Fructose – the good, the bad – and the malabsorbed...
Michelle Berriedale-Johnson explains.
When the Glycaemic Index first hit public awareness in the 1990s, fructose was hailed as the great white hope of diabetics. The index measured the speed at which foods, mainly carbohydrates, were converted into glucose in the body and fructose came very low on the index as it ‘converted’ very slowly into glucose.
Glucose provides energy that the human body and brain need to function. It is absorbed from the gut into the blood stream and thence into the liver where it is converted and stored as a substance called glycogen. Glycogen is released back into the blood stream to be converted into energy by whichever part of the body is in need of energy as, when and in the quantities that it is needed. This process is monitored and regulated by the hormone insulin.
If the release is not properly controlled either too much glucose (sugar) ends up in the blood which becomes sticky. Gradually it will clog up the tiny veins in the eyes, kidneys and extremities and, in due course, larger veins leading to the main organs such as the heart. If too little glucose/glycogen gets into the blood the body does not have enough energy to function at all. Insulin ensures that the glucose from the food that we have eaten, is converted, stored and released in the right amounts, as needed, to fulfil our bodies’ and brains’ energy requirements.
The problem for diabetics is that they either do not produce any insulin, or do not produce the right amount at the right time to ensure the correct release of energy-providing glucose (or glycogen) into the blood stream. So, if they eat a lot of sweet and/or high carbohydrate foods that convert very quickly into glucose, the glucose will flood the blood stream making the blood far more sugary and sticky than is healthy. Ergo, diabetics were advised to live on very low sugar, savoury diets – bad news for diabetics with sweet teeth!
So the fact that fructose came very low on the glycaemic index and did not convert quickly into glucose was good news for diabetics because it meant that fructose-sweetened foods would not immediately turn into glucose and send their blood sugar levels rocketing up. Moreover, fructose actually delivered more sweetness that sucrose (table sugar), so in calorific terms, they would need less of it so had less chance of putting on weight – although this theory did not hold water for the vast majority of fructose sweetened products as, once you heat fructose, its sweetness level reduces to that of sucrose.
And finally, as far as the food industry was concerned, fructose was cheaper than sucrose or other sweeteners…
However... All was not quite what it seemed. Why was fructose so low onthe glycaemic index? What made it different from other sugars?
Unlike glucose, fructose is not controlled by insulin, nor can it be converted, as can glucose, into energy in any part of the body; fructose can only be processed by the liver.
Moreovoer, unlike glucose, eating fructose does not suppress the release of the stomach hormone, ghrelin. Grehlin is the hormone makes you feel hungry, so, if it not suppressed when you eat, you are going to go on feeling hungry and go on eating fructose-sweetened foods whereaas you might have stopped eating glucose-sweetened foods because you felt full.
And strangely, although it is a carbohydrate or sugar, fructose is processed by the liver as a fat so, rather than being store in the liver as harmless glycogen, it gets stored as potentially harmful fat. Thus excess fructose consumption leads to excess fat being stored in the liver which can cause inflammation (with all its attendant ills) and liver disease.
So far from fructose helping diabetics keep their weight down, it might well add to their risk of obesity. Indeed, because excess fructose is stored by the body as excess fat, excess fructose consumption could cause problems right across the population in terms of obesity, heart disease, stroke etc.
The food industry and fructose supporters in general defend fructose on the grounds (quite correct) that if fructose is ingested in equal quantities with glucose (which it is in sucrose, made up of 50% glucose, 50% fructose) it can be metabolised perfectly safely. But the most widely used sweeteners, because of the relative cheapness of fructose, use a higher proportion of fructose than glucose.
But what about fruit?
Yes, fruit does contain fructose (along with glucose) although in varying amounts depending on the fruit. But in fruit the fructose comes bound up in fibre and other carbohydrates and proteins which need to be broken down in the digestive system before it can access the fructose, so you are never getting the pure fructose ‘hit’ of high fructose sweetener.
However, there are two other health problems relating to fructose which have nothing to do with any of the above!
The first is a rare but potentially life-threatening condition, hereditary fructose intolerance, in which the liver does not produce the enzyme (fructose 1-phosphate aldolase) which is essential for it to be able to metabolise fructose. As a result the fructose accumulates in the liver and prevents the conversion of glucose into glycogen (see above) thereby starving the body of glycogen/energy and causing severe hypoglycaemia or low blood sugar. If this is not treated it can lead to coma and, ultimately, death. Its incidence is thought to be about 1 in 22,000.
