Can we blame heart disease, hypertension and cancer on the toxic properties of sugar?

IS SUGAR toxic? That was the question posed by a recent article in the New York Times,which showcased the arguments of Robert Lustig, professor of paediatrics in the Division of Endocrinology at the University of California.

In a 90-minute video on YouTube called Sugar: The Bitter Truth, Lustig delivers a lecture that points to the sugar fructose as a particular villain when taken in excess.

Fructose occurs naturally in fruits. But, more significantly in this case, it is also a component of sucrose (the granulated stuff you might put in your coffee or tea, and in cakes, biscuits and sweets), and it is particularly concentrated in a sweetener that’s called high-fructose corn syrup in the US.

"If Lustig is right, then our excessive consumption of sugar is the primary reason that the numbers of obese and diabetic Americans have skyrocketed in the past 30 years," writes the New York Timesarticle's author, Gary Taubes. "But his argument implies more than that. If Lustig is right, it would mean that sugar is also the likely dietary cause of several other chronic ailments widely considered to be diseases of western lifestyles – heart disease, hypertension and many common cancers among them."

The link between excessive sugar intake and such chronic diseases is “fairly compelling but more indirect than direct”, says John Reynolds, professor of surgery at Trinity College Dublin and St James’s Hospital.

Eating excess sugar can increase the risk of obesity, which is associated with chronic inflammation, resistance to insulin and a constellation of conditions known as the “metabolic syndrome”, and this can in turn push up the risk of chronic disease, he says.

“It’s absolutely clear that insulin resistance and metabolic syndrome as components of obesity may be associated with increased cancer risk, as well as being major players for diabetes and cardiovascular disease,” he says, noting that even if you are not obese it’s still possible to be insulin-resistant.

“The western diet is one that very much increases insulin levels and promotes insulin resistance.

“If you have insulin resistance, you will have higher insulin levels in the blood, you will have higher levels of insulin-like growth factor in the blood, and higher levels of those mediators can have pro-cancer and pro- inflammation properties.”

Studies on animals show that taking high levels of fructose, in particular, appears to stress the liver, he notes.

“In a laboratory set-up, if you want to cause fatty liver – if you want to change the liver and make it fatty and dysfunctional and get it inflamed – you can give nothing better that is in our diet than fructose.”

Eating fructose in its natural state – within unprocessed fruit – is a different matter because of the food matrix, says Reynolds.

“There’s not the slightest bit of evidence that natural fructose within good whole foods is harmful,” he says. “And other carbohydrates you get from potato and bread or other starches are metabolised differently. They contain mainly glucose, and they would be metabolised by every cell in the body rather than challenging and sometimes damaging the liver.”

However, proving the case against excess fructose in humans is not straightforward, according to Reynolds. “It’s very difficult to do the studies in humans because there are so many variables in terms of what else we eat, what else we put into our body, how we exercise and so on,” he says.

“The industry would have hidden behind this – certainly in America – for many years, saying you can’t prove it conclusively. Most people who know this area would have concerns, but it’s very difficult to make a completely linear, 100 per cent copper-fastened argument in humans, because of all the other variables. Having said that, it’s a very compelling case.”

So is sugar toxic? “Sugar is clearly not a toxin as we know it, but if part of the sugar that is most commonly used can directly damage an organ [the liver] so vital and so important in terms of our metabolic responses in the prevention of diabetes and heart disease and now cancer, chronically it is toxic in that regard and it’s probably not an unreasonable term,” says Reynolds.

Consumers in Europe who want to avoid high-fructose corn syrup in products should scan food labels for “isoglucose” or “glucose-fructose syrup”, because those are the terms used here, according to dietitian Dr Mary Flynn, chief specialist in public health nutrition with the Food Safety Authority of Ireland.

However, she notes that we are less likely to encounter it on this side of the Atlantic. “The reality is that in Europe we don’t use it very much,” she says.

Flynn is also keen to stress that the fructose in whole fruits is not a problem. “Because fruit is so bulky, you wouldn’t be eating that much fructose, but if you are eating cakes and biscuits, they are more dense, they are not as bulky and you can eat quite a lot more of them, you are going to get a lot more fructose – it’s an abnormal scenario,” she says.

And she suggests that fruit juices should be diluted with water. “[In fruit juice] you lose the matrix, you lose the bulk of the fruit,” she says. “I would say to parents about giving children juice: if you do use it try to dilute it and go very easy on it.”

Other ways to tackle excess sugar consumption include cutting down on cakes, sweets and biscuits and sugary soft drinks, and avoiding putting sugar in tea and coffee.

“Where sugar can be used sensibly is where it helps make something palatable that is good for you, like stewed fruit, high-fibre cereals or [a thin spread of jam on] brown bread,” says Flynn.

Consultant endocrinologist Prof Donal O’Shea, who runs the obesity clinic at Loughlinstown Hospital in Dublin, notes the problems of taking too much of any one nutrient on board, including sugar.

“The bottom line is that many of the nutrients, vitamins and minerals we need as essential are toxic in excess. For example, iron – we die without it, but too much and we get diabetes, cancer and liver failure,” he says.

“When we were primal organisms we ate only when hungry, and therefore overload wasn’t an issue.

“Now we have excess and see what too much of a ‘good’ thing does.

“Sugar is essential for life, but excess causes problems – whether a direct toxin or not isn’t clear, but I suspect it does have direct toxic effects, and certainly the change to fructose as a main sweetener back in the 1970s in the US was a very understudied change,” says O’Shea.