Low Fructose Diet
by Sue Kira, Naturopath & Clinical Nutritionist
Fructose, or fruit sugar, is a simple monosaccharide (simple sugar) found in many plants including fruits, fruit juices, and corn, where it is often bonded to glucose to form the disaccharide sucrose. Fructose is also found in high amounts in agave, maple syrup, dates, and honey.
Unlike glucose, fructose is mostly metabolised by the liver and any excess over what the body needs is stored as fat. A high-fructose diet puts unnecessary stress on the liver and can increase the risk of non-alcoholic fatty liver disease.
Studies have also shown that fructose doesn’t trigger your appetite hormone (Grehlin) which tells us when to stop eating. This means you can effectively polish off lots of foods high in fructose without feeling full, unless the fructose is bound to fibres such as those found in some whole fruits.
Another good reason to be on a low fructose diet is if you have a small intestine bacterial overgrowth (SIBO), FODMAPs sensitivity, candida, or other bacterial or fungal overgrowth, as fructose will feed up these organisms and make them harder for the body to eradicate.
How is fructose different to other types of sugar?
There are many different types of sugars we can consume including: lactose, the sugar found in dairy products; maltose from malt; galactose, the sugar found in breast milk; and many others. But the main ones are sucrose, fructose, and glucose. Basically, any food that ends in ‘ose’ is a type of sugar. For simplicity, we call them ‘sugars’ and they are all classified as carbohydrates.
Many people know that sugar is found in all sorts of foods, both added and occurring naturally, but not everyone knows how the different types of sugars act or digest in the body.
Often our taste buds don’t know the difference between the types of sugars – but our body sure does. Gram for gram they all contain the same calories but they are processed differently.
Basically, we have two types of sugar carbohydrates and these are called monosaccharides and disaccharides. The monosaccharides have one simple sugar unit and the disaccharides have two. Both fructose and glucose are classified as monosaccharides and linked together they make the disaccharide sucrose.
Glucose is the preferred energy for the body
Glucose is often called ‘blood sugar’ because it circulates throughout the body to either be used immediately for energy or to be stored in muscles and liver cells for use later. The body uses the enzymes glucokinase and hexokinase to obtain glucose from the carbohydrates such as grains, fruits and vegetables that we eat. Insulin is a hormone the body uses to facilitate the entry of glucose into cells.
Fructose has different metabolic pathways
Most of our fructose comes from eating fruits and vegetables but it can also be added to many foods and drinks. It is often found in high concentrations in soft-drinks/sodas and other fruit flavoured drinks and snack products like muesli bars etc.
You may have heard of high fructose corn syrup – yes, that’s fructose, but in high concentrations. Manufacturers use this form of sugar because it can taste sweet without being ‘sickly sweet tasting’ and because it is cheap to obtain and therefore sold at a good profit margin.
High fructose corn syrup was once a processing by-product that was sold off cheaply to dispose of it, but now production is focused on purposely making fructose to add to many processed foods.
Fructose cannot be directly used by the body and must be broken down by an enzyme called fructokinase, which many people are deficient in. This can create fructose malabsorption or intolerance. Also, because of the differing pathway, fructose is not what the body likes to use to fuel the brain or muscles, so most of it is stored as fat in the liver.
This often creates ‘fatty liver disease’, leaving the body with an insufficient quantity of the right fuel to run off, creating a hunger for more sweet foods.
Fructose also doesn’t stimulate the secretion of insulin that normally regulates sugar levels in the blood and puts the needed sugar into the cells, which is why fructose just goes straight to the liver to be stored and not usually used.
Sucrose is made of glucose and fructose
We know sucrose as the common table sugar that is often added to tea, coffee, cakes, biscuits etc, but it is also found in all our fruits and vegetables in varying amounts. The enzyme beta-fructosidase divides the sucrose into the glucose and fructose molecules, and then each molecule is taken where it needs to go – glucose into the blood to then go into our cells, and the fructose to the liver to be stored as fat.
Why is fructose so bad for us if it’s a natural sugar?
Fructose is metabolised by the liver
As strange as it may seem, fructose can’t be used by the body for energy directly, only as stored energy. It is stored in the liver as fat and then, if we run out of energy, our body can get some from our fat stores. This is inefficient, so our body mainly just stores it.
