Why fructose needs to be consumed with caution

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Last Friday’s blog was about the UK’s Food Standard Agency’s (FSA) ‘traffic light’ food labelling scheme, or more specifically, the idiocy of it. One of the anomalies I mentioned was the fact that fruit manages to ring up a red light according to the FSA scheme, on account of its sugar content. That got me thinking about fruit’s predominant sugar ” fructose. This form of sugar is finding its way as an added ingredient in increasing amounts into our diet, generally in the form of what is known as ‘high fructose corn syrup’.

The burgeoning presence of fructose in our foods generally elicits a relaxed reaction from dieticians and health professionals. Part of this may relate to the fact that fructose is said to not cause blood sugar levels to rise like sucrose (table sugar). Fructose has traditionally been seen in a favourable light as an added sweetener, and has generally been recommended as the sugar of choice for diabetics. I also wonder whether the fact that fructose can be found in fruit also gives it a ‘healthier’ image than table sugar.

Despite the laissez-faire attitude many have to fructose, there is a considerable body of evidence that we need to be as wary of this specific form of sugar and sucrose. One recent study, for instance, published in the American Journal of Clinical Nutrition [1], fed seven men a diet containing less than 20 g of fructose for a period of 4 weeks. Subsequently, these men were fed a very similar diet, though this time it contained substantially more fructose (actually 1.5 g of fructose for each kg of their body weight).

The higher fructose diet was found to bring about a very significant increase in the levels of blood fats known as ‘triglycerides’ ” a biochemical change generally viewed as a sign of worsening health, particularly with regard to heart disease. Also, the fructose-rich diet was found to cause an increase in blood glucose levels, something that has obvious relevance for those of us keen to keep a lid on blood sugar levels, particularly diabetics.

The study did not, however, find changes in some other measurements including fasting insulin levels and body weight. However, it should be borne in mind that this study was very small and was conducted over the relative short term too. A longer-term study in a larger group of individuals might have yielded even worse results as a result of fructose-feeding.

To support this, there is other evidence which shows that fructose can impair the body’s ability to handle sugar, as well as reduce the effectiveness of insulin [2]. What is more, animal experiments reveal long-term consumption of fructose can indeed lead to elevated levels of both sugar and insulin.

It should also be borne in mind that many studies into the health effects of fructose have been performed in healthy subjects. This most recent study is a case in point. I shudder to think of what the results might be in test subjects who have a problem with their capacity to handle sugar in the system (such as diabetics and sufferers of the condition ‘metabolic syndrome’).

One of the main food sources of fructose is fruit. Most fruits release their sugar relatively slowly into the bloodstream, which limits their capacity to disrupt the body’s biochemistry and impair health. However, once fruit is juiced, the sugar becomes more available and therefore destabilising. The sugar concentration of fruit juices is the same as soft drinks. And just like soft drinks, fruit juice can be glugged down in considerable quantity quite quickly. This can only add insult to the injury of fruit juice’s highly sugary nature.

I have previously expressed some doubt about the ‘healthy’ image fruit juice has [3]. These recent findings regarding fructose only serve to redouble my concern. I do believe that fruit juice enjoys a healthy reputation that is thoroughly underserved. Some of this reputation depends, I suspect, on some suspect ‘science’. For those of you who missed it, let me draw your attention to research published recently which has found that industry-funded studies into fruit juice and other beverages are almost 8 times more likely to report positive results compared to those which were independently funded [4].

Another form that fructose finds its way into our diet is as a food additive. For instance, soft drinks are often sweetened with high fructose corn syrup. The evidence suggests that fructose as a food ingredient is likely to have highly corrosive effects on health in the long term, particularly for those who have difficulty handling sugar in the system. That’s a shame, considering that it fructose is often used as a sweetening agent in many speciality foods designed specifically for diabetics.

While fructose generally enjoys a healthy reputation, there is good evidence that its effects in the body are none too sweet. By all means include some fruit in your diet, but I would generally advise that fruit juice and foods with added fructose be treated with considerable caution.


1. Le KA, et al. A 4-week high fructose diet alters lipid metabolism without affecting insulin sensitivity or ectopic lipids in healthy humans. Am J Clin Nutr 2006;84(6):1374-9

2. Elliott SS, et al. Fructose, weight gain and the insulin resistance syndrome. Am J Clin Nutr 2002 76(5):911-922

3. Is fruit juice healthy?

Study reveals the potential for the food industry to pervert the course of science

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