Photograph by Richard Nowitz, Nat Geo Image Collection
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A wall of soda can be a formidable one for our diets.
Photograph by Richard Nowitz, Nat Geo Image Collection
The Plate

Sugary Drinks May Make Us Crave Extra Calories

Some kinds of sugar, it turns out, make us lose all willpower when it comes to fattening food.

A recent study by Kathleen Page and colleagues found that a dose of fructose in your drink may make you yearn for snacks. A slug of glucose, on the other hand, leaves you feeling full, food-resistant, and craving-free.

Fructose and glucose are simple monosaccharides which are often found together in nature–in everything from honey and maple syrup to fruits and vegetables. Bound together–a molecule of fructose plus a molecules of glucose–they form the disaccharide sucrose, commonly known as table sugar. Chemically this sugary pair looks a lot alike– but it turns out that their tiny molecular differences lead to major physiological effects.

In Page’s experiments, 24 volunteers guzzled cherry-flavored beverages spiked with either fructose or glucose, then were subjected to a battery of tests. For example, participants were shown pictures of yummy, high-calorie foods (candy, cookies, pizza, and juicy burgers) while their brains were scanned via functional magnetic resonance imaging. Fructose drinkers–on a scale of one to ten–reproducibly rated themselves as hungrier than glucose drinkers; and their brains, while viewing luscious pictures of food, showed greater activity in regions associated with attention and reward. Glucose drinkers, given a choice between a fattening snack and a reward of cold cash, to be delivered in a few weeks, went for the money. Fructose drinkers went for the food.

The conclusion, the researchers predicted darkly, is that fructose consumption “may promote feeding behavior.” In other words, a glass of fructose-heavy Hawaiian Punch or Mountain Dew may make you want to stuff your face.

Page’s results are the latest in a substantial list of literature fingering fructose as the possible culprit behind the current epidemic of obesity and Type 2 diabetes. Fructose has also been shown to boost levels of triglycerides and LDL (bad) cholesterol in the blood, suggesting a link to cardiovascular disease, and has been implicated as the evil genius behind hypertension and some common cancers.

Such effects have led many physicians and nutritionists to warn people off sugar; and anti-sugar spokesperson Robert Lustig, an expert on childhood obesity at the University of California, San Francisco, School of Medicine, has dubbed sugar an outright poison. (See Lustig’s no-holds-barred lecture Sugar: The Bitter Truth.)

Fructose isn’t inherently horrible stuff, and in nature it’s generally made available in small manageable quantities.  An apple, for example, contains about 10 percent fructose, and an apricot, about one percent. Processing and refining, however, have vastly upped the concentrations of fructose we’re now getting in our food and drink. While sugar cane is about 5 percent fructose, refined table sugar is 50 percent. High-fructose corn syrup is 55 percent fructose and agave nectar, often touted as a natural alternative to table sugar, is 70 to 90 percent fructose.

Fructose is the sweetest naturally occurring sugar – about twice as sweet as glucose, the essential sugar that serves as our body’s prime energy source–and our bodies metabolize the two very differently. Unlike glucose, which is metabolized by every cell in the body, fructose is processed primarily by the liver, where it’s converted to fat–a negative for us, since a build-up of fat in the liver is associated with insulin resistance and heart disease.

Furthermore, fructose doesn’t stimulate insulin secretion or leptin production, which together send signals to the brain that suppress appetite by telling the body it’s had enough to eat. That means that fructose may induce us to keep chowing down when we don’t need to, which in turn can lead to weight gain.

If sugar is poison, as Lustig argues, we’re in trouble–because there’s no doubt that, collectively, we’re eating a lot more sugar than we used to. Over the past few decades, sugar consumption has skyrocketed, mostly due to “added sugars” in a wide range of foods and drinks. Often, annoyingly, this is hidden sugar: barbecue sauce, for example, has 13 grams of sugar in every two tablespoons; a carton of fruit yogurt contains 19 grams of sugar; a handful of raisins, 24 grams, and a glass of Reisling, six grams.

All told, the average American nowadays consumes about 130 pounds of sugar a year. Soft drink consumption, which is where we get most of our fructose, amounted to a mere 90 servings a year (2 servings per week) in the 1940s; as of 2000, it was up to 600 servings a year (2 servings a day). And nutritionists caution that switching to fruit juices won’t help us here: a glass of apple juice is 66 percent fructose.

The research isn’t all in yet on fructose; and not all scientists agree that sugar is the root cause of the current obesity epidemic and its accompanying battery of obesity-related diseases. Some argue that lifestyle and just plain overeating are major contributors.

On the other hand, most agree that our national sugar splurge isn’t doing us any good weight-wise. The World Health Organization recently reduced its recommended sugar intake allowance from 10 percent to 5 percent of total daily calories–which, for a normal adult, works out to about six teaspoons (25 grams) of sugar a day.

The study was published earlier this week in the journal Proceedings of the National Academy of Science.