The only treatment for genuine fructose intolerance is strict avoidance of fructose.
Fructose malabsorption – or Fructmal
Far more common (thought to affect as many as 30-40% of the population to a greater or lesser degree) is fructose malabsorption.
In this condition, which greatly resembles what is known as lactose intolerance but is in fact also a malabsorption problem, the sufferer is deficient in the ‘transporting protein’ GLUT-5 which, in a healthy person, enables the fructose to be absorbed through the gut and into the liver – see above. As a result the fructose travels on through the intestines and ends up in the colon where bacteria break it down into short chain fatty acids and gases which cause the bloating, flatulence and diarrhoea that are typical of fructose malabsorption.
Absorption of fructose through the gut is greatly improved if the fructose is taken with glucose (one molecule of glucose will ‘transport’ one molecule of fructose with it through the gut into the liver). Sucrose, or ‘normal’ sugar, is made up of equal parts of fructose and glucose. So those with fructose malabsorption can often tolerate normal sugar when they cannot tolerate fruit or high fructose sweeteners such as honey, maple syrup or HFCS.
But, sugar alcohols (sorbitol, mannitol, xylitol and all other sweeteners ending in ‘ol’) actually make fructose absorption more difficult, so they too can cause problems. See the table linked to below for ‘safe’ and ‘to avoid’ sweeteners. But be aware that sorbitol can also occur naturally – in fruit such as apples – thus making the nation’s favourite fruit doubly lethal for ‘fructmal’s.
As with lactose malabsorption, you can inherit the condition but it can also be caused by:
You can become a fructose malabsorber (or a fructmal) at any time in your life but, more encouragingly, although it can be a lifelong condition, for most people it will resolve as their general health and, particularly, their digestive health improves.
Symptoms and diagnosis
Mainly digestive – bloating, flatulence, gurgling, abdominal pan and diarrhoea
Less commonly – depression, anxiety, fatigue, headache, brain fog, constipation, weight loss or sugar craving and, as a result of constant diarrhoea, anaemia and nutrient deficiencies
Formal diagnosis is via a hydrogen or a methane breath test. Since our digestions produce neither of these gases naturally, if they are present in our breath it normally comes from fermentation in the large intestine caused by the breakdown of fructose (or lactose) by gut bacteria.
However, it may not only be fructose, and therefore fruits and sweeteners, that give you problems…
Fructans are chains of fructose molecules which end in a glucose molecule and they can cause the same problems to a fructose malabsorber as fructose itself.
Fructans are found in many grains and vegetables including:
However, you will be relieved to hear that the following grains are fine:
But, be warned, the glucose/dextrose trick does not work with fructans.
As with fructose, only trial and error will tell you what you are able to tolerate and in what quantity.
As with most non-organic digestive complaints the best approach is to identify and then eliminate, for a period, the problematic foods. Once some degree of stability has been achieved, you can establish your own thresholds and triggers – and these will vary from person to person. Although some high-fructose foods (most fruits and many sweeteners) will cause reactions in most people, there are no hard and fast rules and you will need to establish a diet for yourself by trial and error. Sorry…
The most successful way to establish your tolerance levels is to go ‘cold turkey’ for, say, a month, excluding all foods that contain either fructose or fructans. Assuming that you feel better (if you do not you should consult your doctor immediately as there may be some other cause for your un-wellness) then introduce foods containing fructans or fructose one by one, giving yourself several days on each new food before you try another as it can sometimes take 36 hours or more for the reaction to work through your system.
For initial guidance there is a helpful list of foods to avoid and those that are ‘safe’ at HealthyHype.com.
Remember also that a glucose (also known as dextrose) molecule will ’transport’ a fructose molecule through the gut wall, so you may be able to tolerate certain relatively high fructose foods provided you combine them with high glucose/dextrose food – such as a dextrose tablet. However, this only works if the levels of glucose/dextrose are as high, if not higher than the levels of fructose – if lower, the excess fructose will still not be absorbed – and you can end up by consuming a good deal more sugar than you might wish to.
At the same time you should do all that you can to improve your intestinal function. Consulting a nutritionist/nutritional therapist who would test your gut function and suggest appropriate supplementation or dietary manipulation can be very helpful and can often be done on line or on the phone. If you do not know one there is a short list in the Supplement section of our site. Alternatively the Biolab Medical Unit has a list of medical doctors specialising in nutrition or you could consult BANT (the British Association for Nutritoinal Therapists) who also have lists of qualified practitioners.
First published in September 2010