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New look at sugar, fat
You've heard the lowdown on fat, that some forms are better for you than others. Now, just in time for the holidays, get ready for a new perspective on sugars.
Nutrition researchers studying how the body metabolizes foods have found that, contrary to old beliefs, all sugars are not the same. The difference is not so much between honey and table sugar, or molasses and maple syrup, but between sugars at the molecular level -- in particular, between fructose and glucose.
Fructose, it seems, is more readily converted to fat than is glucose.
"It's been implicated in potentially contributing to weight gain and to increases of triglycerides," said Peter Havel, an associate professor of nutrition at the University of California, Davis, and co-author of an article on the role of fructose in weight gain, published in the latest issue of the American Journal of Clinical Nutrition.
The matter is by no means settled, as only a few studies have been done on how the human body processes fructose. But as the country's obesity problem spreads, the question is growing in relevance.
The use of fructose to sweeten foods has soared since the 1970s, when developments in corn processing produced high-fructose corn syrup, the predominant sweetener in American foods today. Corn syrup has a long shelf life, making it a popular ingredient for processed foods and helping contribute to the proliferation of such foods.
Not just a product of modern industry, fructose is a sugar naturally found in plants, especially fruit. However, the amount of fructose in a serving of fruit is small -- typically one-fourth to one-half the amount in a can of soda pop.
"Naturally made fructose would have the same effect (in the body) as chemically made fructose, but the fact is that most people don't eat so much fruit that it would make a major difference," Havel said.
"The other thing about fruit is that you get fiber, vitamins, minerals and antioxidants," he added. "We would by no means discourage people from eating fruits and vegetables."
Apart from reducing the amount of sugary foods in the diet overall, avoiding fructose is difficult. For one thing, nutrition labels don't offer a breakdown of fructose/glucose content.
In addition, fructose is a component of a variety of sweeteners, whether natural or manufactured. Sucrose, the table sugar extracted from canes and beets, for example, is naturally composed of equal parts glucose and fructose.
Honey has a combination of fructose, sucrose and glucose, and generally is richest in fructose, according to Charles Baker, a biochemist and vice president of scientific affairs for the Sugar Association. And since fructose tastes sweeter than an equal amount of glucose, honey tends to taste sweeter than granulated sugar, Baker said.
The extra sweetness of fructose is one reason -- among many -- for the rise in popularity of high-fructose corn syrup: It takes less to confer the same amount of sweetness, so it is cheaper to use than granulated sugar.
To complicate matters, high-fructose corn syrup isn't completely fructose. Available blends are manufactured to range from 42 percent to 90 percent fructose. Soft drinks, ice creams and other frozen desserts typically are sweetened with a 55 percent fructose formulation.
But even a formulation that contains only 5 percent more fructose than table sugar, such as soda sweetener, can have an effect on body weight over the long term, Havel said.
"In the mismatch between energy expenditure and intake that results in obesity over time, you don't need that much of a (difference)," he said. "Just increasing the fructose a little bit every day over years might cause people to gain weight."
The mechanism that can lead to weight gain is complex. First, fructose is metabolized, or broken down, almost exclusively in the liver, where it readily is converted into glycerol and fatty acids that become triglycerides, according to Sheldon Reiser, a retired U.S. Department of Agriculture nutrition researcher and a pioneer in the study of fructose metabolism.
Scientists believe that high triglyceride levels increase the risk for cardiovascular disease.
Glucose, by contrast, is metabolized throughout the body. When it enters general circulation -- causing the well-known rise in blood sugar -- glucose is delivered all over, including to the pancreas. There, it triggers the secretion of insulin, a hormone that helps glucose enter cells to be used for energy.
The insulin, in turn, stimulates the production of leptin, a hormone produced by fat cells that regulates appetite. Leptin signals the brain to eat less, and to step up the metabolic rate. "People or (other) animals that don't have any leptin overeat and become grossly obese," Havel said.
The glucose, in concert with insulin, also seems to lower the production of yet another hormone, ghrelin, (GRELL-in) which has been shown to induce hunger and lead people to eat more.
Fructose, by contrast, does not stimulate the production of insulin or leptin, and also appears not to decrease ghrelin, Havel said. So in addition to being more readily converted to fat, fructose doesn't activate the body's hormonal controls on weight gain.
Sugars, whatever the form, are not inherently bad for the body. In fact, the brain requires glucose for fuel. Life as we know it would not exist without sugars.
Plants, through photosynthesis, take the energy of sunlight and convert it to chemical energy in the form of sugars -- molecules of carbon, hydrogen and oxygen. Animals get their sustenance by eating plants or the flesh of animals that eat plants.
One form of sugar -- ribose -- makes up the backbone of our DNA.
Astrophysicists were excited two years ago when they discovered evidence of a simple sugar in a giant cloud of gas and dust near the center of the Milky Way. The sugar they found is glycolaldehyde, a sub-unit of ribose and glucose.
Scientists study the chemistry of interstellar clouds in part to understand how biological molecules formed early in Earth's history. The abundance or scarcity of sugars in space also is a clue to the likelihood of finding extraterrestrial life.
Although life itself depends upon sugars, that doesn't mean more is better. Modern, industrialized society has produced too much of a good thing, and at no time is the excess more evident than during the holidays.
Most nutritionists say that it isn't practical to select sweeteners based upon their fructose or glucose content, given the breakdown isn't specified on labels.