What is fructose?
Fructose is a monosaccharide, or simple sugar. It is mainly found in fruit, which is why it is also called fruit sugar. It can be supplied to the body as free simple sugar, in the form of disaccharides (sucrose) or fructans [1,2,3].
Is fructose harmful?
Fructose is metabolised in two pathways. One takes place in muscle and adipose tissue, the other in the liver. In a healthy person, these mechanisms work properly and do not generate any worrying symptoms. However, in people with impaired fructose metabolism and absorption in the intestine, the consumption of high-fructose products can cause many unpleasant or even dangerous effects [2,4].
Excess fructose in the diet can adversely affect health and promote the development of, among others:
- type 2 diabetes,
- non-alcoholic steatohepatitis,
- non-alcoholic steatohepatitis,
- certain cancers (particularly of the liver, pancreas and colon),
- cardiovascular diseases
- kidney disease
- neurological disorders (e.g. Alzheimer’s disease),
- psychiatric disorders (e.g. neuroses, depression) [1,5].
Causes of fructose intolerance
Fructose metabolism disorders are genetically determined. These include:
- spontaneous fructosuria,
- congenital fructose intolerance,
- hepatic fructose-1,6-biphosphatase deficiency.
All these defects are commonly referred to as fructosemia. However, only congenital fructose intolerance is correctly referred to by this term [6].
What does fructose intolerance lead to? Symptoms
The first symptoms of fructose intolerance usually appear after the onset of consumption of products rich in this sugar. Fructose sugar metabolism disorder most often becomes apparent in infants fed modified mixtures sweetened with sucrose. However, in breastfed infants, the first symptoms of the disease may appear during the expansion of the diet and the introduction of fruit juices [6].
In some children, the condition produces few symptoms and follows a milder course than in adults. Lack of appetite and delayed physical development usually appear first. In contrast, the predominant symptoms of fructose intolerance are severe hypoglycaemia (severe drop in blood glucose levels) and vomiting. These appear shortly after consumption of a fructose-containing product [6].
Continued consumption of fructose can result in:
- enlargement of the liver,
- jaundice,
- haemorrhages,
- apathy,
- excessive sleepiness,
- convulsions,
- urinary tract infections [6].
Non-compliance with the diet in people with fructose intolerance leads to recurrent hypoglycaemic states and worsening of liver and kidney failure. In extreme cases, it can even contribute to death [6].
Unfortunately, through the non-specific symptoms of the disease, it often remains undiagnosed. However, the incidence of acute episodes of the disease decreases with age. It is sometimes the case that the diagnosis of fructose intolerance in children does not occur until preschool or school age [6].
Gastrointestinal symptoms in fructose malabsorption syndrome include:
- abdominal pain,
- flatulence,
- excessive gas production,
- constipation,
- diarrhoea,
- nausea
- vomiting [6].
Does fructose intolerance go away?
Fructose intolerance has a genetic basis and is therefore an incurable disease. It requires adherence to a diet that excludes or restricts this sugar throughout life [6].
Which products contain fructose?
Most fructose in the form of free sugar and disaccharide (sucrose) can be found in:
- juices and fresh fruit (apples, pears, melons, papayas, watermelons),
- dried fruit (apples, plums, cherries, apricots, figs, dates, sultanas),
- jams, preserves, marmalades, jellies,
- honey,
- fructose-sweetened drinks (soft drinks),
- highly processed products with added fructose [2,4].
In contrast, the highest fructan content is characterised by:
- garlic,
- chicory
- onions,
- asparagus,
- artichokes,
- leek,
- certain cereals (wheat, barley, oats) [2,4].
Fructose intolerance – what to avoid?
In addition to its main sources, fructose can be found in foods in the form of syrups (fructose and glucose-fructose). These are used in the food industry to sweeten drinks, sweets, cakes or baked goods. Maple syrup, agave syrup and honey can also be added to meals. These are rich sources of fructose sugar and should be eliminated from the menu [2].
Fructose is also in some:
- mustards, ketchups, prepared sauces, marinades and ready-made soups,
- breakfast cereals,
- flavoured milk and milk products with fruit and fruit flavouring (yoghurt, buttermilk, fruit or flavoured cottage cheese),
- marinated meats,
- dietetic and diabetic products,
- medicines – in the form of syrups, tablets and capsules [2].
It is worth noting that the sugar in confectionery is sucrose, which is broken down in the body into glucose and fructose molecules. Thus, it too is an important source of fructose in the daily diet [2,3,4].
What should the diet in fructose intolerance look like?
The main premise of this diet is the complete exclusion or restriction of fructose. Therefore, products that are a source of fructose sugar should be strictly avoided.
The diet should be based on fresh and unprocessed foods. Allowed products are:
- low-fructose fruits (citrus, plums, papaya, avocado, rhubarb, berries, bananas, kiwi, pineapple),
- selected vegetables and mushrooms (celery, radishes, spinach, peas, broccoli, potatoes, chanterelles, boletus, mushrooms),
- dairy products (milk, cheese, quark, yoghurt, buttermilk),
- cereal products (pasta, rice, groats, bread containing no fructose),
- fresh meat and meat products,
- eggs
- fresh fish,
- fats (butter, margarine, vegetable oils),
- drinks (water, black and green tea, unsweetened coffee, fruit juices diluted with water) [2,3,4].
Various types of fructose-free condiments can be used when preparing meals (e.g. salt, pepper, fresh herbs). In contrast, glucose should be chosen to sweeten meals.
When grocery shopping, it is important to pay attention to the ingredients of products, especially highly processed ones. Reading labels is key to avoiding incorrect choices. Sometimes products that we do not suspect may contain significant amounts of fructose.
It is important to remember that each person with fructose hypersensitivity has a different fructose tolerance threshold. For some sufferers, symptoms appear just after consuming even small amounts of food containing fruit sugar. For others, however, it is only when larger amounts are consumed or after a longer period of time that symptoms occur.
Therefore, dietary recommendations and menus should always be chosen on an individual basis. It is advisable to consult a qualified dietician who will help you to arrange your diet. This will help to ensure a correct balance of meals and reduce the risk of deficiencies [2,3,4,7].
