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First, what is fructose? Fructose, or fruit sugar, is a simple sugar (monosaccharide) found in plants.  It is found as free fructose and also bonded with glucose (to form a disaccharide) as sucrose.  

It is the sweetest tasting of all the naturally occurring carbohydrates and this, along with other physical property advantages, makes it the obvious choice of food manufacturers for increasing palatability, especially in reduced-fat recipes.

Fructose has a very low glycaemic response (19 as compared to 100 for glucose).  This means that it does not trigger an insulin response.  Therefore, fructose does not cause the dramatic sugar high followed by sugar low response that free glucose and glucose from starch does.  Read this article on glycaemic index for more information. This means that it is the sugar of choice for diabetics, who need to carefully manage their blood sugar levels.  

Fructose is absorbed by the small intestine (when it is in the form of sucrose, the enzyme sucrase breaks the sucrose into fructose and glucose and then they are each absorbed) and sent to the liver to be metabolised into glycogen (the short term sugar storage medium), free fatty acids and triglycerides.  When large amounts of fructose are consumed, more of these free fatty acids and triglycerides will also be produced with possible negative health effects (1).

Why Fructose is good food, not evil toxin!

Fructose has been getting a bad press recently - with major conspiracy theories on big industry trying to poison us with this evil toxin!

But is fructose really the worst thing you can eat?

Is fructose really a great food and why has it gotten such a bad public image?

Problems with fructose

Fructose-containing foods may still have a high glycaemic load

High fructose syrups and many other sources of fructose also contain large amount of glucose, either as sucrose or as free glucose.  For example, high fructose corn syrup is actually 55% fructose and 41% glucose and has been found to have a glycaemic index in the range of 58 to 73 (4).  

In contrast, an apple is about 66% fructose and 33% glucose - hence its relatively low glycaemic index of 39.  Furthermore, due to its fibre content, an apple actually has a glycaemic load (a measure of the glycaemic effect of the actual carbohydrate content) of only 6.

The glycaemic load of an equivilant serving of high fructose corn syrup would of course be 58 to 73.

So fructose that is not consumed as part of a whole fruit or vegetable, will still result in a very high glycaemic load and a large insulin response.

So fructose is great:

Consumed as part of whole fruits and vegetables

People who are sensitive to fructans should consume fruits and vegetables that are lower in fructans (follow the FODMAP diet).

N.B. I have found that many people can begin to reintroduce fruits and vegetables      that are on the FODMAP list and benefit from their nutritional value and ease of restricted diet, once they have followed the strict FODMAP diet and allowed their gut to recover, so long as they continue to avoid the FODMAP grains - wheat, barley and rye.

Natural sweeteners that are high in fructose, include honey and agave syrup and these can be used in moderation.

Foods high in added sugar, whether in the form of sucrose, syrup, starch or glucose, should be consumed only in small amounts, especially in individuals who are over-weight and trying to lose weight.

References

1. Bantle JP, Raatz SK, Thomas W, Georgopoulos A (2000). Effects of dietary fructose on plasma lipids in healthy subjects. Am. J. Clin. Nutr. 72 (5): 1128–34.

2. John S. White. Consumption and Metabolism. Adv Nutr March 2013 Adv Nutr vol. 4: 246-256, 201E

3. Heinz F, Lamprecht W, Kirsch J. Enzymes of fructose metabolism in human liver. J Clin Invest. 1968 Aug;47(8):1826-32.

4. Taiwan Yi Xue Hui Za Zhi. 1989 Sep;88(9):883-5. Effects of high-fructose (90%) corn syrup on plasma glucose, insulin, and C-peptide in non-insulin-dependent diabetes mellitus and normal subjects.

5. Sievenpiper JL, de Souza RJ, Mirrahimi A, Yu ME, Carleton AJ, Beyene J et al. Effect of fructose on body weight in controlled feeding trials: a systematic review and meta-analysis. Ann Intern Med 2012; 156: 291–304.

6.S Chiu, J L Sievenpiper, R J de Souza, A I Cozma, A Mirrahimi, A J Carleton, V Ha, M Di Buono, A L Jenkins, L A Leiter, T M S Wolever, A C Don-Wauchope, J Beyene, C W C Kendall, D J A Jenkins. Effect of fructose on markers of non-alcoholic fatty liver disease (NAFLD): a systematic review and meta-analysis of controlled feeding trials. European Journal of Clinical Nutrition, 2014;68, 416–423

7. Abdelmalek MF, Suzuki A, Guy C, Unalp-Arida A, Colvin R, Johnson RJ et al. Nonalcoholic steatohepatitis clinical research network. Increased fructose consumption is associated with fibrosis severity in patients with nonalcoholic fatty liver disease. Hepatology 2010; 51: 1961–1971

8. Vos MB, Lavine JE. Dietary fructose in nonalcoholic fatty liver disease. Hepatology. 2013 Jun;57(6):2525-31.

9. Fujisawa T, Mulligan K, Wada L, Schumacher L, Riby J, Kretchmer N (1993). The effect of exercise on fructose absorption. Am. J. Clin. Nutr. 58 (1): 75–9.

