar was absorbed better - however, few studies investigated if symptoms could be improved. 

Recently, we have investigated if adding glucose to pure sugar solutions and to whole foods as we eat them in a typical diet can improve absorption and symptoms, but the outcome is not promising. Symptoms were not improved when glucose was added to fructose, and glucose makes no difference to fructans, such as in onions and wheat.

So, don’t bother adding glucose. It is unlikely to improve your symptoms and is only increasing your overall intake of sugar which is not a great idea. Just choose lower fructose foods as alternatives.
 

Diet Tips: RESEARCH UPDATE: How important is diagnosis of Fructose malabsorption in the FODMAP approach?

By Lyndal McNamara and Dr Jaci Barrett

Background-

New research regarding breath testing and its use to diagnose fructose malabsorption in IBS has just been published. Researchers examined existing data from several prospective clinical trials involving two groups of IBS patients to see if fructose breath test results were the same when repeated over time.1  The studies also investigated reporting of gastrointestinal symptoms during fructose breath testing and the relationship between test outcome (‘positive’ or ‘negative’ for fructose malabsorption) and the experience of symptoms.1

Study 1 - 41 IBS patients - two fructose breath tests performed, with the second test completed ≥2 weeks after the first

Study 2 - 36 IBS patients - completed a single fructose breath test and kept gastrointestinal symptom diaries the day before and the day of the test

Patients in both studies followed a diet low in fibre and FODMAPs for 24 hours and were required to fast overnight prior to each breath test. Participants consumed a drink containing 35g of fructose in 200ml of water for each test and breath samples were collected every 15 or 20 minutes for up to four hours.

Results-

Study 1 – is a fructose breath test result reproducible?

Major findings

• 30% of participants, positive for fructose malabsorption in their first breath test, tested negative on the subsequent test ≥2 weeks later
• The was no relationship between the amount of breath hydrogen produced in the first and second fructose breath tests

Study limitations-

• Some study 1 participants had their second breath test performed >19 weeks after the first. This relatively long period of time between tests means that changes in diet or medications may have affected gut bacterial populations and therefore influenced the results.
• This study included a small number of participants and retrospectively analysed data from existing studies. A larger, prospective study that collects data over a longer test period would be useful to further validate these findings.
• 
  

Study 2 – are symptoms on a breath test related to breath hydrogen response?

Major findings

• Those who tested ‘positive’ for fructose malabsorption were more likely to report symptoms during a fructose breath test than those who tested ‘negative’
• This was NOT significant when test day symptom scores were adjusted to account for the participants’ usual level of symptoms (pre-test day symptom scores)

Study limitations-

• Gastrointestinal symptom data from study 2 was only collected during the breath-testing period and not the remainder of the day, so may not have captured participants who had a delayed symptom response.

Further data-

In 2013, a group of researchers in the UK conducted a study looking at the effects of fructose in the gut.2 They were examining the osmotic effect of fructose (how much water fructose moves through the gut) and did this using superior MRI technology. They demonstrated some very interesting findings:

• Fructose has an osmotic effect, delivering water through the small intestine
• Fructose has this effect regardless of whether the person has a positive or negative fructose breath test
• The authors conclude – fructose has an osmotic effect and this does not depend on breath test outcomes
• Clinical significance – a fructose breath test is not helpful in identifying fructose as a trigger.

This figure demonstrates the increase in small bowel water content (SBWC) seen on consumption of fructose in individuals with a positive fructose breath test (blue) and a negative fructose breath test (red).2 You can see there is no difference. Fructose increases SBWC regardless of breath test outcomes.

Take home messages-

• Fructose breath test results are not reproducible, i.e. you can have a positive breath test one day, and can have a negative breath test just weeks later
• There seems to be a poor correlation between fructose breath test outcome (i.e. a ‘positive’ or ‘negative’ result for malabsorption) and the experience of symptoms in individuals with IBS
• Fructose can contribute to symptoms regardless of whether a breath test is positive or negative
• The results of this and previous research DO NOT support the use of routine fructose breath tests for the diagnosis of fructose malabsorption or to guide management in people with IBS.

References:

1. Yao CK, Tuck C, Barrett J, Canale K, Philpott H, Gibson P. Reproducibility of lactulose and fructose breath hydrogen testing and impact on clinical utility. Journal of Nutrition & Intermediary Metabolism. 4:29.
2. Murray K, Wilkinson-Smith V, Hoad C, Costigan C, Cox E, Lam C, et al. Differential effects of FODMAPs (fermentable oligo-, di-, mono-saccharides and polyols) on small and large intestinal contents in healthy subjects shown by MRI. The American journal of gastroenterology. 2014;109(1):110-9.

Diet Tips:Adding glucose to high FODMAP foods – does it really help?

By Marina Iacovou (PhD Candidate and Accredited Practising Dietitian)

In an earlier blog post this year (http://bit.ly/1WvWU5Y) we speculated that although early studies suggested adding glucose to fructose may improve tolerance, the strategy was not looking promising – instead all it was doing was adding more sugar to the overall diet.  There was confusion around this strategy with individuals adding glucose to all FODMAPs, not just fructose, which is the only FODMAP that could benefit from glucose addition. Consuming glucose tablets became common practice for many people when eating onion and garlic, foods rich in fructans, not fructose, which scientifically will not improve symptoms.

We can now confidently tell you that adding glucose to foods to help with the absorption of FODMAPs, even fructose, does not work.

One of our PhD candidates, Caroline Tuck, has published work on this very topic. If you want to read the full publication, please click here.

A brief summary of the study and its findings:

• Patients with a positive fructose breath test (“fructose malabsorption”) and functional bowel disorders such as IBS, were recruited. Healthy participants were recruited as a reference/control group
• Participants were provided with 6 different sugar solutions (PART A) – some acted as control solutions (glucose only) and others were combinations of fructose and glucose, and fructans and glucose which were compared to fructose alone and fructan alone
• Additional studies (PART B) were undertaken. where glucose was added to whole foods high in excess fructose
• It was a series of two randomised controlled, double or single-blinded crossover trials – this means that the participants (and in part A the researchers) were not aware what solutions were being consumed and the solutions were given in a random order. As such the results are more reliable
• Overall, breath hydrogen scores and patient symptoms did not improve with the addition of glucose to fructose or fructan solutions, or to whole foods
• Breath hydrogen responses in healthy participants were similar to patients with functional bowel disorders – showing that “fructose malabsorption” is normal
• The results of the study, question the reliability of breath hydrogen tests in a clinical setting. 

So we can conclude that: 1) breath hydrogen tests, particularly to fructose, are unreliable – this is consistent with a previous study we posted about http://bit.ly/2fSaVcn and 2) Overall, the strategy of adding glucose to drinks or foods to lessen the effects of dietary FODMAPs on functional gastrointestinal symptoms, has no evidence.

Some of you may have been doing this already – taking glucose tablets with high fructose foods. If you really feel it helps, you can continue, but remember these points:

• Your total sugar intake will increase significantly
• Your tolerance to FODMAPs, including fructose, can fluctuate over time, so maybe this strategy is working because your tolerance to fructose has improved and you can drop the glucose without significant symptoms

Something to consider…

Tuck CJ, Ross LA, Gibson PR, Barrett JS, Muir JG. Adding glucose to food and solutions to enhance fructose absorption is not effective in preventing fructose-induced functional gastrointestinal symptoms: Randomised controlled trials in patients with fructose malabsorption. J Hum Nutr Diet. 2016.