WE KNOW about good and bad fats. Now suspicion is growing that not all sugars are created equal either. Overweight adults who consume large amounts of fructose have been found to experience alarming changes in body fat and insulin sensitivity that do not occur after eating glucose.
Pure fructose is found in fresh fruit, fruit juice and preserves. But much of it sneaks into our diets though high-fructose corn syrup (HFCS) in soft drinks - which gets broken down into 55 per cent fructose and 45 per cent glucose in the body - or via sucrose (ordinary sugar), which is broken down into the same two sugars.
Fears that fructose and HFCS are fuelling the obesity epidemic and triggering insulin resistance and diabetes have been circulating for years (New Scientist, 1 September 2001, p 26), but there have been few direct investigations in humans.
So Peter Havel at the University of California, Davis, persuaded 33 overweight and obese adults to go on a diet that was 30 per cent fat, 55 per cent complex carbohydrates and 15 per cent protein for two weeks. For a further 10 weeks, they switched to a diet in which 25 per cent of their energy came from either fructose or glucose.
In those given fructose there was an increase in the amount of intra-abdominal fat, which wraps around internal organs, causes a pot belly and has been linked to an increased risk of diabetes and cardiovascular disease. This did not happen with the group who consumed glucose instead, even though both gained an average 1.5 kilograms in weight.
Those who consumed fructose also had raised levels of fatty triglycerides, which get deposited as intra-abdominal fat, and cholesterol. Their insulin sensitivity also fell by 20 per cent. Glucose appeared to have no effect on these measures. Havel presented the results at a meeting of the Endocrine Society in San Francisco last week.
Because Havel's test looked only at pure fructose, not HFCS or sucrose, it is not yet clear whether these substances are to blame for obesity and diabetes. "The question is, what is the amount of HFCS or normal sugar you need to consume to get these effects?" says Havel, who is planning a long-term study to find out. But he says it's not too soon for people with metabolic syndrome - the blend of conditions including belly fat and insulin resistance that raise the risk of diabetes and cardiovascular disease - to avoid drinking too many fructose-containing beverages.
PepsiCo, which sponsored Havel's research, disagrees. "This is a very interesting and important study," says a spokeswoman. "But it does not reflect a real-world situation nor is it applicable to PepsiCo since pure fructose is not an ingredient in any of our food and beverage products."
In a separate study, Havel's team compared the immediate effects of consuming a meal in which 25 per cent of the energy came from one of HFCS, sucrose, fructose or glucose. Blood triglyceride levels were all elevated to a similar level 24 hours after consuming fructose, sucrose or HFCS, but not glucose (The American Journal of Clinical Nutrition, vol 87, p 1194), suggesting that all three substances may have similar, negative health impacts. Longer-term studies are needed to confirm whether the triglycerides produced by sucrose or HFCS have similar effects to fructose on abdominal fat and insulin resistance.
"It adds to what we have known for a long time," says Francine Kaufman at the Keck School of Medicine in Los Angeles. "It's probably not a good idea to consume too much sugar."
Linda Geddes, New Scientist reporter, San Francisco
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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 28 JUNE 2008.
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