Fructose has been accused of being the root of all dietary evil but at a symposium in Sydney this week Canadian researcher John Sievenpiper argued that fructose has been misjudged.

John Sievenpiper knows a fair bit about dietary carbohydrates and health. His team in Toronto recently conducted systematic reviews and meta-analyses evaluating the effects of fructose on body weight, blood pressure and glycemic control in humans. He discussed some of the findings at the ‘Sweet Symposium’ on 2 December.

The fructose-centric view

Sievenpiper reviewed the rapid evolution of what he called the ‘fructose-centric view’ in modern nutrition, which positions fructose as a central driver of the obesity epidemic and cardiometabolic risk. Certainly several lines of evidence had raised suspicions about fructose – apparent increases in fructose consumption were associated with increases in obesity in population studies; animal studies showed fructose had adverse metabolic effects; there were similar findings in some human studies; and an articulate anti-fructose champion, Robert Lustig, emerged and took his simple ‘fructose is toxic’ message to the general public. And the media loved it – even the ABC’s ‘science’ program Catalyst jumped on the bandwagon and pushed the idea around. In less than a decade the fructose hypothesis became an anti-fructose doctrine with a chanting chorus of true believers.

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Concerns about the fructose hypothesis

Sievenpiper outlined several concerns he had about the fructose hypothesis. The first was the emphasis that had been placed on epidemiological links between the consumption of high fructose corn syrup and the increasing rates of obesity in the United States. Such associations are low quality evidence.

Also, the apparent link between increasing fructose intake and obesity rates was an American phenomenon. Sievenpiper noted the Australian Paradox i.e. rising obesity rates against a background of falling intakes of sugar, and there were similar paradoxes in the United Kingdom and Canada. A recent American Paradox had also emerged – falling intakes of sugar among children and younger adults in the US between 1999 and 2008, yet little or no reduction in dietary energy intakes. A plethora of paradoxes.

A second concern was the over-reliance on animal data when describing the metabolic effects of fructose. And the amount of fructose fed in some of the animal studies was exceedingly high – up to 60% of calories and therefore way beyond the amount typically consumed by humans (about 9% of calories in the US diet).

Sievenpiper’s third concern was that many of the human studies failed to fully account for the confounding effect of dietary energy. If feeding someone excess calories in the form of fructose produces metabolic disturbances, what’s the core problem? Is it the fructose, or is it the excess calories?

Fructose and cardiometabolic risk

The fructose-centric view states that ingested fructose ‘turns straight to fat’ i.e. fructose is rapidly taken up by the liver and converted to triglycerides, thereby contributing to fatty liver, insulin resistance and increased triglyceride levels in the blood. This may be what happens in rats but it’s different in humans – most dietary fructose (about 50%) is converted to glucose and ends up in the circulation, and another 15% or so is stored in glycogen. A further 25% is converted to lactate, so only a minor portion of ingested fructose (1-3%) is converted to fat in human beings.

To tease out the effects of fructose and excess calorie consumption Sievenpiper reviewed two types of human studies – ‘substitution’ trials in which the effects of fructose were assessed with total calorie intake held constant, and ‘addition’ trials in which extra calories in the form of fructose were added to diets. The addition trials observed higher body weight, higher triglycerides, higher total cholesterol, more fatty liver and higher uric acid levels – all the bad things that fructose is supposed to do.

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But the substitution trials painted a very different picture – none of these problems was evident and there was even a suggestion of benefit of fructose in relation to blood pressure and glycaemic control.

The conclusion that Sievenpiper drew from this work is that the addition trials show … effects which appear more attributable to the excess energy than the fructose itself. In relation to the obesity epidemic he argued that … attention needs to remain focused on reducing overconsumption of all caloric foods. Lower calorie intake to control body weight? What a radical idea!

Robert Lustig has argued that fructose is bad but glucose is good and should replace fructose where possible. Sievenpiper cited a recent review by David Ludwig that addressed this proposition directly. And the answer:

… the recommendation to replace fructose with glucose lacks an evidence basis. Rather, public health efforts should focus on reducing intakes of all highly processed carbohydrates, not just refined sugar.

‘Highly processed carbohydrates’

Ludwig doesn’t distinguish between refined sugar and refined starch; he bundles them together as ‘highly processed carbohydrates’. Sievenpiper cited a recent systematic review of 25 controlled trials supporting this view that concluded:

… it would appear that a moderate dietary sucrose intake at levels up to 25% of energy appears to have no significant adverse effects on lipid or carbohydrate metabolism in normal healthy adults when substituted for starch …

These findings are not new – they are consistent with those of the Institute of Medicine in the US and the European Food Safety authority that were discussed in an earlier post. But they beg the question: if substituting refined sugar for refined starch in diets up to the quite high level of 25% of energy has no significant health effects why do our nutrition authorities continue to use sugar or added sugar as a criterion for assessing the nutritional quality of foods and diets?

The last word

The fructose doctrine suggests that many of our dietary ills stem from just 8-9% of daily calories contributed by fructose. The real problem – highly processed carbohydrates – is much bigger than that.

Further reading

If you are looking for more insight into John Sievenpiper’s views on fructose check out his interview with American science writer David Despain.