Excess fructose can damage the liver, create fatty liver and be equally as toxic to the liver as ethanol (alcohol). Because of excess fructose in the body, there can also be issues with insulin resistance which can be a real problem for diabetics and those who have problems with weight control.
Fructose creates bacterial overgrowth
Even though we can’t use fructose directly for energy, any pathogenic gut bacteria are fed well by fructose, which creates potential overgrowths of the wrong type of bacteria in our intestines and unbalances the good bacteria levels.
Fructose reacts with fats and creates oxidation and inflammation
If you didn’t already think fructose was bad enough, fructose also reacts with polyunsaturated fats and proteins creating compounds called ‘advanced glycation end products’ which create oxidation (free radical damage) to our cells which can lead to inflammation and potentially create disease in the body.
The acronym for advanced glycation end products is ‘AGE’ and for a very good reason, because it ages our cells and makes us age prematurely – not good for anti-aging efforts.
Fructose can increase cardiovascular risks
By damaging the liver, fructose can create a shift in our blood lipid profile (cholesterol) which can increase cardiovascular risk (the liver makes cholesterol).
In the ‘old days’ they thought that saturated fats created heart disease, but these days scientists recognise that it is the combination of fats and sugars, or more accurately, it is the polyunsaturated fats (yes, the ones they told us to change over to) and fructose sugar that combine to create the oxidation on the walls of arteries that blocks them up and creates problems.
All sugars add to this serious health issues, but fructose is seven times more likely to create this damage than other sugars.
Fructose can affect our brain and memory
Fructose is the type of sugar that can create a type of ‘brain fog’, which affects memory and attention and the ability to think clearly. Who needs that?
Fructose can create leptin resistance and make us eat more
Apparently, fructose can cause leptin resistance. It is the leptin hormone (called Grehlin) that regulates our metabolism and appetite. With leptin resistance, the ‘off’ switch for eating doesn’t happen as easily and we can be more prone to overeat.
This especially happens with fructose containing foods and drinks, but if we have leptin resistance it can also affect how we eat other foods. Thus, fructose can make it easier to get fat, in an unhealthy way.
Some people want to get fatter, but fat on the liver is not healthy nor is it healthy to have too much fat anywhere. We need some fat to make our hormones etc function properly but not excess fat. The amount of high fructose corn syrup in many processed foods and drinks is certainly a big contributor to today’s obesity epidemic.
Fructose is bad for our kidneys
Fructose is also bad for our kidneys. Fructose can increase our uric acid production which can cause kidney stones, gout, and even create or increase high blood pressure (hypertension).
Fructose may increase cancer risk
Cancer cells love fructose as a primary food source, which is not good unless we are trying to pump up the cancer cells ready for chemotherapy. I wondered why kids with cancer were given lollies to eat until I discovered the reason. But excessive sugar/fructose is only useful during treatment times, otherwise the above factors can come into play.
Fructose malabsorption and intolerance
Fructose is quite hard to digest at the best of times, but some people have more trouble than others, especially if their liver is not good or they lack digestive enzymes. The undigested fructose will be fermented by bacteria in the gut which produces gas that creates bloating, abdominal distension, cramps and sometimes diarrhea. This is called fructose malabsorption.
There is also a condition called fructose intolerance which is genetic and is often picked up at infancy and is much more serious, often leading to seizures, liver disease, or even retardation of mental functions. Fortunately, it can be treated by avoiding fructose.
Apart from the gastrointestinal symptoms mentioned above, both fructose malabsorption and fructose intolerance can lead to nutritional deficiencies, especially if diarrhea is part of the symptom picture for prolonged periods of time.
The main nutrients that can be deficient in these cases are iron, B12, B6, folate, calcium, magnesium, vitamin E and vitamin C. Both the malabsorption and the loss of nutrients from diarrhoea can lead to other symptoms related to these deficiencies which can be many and varied but commonly include fatigue, mood changes, headaches, brain fog and even constipation for some.
Fructose malabsorption testing
There is a simple ‘Hydrogen Breath Test’ that can determine fructose malabsorption which can be done as a standalone test or part of a compilation of breath tests for SIBO (small intestine bacterial overgrowth) which tests not only hydrogen but also methane.
