Forty years ago, British nutritionist John Yudkin wrote a book about sugar. Titled Pure, White and Deadly, Yudkin argued that consumption of sugar, not fat, was driving the epidemic of heart disease.
But the evidence he put forward was not convincing, so Yudkin’s book (and reputation) fell from favour.
Studies conducted in the past decade or so have led to something of a revival of the idea that excessive intakes of sugar (or the component fructose) may be contributing to a host of health-related conditions that are collectively known as the “metabolic syndrome”. These conditions include obesity, high blood pressure, insulin resistance (a forerunner of type 2 diabetes) and high blood lipids (including cholesterol). Metabolic syndrome is strongly associated with heart disease and type 2 diabetes, among other chronic diseases.
With the rise of obesity, the spotlight has recently been on sugar as a potential driver of the epidemic.
In 2003, the World Health Organization (WHO) recommended that “free sugars” should provide less than 10% of total energy intake. Free sugars refers to all sugars added to the food by the manufacturer, cook or consumer, plus sugars naturally present in honey, syrups and fruit juices.
The WHO plans to update its recommendations, so a review was conducted of the scientific literature on the effects of sugar consumption on body weight. The results were published last week in the British Medical Journal (BMJ).
In summary, the authors found there was a small but statistically significant weight gain (around 0.8 kilograms) associated with increasing sugar intake among adults. Consistent with this finding, a similar (and also significant) weight loss was associated with a reduction in sugar intake.
Increased sugar intake was also associated with significant weight gain in children. But contrary to the finding for adults, decreased intake of sugar was not significantly associated with reduced body weight in children. However, this was attributed to poor compliance: the children did not always adhere to their assigned levels of sugar intake.
A key conclusion was that consumption of sugar-sweetened beverages was particularly likely to lead to increased body weight in both adults and children. This confirms previous findings that sugar in liquid form is closely associated with weight increase.
In an editorial accompanying the BMJ paper, two experts comment that,
Sugar in this form does not induce satiety to the same degree as it does in solid form, which makes overconsumption easier. Because of this, and the large amounts of sugar consumed in drinks, reducing the intake of sugar sweetened drinks is a high priority.
The BMJ paper also reported that when energy intake is kept constant, replacing sugars with other carbohydrates, protein or fat does not lead to significant weight reduction. So it appears that it’s the over-consumption of total kilojoules that leads to weight increase when sugar intake is high, possibly because sugars consumed in liquid form are less likely to leave you feeling full.
The authors do not discount the possibility that the increase in weight may be due to particular metabolic effects uniquely attributable to fructose, as hypothesised by Robert Lustig, a US professor of paediatrics. But the editorial accompanying the BMJ paper argues against this “fructose hypothesis”, stating that “there is no good evidence that replacement of fructose with glucose would be beneficial.”
The editorial also points out that “… many starchy foods, particularly highly processed grains and potato products, have a high glycemic index, raising blood glucose and insulin more rapidly than an equivalent amount of sucrose.”
So does the BMJ paper change our understanding of what is driving the obesity epidemic?
The short answer is no. Rather, the paper provides additional evidence that high intakes of sugar are almost certainly harmful to health, not least through their tendency to promote weight gain.
The authors also make it clear that, contrary to the belief of many members of the public, sugar intake is almost certainly not the only driver of the obesity epidemic. They point out, for example, that the effect of varying sugar consumption on body weight is relatively small, and that this is unsurprising, “owing to the multifactorial causes of obesity”.
So what should we do with this knowledge?
Clearly, avoiding excessive sugar intake would be prudent, but this has been recommended for as long as dietary guidelines have been in existence in Australia (and in most other nations). The current wording of the relevant Australian guideline is, “Consume only moderate amounts of sugars and foods containing added sugars”.
This wording is likely to be altered when the revised guidelines are published next month. At present, the draft revised guideline reads: “Limit intake of foods and drinks containing added sugars. In particular, limit sugar-sweetened drinks.”
That sounds like good advice to me!