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The sugar fructose has been much in the news in recent years, targeted by food campaigners such as Robert Lustig, founder of the Institute for Responsible Nutrition. They believe that our diets are far too high in sugar, especially in fructose, which unlike its close cousin glucose is not essential for the body and brain. Fructose is sweeter than both glucose and sucrose (table sugar, a combination of fructose and glucose). It is found naturally in honey and in fruit, such as dates, figs, apples, pears and grapes, which have very high levels of fructose.

Evolution of taste

Humans have evolved to find the sweet taste of fructose rewarding, a signal of high-energy food in a world where calories were not as easily available as they are now. Sweet foods were a rare treat, not an everyday food, until industrial processing and the slave trade brought sugar into Western culture. This was especially true for infants, as milk contains little fructose. Moreover, naturally-occurring sweet foods contain many other beneficial nutrients, such as fibre. Although such foods are energy-dense, the sugars in them are not immediately available once the fruit is eaten, but must be extracted. This takes time, reducing the impact on the body, which is why people can eat plenty of fruit without becoming overweight.

’empty calories’

Modern food technology, however, has detached fructose from its origins, using it as a sweetener in foods such as high fructose corn syrup, whose nutritional value is low. Hence the apparent paradox that people who eat a lot of junk food can be both obese and malnourished. Though they provide “empty calories”, these foods are enjoyably “moreish” to eat – or drink; soft drinks are a major source of fructose. They are also supplied to both adults and children. And they provide a source of sugar which is immediately available for processing – including conversion to fat – by the body, and which is often far more than the body requires.

Is fructose bad for me?

Fructose, unlike glucose, does not stimulate the release of insulin. Thus it is not regulated by the physiological mechanisms which control glucose levels. It is mostly converted to glucose by the liver, an energy-intensive process which also produces fat. This may contribute to fatty liver problems akin to those found in alcoholism or cirrhosis. Fructose may also boost levels of the “hunger hormone” ghrelin, whereas glucose stimulates production of leptin, which suppresses appetite. A small recent neuroimaging study in humans found that glucose, but not fructose, activated brain areas involved in appetite regulation and reward processing, possibly via their different effects on insulin.1

Because fructose is taken up and processed almost entirely by liver cells (as other cells, except for sperm, lack the necessary enzyme), some researchers have suggested that high-fructose diets may alter levels of blood triglycerides and cholesterol, raising the risk of cardiovascular disease.2 Others, however, find no evidence of this, for example in a US study of over 25000 people.3 Food campaigners note that between 1970 and 2000, the amount of added sugar in average diets rose by a quarter, while rates of obesity also rose. However, since 2000 fructose consumption has fallen, while obesity has continued to rise. In short, obesity cannot be simply linked to a single food ingredient – if only it were so simple! Nonetheless, the recommendations to eat less sugar make biological sense, since we eat far more of it than our bodies evolved to handle.

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