Fructose is a monosaccharide, being the ‘M’ in FODMAP.

Many people think fruit when they hear the word fructose, which is mostly correct. The main sources of fructose is in fruits. However, fructose is not in all fruits.

AND there are some foods which contain fructose that are easily digested if you have fructose intolerance. But how?

This week, we take a deeper look at the fructose sugar.

What is fructose?

Let’s look at fructose, an important sugar to be aware of if you are on the low FODMAP diet. Fructose is a monosaccharide, which is the ‘M’ in the word ‘FODMAP’. ‘Mono’ means ‘single’, and ‘saccharide’ means ‘sugar’. Simply put, monosaccharide means ‘single sugar’ and these sugars make up many of the carbohydrate containing foods that we eat.

The three types of monosaccharides found in foods are glucose, fructose and galactose, but fructose is the sugar of importance in the low FODMAP diet. This is because, of the three monosaccharides, fructose is the sugar that can be poorly digested and trigger IBS symptoms. That is why fructose is included as a FODMAP to limit when following a low FODMAP diet.

Fructose is present in a range of fruits including apples, mangos, watermelon, pears, peaches and cherries. It is can also be found in honey or used as a substitute for castor sugar in some recipes and food products. Fructose is typically well-absorbed in the greater population. However, for those with IBS, or specifically fructose intolerance, fructose can become a major trigger for their IBS symptoms.

Why is fructose restricted on the low FODMAP diet?

When someone with fructose intolerance consumes too much fructose (or more specifically excess fructose*), they may experience diarrhoea and other unpleasant IBS symptoms. Fructose intolerance is actually quite common, with up to 1 in 3 people being unable to absorb fructose properly!

Fructose requires a transporter known as GLUT5 to be absorbed. People who have difficulty absorbing fructose have a limited supply of this GLUT5 transporter, meaning that only a limited amount of fructose can be absorbed. The remaining or excess fructose continues through the digestive system into the large intestine where it is fermented by the bacteria living in the gut. The process of fermentation that occurs is the ‘F’ in FODMAP. The excess fructose is fermented by the bacteria, producing gas and triggering IBS symptoms such as flatulence and bloating.

Fructose is also osmotic, which means that fructose causes additional water to enter the gut. This excess water affects bowel movements and causes diarrhoea, which is a common symptom with fructose intolerance. This is why it is essential to limit fruits containing excess fructose on the low FODMAP diet. However, it is not recommended to limit all fruits, as there are a range of fruits that can still be suitable on the low FODMAP diet.

Why are some foods that contain fructose well-absorbed, and others poorly absorbed?

*Fructose absorption is an issue in those with fructose intolerance when it is in excess of glucose. What this means is if the amount of fructose is higher than the amount of glucose, it is in excess. When fructose is in excess in those with fructose intolerance, it can remain unabsorbed in the small intestine and move into the large intestine where it is fermented by bacteria.

Some foods may contain fructose, but in a smaller amount than glucose, this means fructose is not in excess of glucose. These foods that contain fructose, but with a higher level of glucose in them, are readily absorbed by those with fructose intolerance. The reason for this is because glucose acts as a facilitator for fructose to be absorbed in the small intestine. That is how those with fructose intolerance are able to consume foods with fructose in them.

How do you know if a food contains fructose in excess of glucose, or vice versa?

Unfortunately, there is no way to determine if a food has more fructose, or more glucose from just looking at the back of the packet. The only way to know if a product is safe for those with fructose intolerance is if the product has been tested and certified for it’s fructose and glucose levels. Any food that carries the FODMAP Friendly logo is safe for those with fructose intolerance as they are low in all FODMAPs, including fermentable monosaccharides like excess glucose.

How do I know if I have fructose intolerance?

There are 2 main methods for determining fructose intolerance:

1. Breath Testing

The first method is completing a Hydrogen/Methane breath test through a GP referral. You must first consume a dose of fructose. The breath test is then used to measure the amount of hydrogen and methane in your breath over a 3 hour period. These gases are produced during the fermentation process, which occurs when the fructose has not been absorbed. However, this test is not a diagnosis of fructose intolerance, but rather a measure of fructose malabsorption. This information can assist in determining your sensitivity to certain FODMAPs. Breath tests should only be completed when other possible gastrointestinal conditions have been ruled out.

2. FODMAP Challenge/Re-Introduction

The second method is completing a FODMAP challenge with the guidance of a FODMAP trained dietitian. The FODMAP challenge involves following a strict low FODMAP diet, and then completing challenges to determine the specific FODMAPs and doses triggering your IBS symptoms. Fructose is one of the FODMAP challenges, so this diet can be used determine fructose intolerance. The strict low FODMAP diet is not a long-term diet, but is used to determine the specific FODMAPs that are triggers and your thresholds for each. Your dietitian will then educate you on which FODMAPs you need to avoid, and which you can re-introduce into your diet.

Do I need to avoid all fruit on the low FODMAP diet?

The short answer is no. While there are some fruits that should be limited on the low FODMAP diet, we encourage you to still consume a variety of low FODMAP fruits on the low FODMAP diet. Fruit is an important part of a balanced diet, as fruits contain a range of vitamins, minerals and fibre which our bodies require for optimal functioning. The Australian Dietary Guidelines recommend for adults to consume 2 serves of fruit daily, and this includes those on the low FODMAP diet!

What fruits are suitable for the low FODMAP diet?

There are a range of fruits that you can have on the low FODMAP diet including: firm bananas, grapes, honeydew (90g), kiwi fruit (150g or 2 small), pineapple (140g or 1 cup), strawberries, blueberries (40g), mandarins and organges (120g). Check out this recipe for a low FODMAP smoothie full of low fructose ingredients!

Have you read last weeks post about lactose – the most common intolerance where up to 90% of countries experience issues with lactose? Check out the blogpost here.