cover article in The New York Times Magazine several weeks ago persuasively reported that our national overindulgence in fructose and other sugars is driving the epidemics of obesity, diabetes and other illnesses. But that much-discussed article, by the writer Gary Taubes, focused on how sugars like fructose affect the body in general. It had little opportunity to examine the related issue of how sugar affects the body in motion. Do sweeteners like fructose — the sweetest of the simple sugars, found abundantly in fruits and honey — have the same effect on active people as on the slothful?
A cluster of new studies suggests that people who regularly work out don’t need to worry unduly about consuming fructose or other sugars. In certain circumstances, they may even find the sweet stuff beneficial.
The unique role that the various sugars play in exercise is well illustrated by a new study published in March in Medicine & Science in Sports & Exercise. It involved a group of highly trained cyclists and their livers. For the experiment, Swiss and British researchers directed the cyclists, all men, to ride to exhaustion on several different occasions. After each ride, they swallowed drinks sweetened with fructose or glucose, another simple sugar often identified as dextrose on ingredient labels. (Some also drank a milk-sugar sweetener.)
The liver is often overlooked when we consider organs integral to exercise, but it is an important reservoir of glycogen, the body’s stored form of glucose. All sugars, including sucrose, or table sugar, and high-fructose corn syrup, which usually consists of almost equal portions of glucose and fructose, are converted into glucose, and stored as glycogen, in the body. Strenuous exercise diminishes or exhausts this liver glycogen, and until those stores are replenished, the body isn’t fully ready for another exercise bout.
In this study, the scientists used magnetic resonance imaging to measure the size of each rider’s liver, before and after the rides. All of the cyclists lost liver volume during their workouts, a sign their livers were depleted of glycogen. But those who afterward drank fructose replaced the lost volume rapidly, showing a 9 percent gain in volume after six-and-a-half hours versus a 2 percent gain among the riders drinking glucose-sweetened drinks. Over all, the researchers concluded, fructose-sweetened drinks were twice as effective as the glucose-sweetened drinks in stimulating the liver to recover.
This finding concurs with a large body of earlier research suggesting that fructose is particularly useful for avid athletes. During long, hard workouts, they can burn through almost all of their stored glycogen and fade. But drink or eat something sugary, and the muscles can keep working.
Interestingly, absorption seems to be best if the sweetener contains both glucose and fructose. A 2008 study of cyclists found that if they downed a sports drink sweetened with glucose during a two-hour bout of moderate pedaling, they rode faster during a subsequent time trial than riders who had drunk only water. But if the sports drink contained both glucose and fructose (in a two-to-one ratio), the riders were 8 percent faster in the time trial than those drinking glucose-sweetened fluids alone. (Most bottled sports drinks on the American market are sweetened with high-fructose corn syrup, so contain glucose and fructose in a closer to one-to-one ratio.)
Does this suggest that those of us who regularly but moderately work out might want to consider sugar loading? Alas, the answer is no. Large amounts of sweetened sports drinks, gels and bars are recommended only for the “serious athlete” who works out for more than two hours at a time, Asker Jeukendrup, director of the human performance lab at the University of Birmingham in England and co-author of both studies, said in an e-mail. “If someone goes for a 30-minute walk, the duration and intensity will be too short” for sugar to make a difference in terms of performance, he said.
But that half-hour stroll could affect how your body responds to sugar, other new science suggests. You may not need Skittles to fuel the walk, but the walk will affect how your body metabolizes the candy, if you do indulge. Activity can “significantly reduce the health risks associated with fructose and other forms of sugar,” said Dr. Richard J. Johnson, a professor of medicine at the University of Colorado Anschutz Medical Campus in Denver, who has long studied fructose metabolism and was an author of a review article last year about fructose and exercise.
Consider, again, the liver. In sedentary people, ingesting large amounts of fructose, which is mostly metabolized in the liver, has been associated with the development of a disorder known as fatty liver. That condition can reduce the body’s ability to respond to insulin, the hormone that helps to control blood sugar. A person with a fatty liver often develops resistance to insulin, becomes less able to control levels of glucose in the blood, and drifts almost inexorably toward Type 2 diabetes.
But exercise can derail this process. A review of recent studies, published in December, concluded that beginning an exercise program could significantly lessen the amount of fat in someone’s liver, even if that person didn’t lose weight during the program.
Moderate exercise — about 30 minutes a day five times a week — also aids in the control of blood sugar levels if a person has developed Type 2 diabetes, according to a comprehensive review published this week in The Journal of the American Medical Association.
Over all, Dr. Johnson said, the “current science suggests that exercise exerts a positive physiological influence” on some of the same metabolic pathways that sugar harms. “Exercise may make you resistant to the undesirable effects of sugar,” he said.
Not that any of us should live on sweets. “Sugar is not all bad,” Dr. Johnson concluded, “but it’s hardly nutritionally good, either.” The best sweet option, he added, is fruit, which comes prepackaged with a small but satiating dose of all-natural fructose.