10. Donahue R, Attaluri A, Schneider M, Valestin J, Rao SS. Absorptive capacity of fructans in healthy humans: a dose response study. Gastroenterology. 2010;138:S709.

11. Fedewa A, Rao SSC. Dietary fructose intolerance, fructan intolerance and FODMAPs. Current gastroenterology reports. 2014;16(1):370

What is fructose?

So when is fructose a great food choice?

But doesn't fructose make you fat?

In the same way that starch is a polymer of glucose molecules, fructans are polymers of fructose and glucose molecules.  Fructans are contained in large amounts in onion, garlic, asparagus, coconut flesh, wheat, barley, rye (old style spelt wheat is low in fructans), leek, watermelon, chicory and also in grass (where they can cause metabolic problems including laminitis in horses).

These foods are identified in the FODMAP diet as problematic for people who are sensitive to fructans.  

The human body has only limited ability to break down these fructan polymers and therefore only absorbs 5 – 15% of fructans in the small intestine (10). The mechanism for malabsorption and intolerance is related to the lack of enzymes to fully digest the complex sugars (polysaccharides).  It results in the malabsorbed fructans reaching the colon, where they are then fermented. Furthermore, the small molecule of fructans draws more water into the intestine which can also result in bloating and diarrhea.  

The consumption of fructans is believed to have increased due to high fructan-containing diets being very common in the Western diet, especially as more wheat-based products (breakfast cereals, pasta and bread) are consumed. This, alongside the gut porosity enhancing and irritating effects of gliadin (part of the gluten protein in wheat) may have increased the number of people who are intolerant or sensitive to fructans (11).

The problem with Fructans

Many studies have indeed shown that the majority of fructose fed to rats is converted to dangerous free fatty acids and triglycerides that enter the blood stream and cause high blood cholesterol levels and high body fat levels.  

However, these studies were of rats, not humans. Several studies have shown that fructose does not cause these same effects in humans.

Is too much fructose bad for your heart?

However, fructose metabolism in your liver will also tend to produce more free fatty acids and triglycerides.  These ultimately end up as part of VLDLs (very low density cholesterol) and LDLs (low density cholesterols) which are known to increase the risk of developing atherosclerosis and heart disease.

A direct link between fructose consumption and risk of heart disease is still not clear (as unfortunately most studies have been carried out on mice and rat models, which have been shown to have very different levels of fructose metabolism to humans), but several studies have shown a link between excessive fructose consumption and levels of triglycerides in men (1, 6, 7, 8).

Glucose and starch - the insulin response

Glucose is also absorbed by the small intestine, but blood glucose levels are monitored by the body and regulated by several hormones, including insulin, glucagon, amylin and adrenalin.  

High blood glucose levels trigger an insulin response which stimulates the liver and muscles to take up glucose where it is metabolised into glycogen and once these stores are full, the liver will convert excess glucose to fat for uptake by body fat cells and the fat cells themselves take up glucose from the blood for fat storage.

Starch is made in plants to store energy and is composed of many glucose molecules linked together to form a polymer, referred to as complex carbohydrates. Starch is broken down into glucose molecules in the mouth and small intestine.

More about the insulin response.

It is important to remember that due to the large amount of starch in the form of bread, pasta, potatoes, cakes, biscuits etc. in the modern average diet, most people consume more than five times as much glucose as fructose (2).

Eating a lot of fructose will have similar effects to eating too much of any of the other dietary sugars and starch - once your glycogen (short term energy stores) are full, your body will divert storage to fat.

We know that excess body fat (and particularly abdominal fat) is a risk factor for heart disease, diabetes (and the associated health problems) and various cancers.  

This is because the metabolism of fructose in the liver of rats is very different (2, 3).

However, what is clear is that too much carbohydrate from any source, whether fructose, sucrose, glucose, lactose or starch, will cause higher blood fat levels, higher body fat levels and obesity. (5)

Again, it is important to note that most of the carbohydrates in the average, modern diet come from starch in the form of bread, pasta, cakes, rice etc. . This starch is simply lots of glucose joined together.

So fructose in itself does not make you fatter than any other food source, however too much of any food will!

Problems with digesting fructose

If too much fructose is taken in at a time, it cannot be absorbed quickly enough by the small intestine and passes into the large intestine. Here it is fermented by the intestinal flora producing carbon dioxide, short-chain fatty acids (which feed intestinal flora and bacteria), hydrogen and various other acids and gases. These can cause bloating, abdominal discomfort or pain, flatulence and diarrhoea.  

The production of short-chain fatty acids provides great food for unhealthy bacteria as well as healthy, friendly bacteria and leads to an overgrowth of unhealthy bacteria and the associated problems.

The absorption in the small intestine is reduced by exercise, so eating fructose too soon after exercise can cause more problems (9).

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