You can either do this test, or if you or a practitioner strongly feel you have fructose malabsorption, you can simply try the diet and see if your symptoms go away. You may need further support with gut healing in which case your practitioner may suggest the Leaky Gut Diet.
As mentioned, you may consider the leaky gut diet with your practitioner’s support and the diet low in FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) as it is possible to be reactive to more than fructose (fructose and fructans are monosaccharides).
Fructose, gluten intolerance, IBS and celiac connection
Many people who have gluten issues, IBS, or even lactose intolerance (lactose is a disaccharide made up of glucose and galactose) also commonly react to fructose. If you have a fructose malabsorption, you may also have a sensitivity to other FODMAP’s and may wish to use the Low Fodmaps Diet for a while and then challenge (reintroduce) the foods to see which you react to the most.
If you only react to the fructose group, then a low fructose diet is the one for you. If you have already been tested via food challenge or the breath testing methods and know which foods you react to, then you don’t have to do the full FODMAPs challenge.
Fructans are fructose molecules joined together with a glucose molecule stuck on the end of the molecular chain. These fructans can cause the same issues as fructose malabsorption. Unlike fructose, fructans are not sweeteners but are found in certain grains, vegetables, and specific fibres used in things like probiotics as ‘pre-biotics’.
Foods High in Fructans to avoid
- Brown rice
- Chicory root (often used as coffee substitute)
- Dandelion leaves and root
- Green beans
- Inulin and fructooligosacharride (FOS) are commonly found in many probiotics and ‘fortified’ foods
- Onion family vegetables (leeks, red, white and brown onions, spring onions, chives, shallots)
- Tomatoes and tomato products like tomato paste
Acceptable Grains Low in Fructans
- White rice
Foods High in Fructose to avoid
- Fruit and their juices such as apple, cherry, grape, guava, lychee, mango, melons (honeydew, cantaloupe/rockmelon and watermelon), orange, papaya, pear, persimmon, pineapple, quince, star fruit, berries in large quantities, such as blueberries and raspberries (small amounts are fine).
- Most dried fruit such as apricots, sultanas, currant, raisins, dates, figs, and health bars with dried fruit.
- Processed fruits such as barbecue sauce, chutney, plum sauce, sweet and sour sauce, tomato paste, fruit from cans or tins because they are usually in pear juice.
- Natural sweeteners such as honey, maple syrup, high fructose corn syrup, corn syrup solids, fruit juice concentrate.
- The sweetener, tagatose, is converted into fructose and can be found in soft drinks/sodas, bottled or packet fruit or vegetable juices, many teas, breakfast cereals and cereal bars with dried fruit, sweets/lollies and chewing gum, jams and marmalades, fondants and fillings for cakes and biscuits and many diet foods. Levulose or ‘invert sugar’ seen on food labels is a sign of fructose content.
- Foods high in sugar/sucrose also have high concentrations of fructose.
- Vegetables are mostly fine, except sugar snap peas, artichokes, and asparagus.
- Alcohol, especially beer and sweet dessert wines such as sherry, port, and muscatel wine. But really, any alcohol is an issue.
- Foods that contain sorbitol (E420 additive number) and xylitol (E967 additive number) are usually poorly tolerated by those with fructose intolerance, so it is best to avoid foods such as ‘diet drinks’, sugar-free gums and sweets, chocolates and desserts that contain these additives.
The fruits should be eaten in small quantities, spread over the day.
- Beetroot/beets (small amounts)
- Bean sprouts
- Blueberries (small amounts)
- Clementine/mandarins & other citrus in small amounts
- Green peas
- Herbs like parsley, coriander
- Raspberries (small amounts)
- Strawberries (small amounts)
- Sweet potato/yams
Note: All proteins and fats do not contain fructose or fructans
Before you commence your diet, see your medical or health care professional for qualified guidance about what foods and supplements are best for your body. While on the diet do not stop any medications or supplements previously prescribed unless advised otherwise by your medical or health care professional.
During the early stages of a new diet, you may experience symptoms such as fatigue, headaches, or body aches, which may occur because your body is detoxifying. However, if you are unsure about a symptom at any time, check immediately with your medical or health care professional.