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Mon 19 Jan 2009 04:00 AM

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Is fruit sugar bad for you?

It doesn't cause spikes in insulin and blood sugar, but large amounts of fruit sugar (fructose) may contribute to overeating.

It doesn't cause spikes in insulin and blood sugar, but large amounts of fruit sugar (fructose) may contribute to overeating.

Most of the sugar we eat gets broken down and absorbed in the small intestine. Swarms of specialized enzymes attack larger molecules and convert them into three simpler sugars: mainly glucose, but also galactose (a part of lactose, the sugar in milk) and fructose.

There are a few more steps involved in breaking down the starches in bread, potatoes, and the like, but ultimately starch shares a similar digestive fate.

Our livers prudently stow away some of the absorbed glucose as glycogen, a molecule that can be turned back into glucose when we haven't eaten for a while. But most of the sugary stuff is distributed right away. Glucose levels in the blood shoot up, and the pancreas gets busy, pumping out the insulin that cells throughout the body need in order to take in glucose and use it for energy.

Oversupply the metabolic pathway for fructose with this sugar, and the liver ends up churning out triglycerides – fat that circulates in the blood.

Post-meal glucose and insulin spikes are perfectly normal and entirely unavoidable. But if they're too big, or come too often, they're harmful. Canadian researchers devised the glycaemic index (GI) to make it easier to compare how different food affects blood sugar levels: the higher the GI number, the bigger the increase in blood sugar.

A number of studies have found that people who eat a lot of high-GI foods - cookies, sweets, bread made with refined flour, and potatoes - have higher rates of diabetes and heart disease.

Specialised metabolism

Pretty much all of the body's cells come equipped with enzymes that allow them to harness glucose. But the enzyme that metabolises fructose, called fructokinase, is found exclusively in liver cells. So although fructose is involved in glucose metabolism indirectly, fructose metabolism is pretty specialised. As a result, if you eat fructose, your blood glucose and insulin levels stay fairly level.

If, as the GI-index research indicates, glucose and insulin increases are a problem, then replacing other sugars in your diet with fructose looks like it might be a solution.

But, alas, it's more complicated than that. For all the mischief that glucose and insulin cause, they do trigger some helpful hormonal changes. Levels of leptin, the hormone that gives us that "full" feeling, go up when insulin surges, and levels of ghrelin, the "hunger hormone," go down.There's fairly good evidence that fructose has just the opposite effect, reducing leptin, so we don't necessarily feel full after a fructose-filled meal, and not lowering ghrelin as much as glucose does, so we stay hungry. For this reason, some experts see high-fructose diets as contributing to overeating.

And there's another problem: Oversupply the metabolic pathway for fructose with this sugar, and the liver ends up churning out triglycerides - fat that circulates in the blood. The same is true of glucose and its metabolic pathway, but a larger amount of glucose is required to cause an oversupply.

Fructose is a naturally occurring sugar. You'll find it in honey, vegetables (in small amounts), and, of course, fruit. Not all the sugar in fruit is fructose, which is one of the reasons - lack of fibre is another - that some fruit, such as watermelon, have a high glycaemic index.

The great American weight problem and the sweetening of the food supply with the suspect syrup happened at about the same time.

But most people's intake of fructose from fruit and vegetables is dwarfed by what they get from sucrose - better known as table sugar, or just sugar - and high-fructose corn syrup. Sucrose is a two-sugar molecule (a disaccharide) consisting of fructose and glucose. High-fructose corn syrup is corn syrup that has been processed to increase the fructose level, which makes it taste sweeter.

The most common variety is 55% fructose and 45% glucose. In the US, high-fructose corn syrup has replaced sucrose as a sweetener, especially in soft drinks, but in many other types of food too.

Experts debate how responsible high-fructose corn syrup is for the American obesity epidemic and the soaring type 2 diabetes rate. Those who see a connection note that the great American weight problem and the sweetening of the food supply with the suspect syrup happened at about the same time. They also point to fructose metabolism and its triglyceride output.

Skeptics counter with several arguments. Sucrose and high-fructose corn syrup contain about the same amount of fructose and glucose (a 50-50 split versus a 55-45 one), so it's questionable whether the metabolic effects are that different. Further, many of the damning metabolic studies have used all-fructose solutions, not high-fructose corn syrup. And they add, the real problem is all the nutritionally empty, added sugar in our diets, not any particular form of that sugar.

Watch the syrup

Where does this leave us? Certainly no one should stop eating fruit out of some misguided fear of fructose. Most varieties of fruit are brimming with nutritional virtue - vitamins, minerals, fibre - and have relatively little fructose and other sugar.

Is high-fructose corn syrup especially harmful? At this point, the case against food and soft drinks sweetened with the stuff is pretty persuasive.

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